Peter B Kelemen
email: peterk@ldeo.columbia.edu

Juan Carlos De Obeso

James A Leong

Marguerite Godard

Keishi Okazaki

Alissa J Kotowski

Craig E Manning

Eric T Ellison

Manuel D Menzel

Janos L Urai

Greg Hirth

Matthew Rioux

Daniel F Stockli

Romain Lafay

Andreas M Beinlich

Jude A Coggon

Nehal H Warsi

Jürg M Matter

Damon A H Teagle

Michelle Harris

Katsuyoshi Michibayashi

Eiichi Takazawa

Zaher Al Sulaimani

Listvenite formation during mass transfer into the leading edge of the mantle wedge: Initial results from Oman Drilling Project Hole BT1B

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Oman Drilling Project (OmanDP) Hole BT1B at 23.364374°N, 58.182693°E, about 12 km southeast of the town of Fanjah in the Sultanate of Oman, sampled serpentinized peridotites and listvenites (fully carbonated peridotites, [START_REF] Halls | Listvenite and related rocks: Perspectives on terminology and mineralogy with reference to an occurrence at Cregganbaun, Co. Mayo, Republic of Ireland[END_REF] at the base of the Samail ophiolite, the basal fault of the ophiolite, and the underlying metamorphic sole, with the intention of investigating mass transfer and deformation in the "leading edge of the mantle wedge" overlying a Tethyan subduction zone. Study of outcrops along the contact between the sole and the peridotite can illuminate processes in and above a subduction zone that are generally inaccessible to direct observation. Of particular interest are (a) the source of footwall fluids, (b) the nature and mechanism of fluid transport along the thrust fault and into the mantle wedge, (c) chemical, mineralogical and rheological modification of the hanging wall by reaction with footwall fluids, and (d) the mechanisms of subduction zone deformation.

Hole BT1B was drilled using cylindrical diamond bits and wireline core retrieval, from March 7 to March 23, 2017. Core recovery was ~ 100% throughout the Hole. Core was shipped to Japan and loaded onto Drilling Vessel Chikyu, where the OmanDP Science Team performed analyses closely following protocols established by the various incarnations of the Ocean Drilling Program (currently, the International Ocean Discovery Program, IODP). Detailed core descriptions, together with drilling history and some background information (Kelemen et al 2020b, Kelemen et al 2020c) are available online at http://publications.iodp.org/other/Oman/VOLUME/CHAPTERS/113_BT1.PDF This paper reports new mapping and structural observations, mineral analyses and modeling, together with a summary of prior observations. It is intended to provide interpretive context for more detailed studies of core from Hole BT1B and the geology of the surrounding region. Such studies in this Special Issue of the Journal of Geophysical Research include those focused on textural and petrologic data for listvenites [START_REF] Beinlich | Ultramafic rock carbonation: Constraints from listvenite core BT1B, Oman Drilling Project[END_REF] and the metamorphic sole [START_REF] Kotowski | Structural and thermal evolution of an infant subduction shear zone: Insights from sub-ophiolite metamorphic rocks recovered from Oman Drilling Project Site BT-1B[END_REF], volume changes during serpentinization [START_REF] Malvoisin | Measurement of volume change and mass transfer during serpentinisation: Insights from the Oman Drilling Project[END_REF], major and trace element geochemistry with relevance to subduction zone mass transfer [START_REF] Godard | Geochemical profiles across the listvenite-metamorphic transition in the basal megathrust of the Semail Ophiolite: Results from drilling at Oman DP Hole BT1B[END_REF][START_REF] Okazaki | Major mineral fraction and physical properties of carbonated peridotite (listvenite) from ICDP Oman Drilling Project Hole BT1B inferred from X-ray CT core images[END_REF], Mg-, Srand C-isotope geochemistry constraining the source of CO2-bearing fluids and the fluid-rock reaction process (de Obeso et al 2021a, de Obeso et al 2021b) and deformation of the listvenites [START_REF] Menzel | Brittle deformation of carbonated peridotite: Insights from listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)[END_REF], Menzel et al 2021).

Section 2 provides geological context for observations of core and surrounding outcrops. Section 3 summarizes methods used to produce results presented for the first time in this paper. Section 4 reports new geological field observations and analytical data, and reviews key results from Kelemen et al. (2020b) in context. Section 5 discusses the pressure, temperature and timing of listvenite formation, the nature and source of the fluids that transformed mantle peridotite into serpentinite and listvenite, the chemical and mechanical processes during these transformations, and deformation of altered mantle peridotite immediately above a paleo-subduction zone beneath the "leading edge of the mantle wedge".

It is hoped that this paper, and the other papers in this Special Issue, will provide a starting point for future investigations of this unique and important site. In this context, readers should be aware that the archive half of the core is currently stored at Petroleum Development Oman where it is available for viewing, the working half of the core is stored at the American Museum of Natural History, where it can be sampled upon request to the Museum, and a huge volume of data from shipboard visual core observations and analytical data is available to anyone at http://publications.iodp.org/other/Oman/OmanDP.html , https://www.icdponline.org/projects/world/asia/oman/, and other sites that can be accessed from there.

Geological setting

Regional geologic context

The Samail ophiolite is composed of oceanic crust formed at a submarine spreading center above a subduction zone. The crustal thickness and composition of the ophiolite is similar to the geophysically and geologically constrained characteristics of fast-spreading, Pacific oceanic crust, with a few km of submarine lavas and sheeted dikes overlying a thicker, gabbroic lower crust (e.g., [START_REF] Christensen | Geology and seismic structure of the sorthern section of the Oman Ophiolite[END_REF], Coleman & Hopson 1981[START_REF] Nicolas | Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust[END_REF] However, the lavas have a trace element "subduction signature" [START_REF] Alabaster | The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex[END_REF][START_REF] Pearce | The Oman Ophiolite as a Cretaceous Arc-Basin Complex: Evidence and Implications[END_REF][START_REF] Pearce | Tectonic implications of the composition of volcanic arc magmas[END_REF], and parental, mantle-derived magmas appear to have contained 0.2 to 2 wt% H2O, substantially more than in primitive mid-ocean ridge basalts [START_REF] Macleod | Moist MORB" axial magmatism in the Oman ophiolite: the evidence against a mid-ocean ridge origin[END_REF]. Beneath the crustal section of the ophiolite, residual mantle peridotites and tabular dunites record polybaric decompression melting, melt extraction, and focused transport of basaltic melt upward to form the crust [START_REF] Braun | Dunite distribution in the Oman ophiolite: Implications for melt flux through porous dunite conduits[END_REF][START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF][START_REF] Kelemen | Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites[END_REF][START_REF] Kelemen | Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF].

Lithologies at the base of the Samail ophiolite

Here we describe the units above and below the basal fault of the Samail ophiolite. We begin with a review of work on the metamorphic sole. Then, we describe the overlying Banded Unit at the base of the ophiolite's mantle section. Finally, we briefly outline the nature of the sedimentary rocks beneath the metamorphic sole.

Beneath the mantle section of the ophiolite, and above allocthonous sediments of the Hawasina Formation, are discontinuously exposed lenses of a "metamorphic sole". Peak pressures and temperatures in the sole generally record hot subduction conditions at temperatures up to 700-900°C [START_REF] Cowan | Structure of the metamorphic sole to the Oman Ophiolite, Sumeini Window and Wadi Tayyin: implications for ophiolite obduction processes[END_REF][START_REF] Ghent | Metamorphism at the base of the Samail ophiolite, southeastern Oman mountains[END_REF][START_REF] Hacker | The conundrum of Samail: Explaining the metamorphic history[END_REF], Searle & Cox 1999[START_REF] Searle | Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains[END_REF], Searle et al 1980[START_REF] Searle | The structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction[END_REF][START_REF] Searle | Petrochemistry and origin of sub-ophiolitic metamorphic and related rocks in the Oman Mountains[END_REF][START_REF] Soret | Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)[END_REF] though at BT1B peak temperatures are 450-550°C [START_REF] Kotowski | Structural and thermal evolution of an infant subduction shear zone: Insights from sub-ophiolite metamorphic rocks recovered from Oman Drilling Project Site BT-1B[END_REF]. Apparently -based on published data and calculations -the sole records a broad range of peak pressures from a possible lower limit of 200 MPa [START_REF] Ghent | Metamorphism at the base of the Samail ophiolite, southeastern Oman mountains[END_REF] or 800 MPa [START_REF] Soret | Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)[END_REF] to a possible upper limit of 1400 MPa [START_REF] Cowan | Structure of the metamorphic sole to the Oman Ophiolite, Sumeini Window and Wadi Tayyin: implications for ophiolite obduction processes[END_REF][START_REF] Searle | Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains[END_REF]. [START_REF] Kotowski | Structural and thermal evolution of an infant subduction shear zone: Insights from sub-ophiolite metamorphic rocks recovered from Oman Drilling Project Site BT-1B[END_REF] report that the sole in core from Hole BT1B records a peak pressure in the range of 800 to 1200 MPa.

The sole contains metasediments and meta-volcanic rocks -including submarine pillow lavas -with the major element compositions of mid-ocean ridge basalts (MORB) as well as alkali basalts and more evolved andesites and dacites [START_REF] Alabaster | The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex[END_REF][START_REF] Ernewein | The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)[END_REF][START_REF] Macleod | Moist MORB" axial magmatism in the Oman ophiolite: the evidence against a mid-ocean ridge origin[END_REF][START_REF] Pearce | The Oman Ophiolite as a Cretaceous Arc-Basin Complex: Evidence and Implications[END_REF], Searle et al 1980). In core from BT1B, alkaline metabasalt compositions in the sole [START_REF] Godard | Geochemical profiles across the listvenite-metamorphic transition in the basal megathrust of the Semail Ophiolite: Results from drilling at Oman DP Hole BT1B[END_REF], Kelemen et al 2020c) are unlike the magmas that formed the crust of the Samail ophiolite. In the ophiolite, the structurally lowest, "Geotimes" or "V1" lavas are similar to normal midocean ridge basalts, though they probably were hydrous and they contain a hint of an arc trace element signature [START_REF] Alabaster | The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex[END_REF][START_REF] Ernewein | The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)[END_REF][START_REF] Macleod | Moist MORB" axial magmatism in the Oman ophiolite: the evidence against a mid-ocean ridge origin[END_REF][START_REF] Pearce | The Oman Ophiolite as a Cretaceous Arc-Basin Complex: Evidence and Implications[END_REF]. Their composition, and that of dunite conduits for transport of primitive melts parental to V1 through the shallow mantle, is consistent with formation of the gabbroic lower crust in the Samail and Wadi Tayin massifs of the ophiolite from primitive V1 magmas [START_REF] Braun | Dunite distribution in the Oman ophiolite: Implications for melt flux through porous dunite conduits[END_REF][START_REF] Kelemen | Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman ophiolite: Implications for the origin of the oceanic lower crust[END_REF][START_REF] Kelemen | Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels[END_REF]. The overlying "Lasail" or "V2" lavas in the ophiolite are incompatibleelement-depleted, with a stronger subduction signature [START_REF] Alabaster | The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex[END_REF][START_REF] Ernewein | The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)[END_REF][START_REF] Macleod | Moist MORB" axial magmatism in the Oman ophiolite: the evidence against a mid-ocean ridge origin[END_REF][START_REF] Pearce | The Oman Ophiolite as a Cretaceous Arc-Basin Complex: Evidence and Implications[END_REF] and include boninites (e.g., [START_REF] Ishikawa | Boninitic volcanism in the Oman ophiolite: Implications for thermal condition during transition from spreading ridge to arc[END_REF] as well as tholeiitic basalts, andesites and dacites. Neither V1 nor V2 lavas are similar to the alkali basalt compositions in the sole. The present day 87 Sr/ 86 Sr ratios in the alkali basalts in core from BT1B range from 0.704 to 0.706 (de Obeso et al 2021a), more radiogenic than MORB. Though their Sr isotope ratio may have been modified by alteration, one possibility is that the alkali basalts are remnants of subducted seamounts with elevated 87 Sr/ 86 Sr, similar to accreted seamounts along the Cascadia margin of North America (e.g., [START_REF] Duncan | A captured island chain in the Coast Range of Oregon and Washington[END_REF].

In addition to metabasalts, regionally the sole contains metasediments, and "exotic limestones", all incorporated by [START_REF] Searle | The structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction[END_REF] in the "Haybi Formation". However, in this paper we informally group the Haybi Formation into an undifferentiated metamorphic sole unit. In the sole sampled by drill core from Hole BT1B, metasediments are clearly distinguishable from the metabasalts based on texture, but some of them are compositionally similar to the metabasalts, perhaps reflecting a volcanoclastic origin, whereas others grade into somewhat odd, low-SiO2, muscovite-bearing lithologies [START_REF] Godard | Geochemical profiles across the listvenite-metamorphic transition in the basal megathrust of the Semail Ophiolite: Results from drilling at Oman DP Hole BT1B[END_REF], Kelemen et al 2020b[START_REF] Kotowski | Structural and thermal evolution of an infant subduction shear zone: Insights from sub-ophiolite metamorphic rocks recovered from Oman Drilling Project Site BT-1B[END_REF].

Above the sole -where it is present -and elsewhere at the base of the ophiolite, the lower few km of the mantle section contains easily visible, meter to 10-meter scale, parallel bands of dunite, harzburgite and (rare) lherzolite, informally known as the Banded Unit [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF][START_REF] Khedr | Petrology and chemistry of basal lherzolites above the metamorphic sole from Wadi Sarami central Oman ophiolite[END_REF][START_REF] Khedr | Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity[END_REF][START_REF] Linckens | Evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation[END_REF][START_REF] Lippard | The Ophiolite of Northern Oman[END_REF], Prigent et al 2018a, Prigent et al 2018b[START_REF] Takazawa | Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)[END_REF][START_REF] Muscat | Melt extraction and metasomatism recorded in basal peridotites above the metamorphic sole of the northern Fizh massif, Oman ophiolite[END_REF]. The lithological contacts in this unit are sharp.

They have low angle dips with respect to the paleo-seafloor, the crust-mantle transition zone, and the basal fault that juxtaposes mantle peridotite with the metamorphic sole. Mylonitic shear zones are present in the Banded Unit , with textures recording deformation at 700-1,000°C [START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF][START_REF] Herwegh | Mechanical anisotropy controls strain localization in upper mantle shear zones[END_REF][START_REF] Linckens | Evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation[END_REF], Prigent et al 2018a). Thus, there is evidence for high strain ductile deformation and transposition of layering at the base of the mantle section, which might have accommodated substantial thinning (Prigent et al 2018a[START_REF] Soret | Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)[END_REF]. Similarly, there is evidence for thinning of the units below the ophiolite [START_REF] Grobe | Multiphase structural evolution of a continental margin during obduction orogeny: Insights from the Jebel Akhdar dome, Oman mountains[END_REF][START_REF] Grobe | Tectonothermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman[END_REF].

While some of the range in temperature and pressure estimates from the sole and the Banded Unit may be due to analytical and methodological uncertainty, or incomplete preservation and sampling of the highest-grade rocks, some may be due to temporal and spatial variability in peak metamorphic conditions. Moreover, the temperature record may be biased toward peak conditions, rather than later cooling. At the initiation of subduction near a spreading ridge, hot metamorphic rocks from the footwall may be accreted to the hot base of the newly formed mantle wedge, whereas as subduction zones grow colder and develop a steady-state thermal structure, cold dense lithologies in the footwall may be subducted [START_REF] Agard | Plate interface rheological switches during subduction infancy: Control on slab penetration and metamorphic sole formation[END_REF][START_REF] Soret | Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)[END_REF]. If so, the metamorphic sole in Oman may record the anomalously high temperatures of subduction initiation, and not the lower temperatures of evolution toward a steady-state subduction zone geotherm. The relatively low temperatures recorded by the sole in BT1B core could represent a point along this cooling path [START_REF] Kotowski | Structural and thermal evolution of an infant subduction shear zone: Insights from sub-ophiolite metamorphic rocks recovered from Oman Drilling Project Site BT-1B[END_REF].

A close correspondence between 96 to 95 Ma igneous ages in the crust, and both 40 Ar/ 39 Ar and zircon U/Pb ages of metamorphic rocks along the basal thrust (ca. 96-94 Ma), indicates that thrusting of the Samail ophiolite over adjacent oceanic crust and nearby pelagic sedimentary units began during formation of igneous crust in the ophiolite (e.g. , [START_REF] Garber | Petrochronology of Wadi Tayin metamorphic sole metasediment, with implications for the thermal and tectonic evolution of the Samail Ophiolite (Oman/UAE)[END_REF][START_REF] Hacker | The conundrum of Samail: Explaining the metamorphic history[END_REF], Hacker & Mosenfelder 1996, Hacker et al 1996[START_REF] Rioux | Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust[END_REF][START_REF] Rioux | Tectonic development of the Samail ophiolite: High precision U-Pb zircon geochronology of crustal growth and ophiolite emplacement[END_REF][START_REF] Rioux | Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U-Pb zircon geochronology[END_REF], Rioux et al 2021b[START_REF] Stanger | Silicified serpentinite in the Semail nappe of Oman[END_REF][START_REF] Styles | The Geology and Geophysics of the United Arab Emirates[END_REF][START_REF] Tilton | Uranium-lead isotopic ages of the Samail ophiolite, Oman, with applications to Tethyan ocean ridge tectonics[END_REF][START_REF] Muscat | Dating the geologic history of Oman's Semail ophiolite: insights from U-Pb geochronology[END_REF] or perhaps even earlier [START_REF] Guilmette | Forced subduction initiation recorded in the sole and crust of the Semail Ophiolite of Oman[END_REF].

The base of the metamorphic sole is truncated by a fault contact with autochthonous, low-grade metasediments of the Hawasina Formation, composed of pelagic clastic units interlayered with limestones [START_REF] Béchennec | The Hawasina Nappes: stratigraphy, palaeogeography and structural evolution of a fragment of the south-Tethyan passive continental margin[END_REF][START_REF] Béchennec | The Hawasina Basin: A fragment of a starved passive continental margin, thrust over the Arabian Platform during obduction of the Sumail Nappe[END_REF]. Relatively high, age corrected 87 Sr/ 86 Sr ratios in the clastic units suggest that they are distal sediments derived from erosion of continental crust (de Obeso et al 2021a, [START_REF] Weyhenmeyer | Origin and evolution of groundwater in the alluvial aquifer of the Eastern Batinah Coastal Plain, Sultanate of Oman : a hydrogeochemical approach[END_REF], while carbonate units record Sr isotope ratios similar to those of Mesozoic seawater [START_REF] Weyhenmeyer | Origin and evolution of groundwater in the alluvial aquifer of the Eastern Batinah Coastal Plain, Sultanate of Oman : a hydrogeochemical approach[END_REF][START_REF] Wohlwend | Late Jurassic to Cretaceous evolution of the eastern Tethyan Hawasina Basin (Oman Mountains)[END_REF]. These sedimentary units were thrust over Mesozoic to Proterozoic rocks of the Arabian continental margin, forming a "rumpled rug" between the autochthon and the ophiolite throughout northern Oman and the eastern United Arab Emirates.

History of the basal thrust of the Samail ophiolite

Where the metamorphic sole is preserved, its tectonic contact with the mantle section of the ophiolite represents a paleo-subduction zone at the base of the ophiolite, which consumed several hundred kilometers of Tethyan basin and then continental crust before coming to rest on the Arabian continental margin [START_REF] Béchennec | The Hawasina Nappes: stratigraphy, palaeogeography and structural evolution of a fragment of the south-Tethyan passive continental margin[END_REF][START_REF] Béchennec | The Hawasina Basin: A fragment of a starved passive continental margin, thrust over the Arabian Platform during obduction of the Sumail Nappe[END_REF][START_REF] Breton | Eoalpine (Cretaceous) evolution of the Oman Tethyan continental margin: insights from a structural field study in Jabal Akhdar (Oman Mountains)[END_REF][START_REF] Cooper | Structure and sequence of thrusting in deep-water sediments during ophiolite emplacement in the south-central Oman Mountains[END_REF][START_REF] Ninkabou | Structure of the offshore obducted Oman margin: Emplacement of the Semail ophiolite and role of tectonic inheritance[END_REF][START_REF] Searle | The northern Oman Tethyan continental margin: Stratigraphy, structure, concepts and controversies[END_REF][START_REF] Van Hinsbergen | Kinematic and paleomagnetic restoration of the Semail ophiolite (Oman) reveals subduction initiation along an ancient Neotethyan fracture zone[END_REF]. During this time, the overlying peridotite was "the leading edge of the mantle wedge".

During coeval metamorphism of the sole and igneous accretion of the crust in the ophiolite, the relative locations of these two units are uncertain. Primitive V1 magmas in the ophiolite, with compositions almost indistinguishable from mid-ocean ridge basalts (MORB), crystallized to form most of the crust, particularly in the southern Wadi Tayin and Samail massifs that were the site of of the OmanDP boreholes. Like MORB worldwide, these magmas probably formed via polybaric decompression melting over a depth interval of 75 km or more (e.g., [START_REF] Allegre | Cortège ophiolitique et cortège océanique, géochimie comparée et mode de genèse[END_REF][START_REF] Asimow | A hydrous melting and fractionation model for mid-ocean ridge basalts: Application to the Mid-Atlantic Ridge near the Azores[END_REF], Bottinga & Allegre 1973[START_REF] Klein | Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness[END_REF][START_REF] Mckenzie | The volume and composition of melt generated by extension of the lithosphere[END_REF]. Thus, the metamorphic sole (recording peak depths less than 40 km) must not have been directly beneath the spreading center during crustal formation and metamorphism. Instead, the ophiolite section and the metamorphic sole were juxtaposed after crust formation and after peak metamorphism in the sole.

Moreover, the structural thickness of the Samail ophiolite measured perpendicular to the paleoseafloor and/or the crust/mantle transition zone, and including both crust and mantle sections, never exceeds ~ 20 to 30 km [START_REF] Boudier | Cross section through the peridotite in the Semail ophiolite[END_REF][START_REF] Nicolas | Accretion of Oman and United Arab Emirates ophiolite: Discussion of a new structural map[END_REF], corresponding to a pressure less than or equal to ~ 800 MPa, beneath ~ 7 km of crust (on average, [START_REF] Nicolas | Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust[END_REF] and up to ~ 20 km of fresh mantle peridotite. Thus, the structural thickness of the ophiolite appears to be at least 10 km less than the depth inferred from the high end of the pressure range recorded by the sole (1300 or 1400 MPa, ~ 40 to 45 km). In addition, there is no evidence for widespread, tectonic thinning of the crustal section of the ophiolite, either by faulting or ductile deformation.

How can these data be reconciled? One possibility is that the peak pressures inferred for the sole are imprecise, and/or that they represent tectonic "overpressures" that don't correspond closely with depth [START_REF] Garber | Petrochronology of Wadi Tayin metamorphic sole metasediment, with implications for the thermal and tectonic evolution of the Samail Ophiolite (Oman/UAE)[END_REF]. In this interpretation, the metamorphism of the sole could have occurred at ~ 800 MPa, corresponding to the current structural thickness of the ophiolite. A second possibility, as noted above, is that the base of the mantle section of the ophiolite underwent tectonic thinning, perhaps via simple shear (Prigent et al 2018a[START_REF] Soret | Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)[END_REF]. A third alternative is that some of the lenses of the metamorphic sole -those recording the highest pressures -migrated updip to reach their current structural position, ~ 25 km below the paleo seafloor. Indeed, upward transport of buoyant footwall lithologies during subduction is recorded in many preserved collisional orogens (e.g., [START_REF] Chemenda | Evolutionary model of the Himalaya^Tibet system: Geopoem based on new modelling, geological and geophysical data[END_REF]. If so, updip migration of the sole with respect to the overlying ophiolite must have occurred after peak metamorphism but during Tethyan subduction, prior to emplacement of the ophiolite and the sole over the allochthonous Hawasina sediments. Finally, of course, it is possible that the current juxtaposition of the sole and the base of the ophiolite may be explained as the result of combinations of these three alternatives.

Subsequent events have affected the wedge, the sole, the underlying sedimentary units, and the faults between them [START_REF] Grobe | Multiphase structural evolution of a continental margin during obduction orogeny: Insights from the Jebel Akhdar dome, Oman mountains[END_REF][START_REF] Grobe | Tectonothermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman[END_REF]. The end of subduction and emplacement of the ophiolite over allochthonous Hawasina sediments and the autochthonous, Arabian continental margin was followed by subaerial erosion, and then by a marine transgression marked by deposition of shallow water limestones at ~ 74 Ma (Al [START_REF] Khirbash | Genesis and mineralogical classification of Ni-laterites, Oman Mountains[END_REF][START_REF] Alsharan | Sedimentological and geochemical interpretation of a transgressive sequence: the Late Cretaceous Oahlah Formation in the western Oman Mountains, United Arab Emirates[END_REF][START_REF] Bailey | Geologic map of Muscat-Ibra area, Sultanate of Oman[END_REF], Hansman et al 2017a[START_REF] Nolan | Maastrichtian to early Tertiary stratigraphy and palaeogeography of the Central and Northern Oman Mountains[END_REF][START_REF] Schlüter | Evolution of a Maastrichtian-Paleocene tropical shallow-water carbonate platform (Qalhat, NE Oman)[END_REF], Wyns et al 1992). Then, large scale uplift deformed all these units, forming the gigantic Jebel Akdar and Saih Hatat domes, cored by Proterozoic continental crust (e.g., [START_REF] Glennie | Late Cretaceous nappes in the Oman Mountains and their geologic evolution[END_REF], Glennie et al 1974b). Saih Hatat uplift and cooling started at about 60 Ma, if not earlier (e.g., [START_REF] Grobe | Multiphase structural evolution of a continental margin during obduction orogeny: Insights from the Jebel Akhdar dome, Oman mountains[END_REF][START_REF] Grobe | Tectonothermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman[END_REF], Hansman et al 2017b). Doming reactivated and cut the basal thrust of the ophiolite in normal faults and shear zones. Some of these younger faults juxtaposed mantle peridotite from the ophiolite with the allochthonous Hawasina sedimentary rocks, and even with the autochthonous platform carbonates of the Arabian continental margin, along tectonic contacts where the metamorphic sole is no longer present. However, despite these complexities, we reiterate that -where it is parallel to banding and foliationthe contact between the metamorphic sole and the overlying Banded Unit at the base of the Samail ophiolite mantle section represents the basal thrust of the ophiolite. In this paper, when we refer to the "basal thrust", we are referring to contacts that preserve these characteristics. In turn, though of course there can have been imbrication and/or subduction erosion, the basal thrust of the ophiolite represents the locus of 100's of kilometers of subduction of oceanic crust, overlying sediments and, ultimately, the Arabian continental margin.

Geology of MoD Mountain

Hole BT1B is on the north side of the wide Wadi Mansah, southeast of "Ministry of Defense Mountain", (MoD Mountain), which is informally named for the military firing range on the south side of Wadi Mansah. The MoD Mountain region is close to the saddle between the northeastward plunging end of the Jebel Akdar massif, to the west, and the westward plunging Saih Hatat massif to the northeast (Figure 1). This region preserves a gently folded, tectonic "stratigraphy" (Figures 2 , 3 and4). The lowest exposed units in this sequence, northeast of Hole BT1B, are diagenetically-altered sedimentary rocks, mostly clastic shales and slates with a few meter to 10 meter scale intercalations of limestone and minor lenses of metavolcanic rocks. These are parts of the Hawasina Formation [START_REF] Béchennec | The Hawasina Nappes: stratigraphy, palaeogeography and structural evolution of a fragment of the south-Tethyan passive continental margin[END_REF][START_REF] Béchennec | The Hawasina Basin: A fragment of a starved passive continental margin, thrust over the Arabian Platform during obduction of the Sumail Nappe[END_REF]. Overlying the Hawasina Formation is one of the most aerially extensive outcrops of the metamorphic sole in Oman and the UAE (e.g., Figure 1 and geologic maps in [START_REF] Soret | Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)[END_REF][START_REF] Wilde | Preliminary study of Tertiary hydrothermal alteration and platinum deposition in the Oman ophiolite[END_REF]. The outcrop area is unusually large in this region because the unit is regionally flat lying, though broadly folded. [START_REF] Nicolas | Accretion of Oman and United Arab Emirates ophiolite: Discussion of a new structural map[END_REF]. Metamorphic sole, in red. Black circles indicate approximate location of listvenites [START_REF] Nasir | Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman[END_REF][START_REF] Stanger | Silicified serpentinite in the Semail nappe of Oman[END_REF][START_REF] Wilde | Preliminary study of Tertiary hydrothermal alteration and platinum deposition in the Oman ophiolite[END_REF]. Red rectangle indicates the approximate location of Figure 3, a geologic map of MoD Mtn and vicinity, which was the site of Oman DP Hole BT1B, and the focus of this paper.

Above the metamorphic sole in this gently folded "stratigraphy" is partially serpentinized peridotite, composed of distinctive, banded alternations of dunite, harzburgite and lherzolite on a scale of meters to tens of meters, characteristic of the "Banded Unit" at the base of the ophiolite [START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF][START_REF] Boudier | Cross section through the peridotite in the Semail ophiolite[END_REF][START_REF] Lippard | The Ophiolite of Northern Oman[END_REF][START_REF] Searle | Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains[END_REF]. Indeed, partially serpentinized mantle peridotites on MoD Mountain, northeast of Hole BT1B have compositions similar to the Banded Unit elsewhere, and distinct from shallower, residual mantle peridotites of the Samail ophiolite emantle section, as discussed in Section 5.5, below. The presence of the Banded Unit overlying the metamorphic sole is another indication that MoD Mountain area exposes the basal thrust of the ophiolite -the paleo-subduction zone -together with the overlying mantle wedge.

In some localities, peridotites at the base of the Samail ophiolite mantle section have undergone 100% carbonation at ~ 100-200°C to form "listvenites" [START_REF] Beinlich | Ultramafic rock carbonation: Constraints from listvenite core BT1B, Oman Drilling Project[END_REF][START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF], Glennie et al 1974a[START_REF] Nasir | Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman[END_REF][START_REF] Stanger | Silicified serpentinite in the Semail nappe of Oman[END_REF][START_REF] Wilde | Preliminary study of Tertiary hydrothermal alteration and platinum deposition in the Oman ophiolite[END_REF], in which all Mg and Ca are in carbonate minerals, and silica derived from olivine and pyroxene has formed quartz or chalcedony. In Oman, such listvenites are most abundant on and around MoD Mountain northeast of Hole BT1B, where they form part of the gently folded tectonic "stratigraphy" we are discussing here. Listvenites on the flanks of MoD Mountain form discontinuous tabular lenses with low angle dips, 10 to 200 meters thick, parallel to the basal thrust and to lithological banding in the peridotite. These lenses occur along the basal thrust -between peridotite and the sole -and enclosed within the peridotite, up to 300 meters above the sole. Contacts in outcrop between listvenite and the surrounding, partially serpentinized peridotite are marked by strongly foliated, 100% serpentinized zones, 1 to > 20 m thick.

The lithological banding in the peridotite and the listvenite lenses dip gently south on the south side of MoD Mountain, north on the north slopes of the mountain, and then south again along and NE of the Kelemen et al. (2020b) wadi bounding MoD Mountain to the north (Falk and Kelemen 2015 and this paper, Section 4.2).

Despite later faulting, these structures define a broad anticline with an axis approximately coincident with the summit ridge of MoD Mountain, and a syncline with an axis roughly coincident with the wadi northeast of MoD Mountain. Listvenites form erosion resistant dip slopes and the tops of small buttes outlining the folded stratigraphy.

Listvenites elsewhere in Oman and the UAE are found along the basal thrust, commonly juxtaposed with, or within a few km of, the metamorphic sole and/or the Banded Unit at the base of the Samail mantle section, as at MoD Mountain. In some other outcrops, listvenites form lenses within broad serpentinite mélange zones at the base of the ophiolite [START_REF] Nasir | Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman[END_REF][START_REF] Stanger | Silicified serpentinite in the Semail nappe of Oman[END_REF]. In contrast, listvenites are not found within the peridotite more than a few kilometers away from the basal thrust of the ophiolite.

A

Methods

Here we describe methods used for new data presented for the first time in this paper. Supplementary

Section S2 provides methods for data illustrated in this paper, but previously reported elsewhere.

Geological mapping

Geological field work to produce the map and cross-section in Figure 4 spanned short visits over about a decade. Contacts and structural measurements were located by GPS at the time of measurement. The topographic profile for the cross section was constructed using Google Earth.

Raman spectroscopy

Analyses of minerals in thin section and rock slabs were conducted at the Raman Microspectroscopy Laboratory, University of Colorado-Boulder with a Horiba Scientific LabRam HR Evolution Raman microscope. Measurements used a 100 mW 532 nm laser, focused through a 50x (0.75 NA) microscope objective onto a ~2 µm spot. The laser power was modulated with neutral density filters to about 15 mW at the sample surface. Multiple (2-10) accumulations were coadded in order to filter spikes and improve signal to noise, and the acquisition time and accumulation number were adjusted to yield appropriate data quality. Data processing was performed using LabSpec 6 software (Horiba Scientific), including correction for instrumental artifacts and polynomial baseline fitting/subtraction.

Raman mapping was performed using a motorized stage with 2 µm step size, and map datasets were fit using classical least-squares fitting with endmember spectra isolated from regions within the map using LabSpec 6 after data processing.

Thermodynamic calculations and modeling

The speciation and chemical mass transfer code EQ3/6 [START_REF] Wolery | EQ3/6, a software package for geochemical modeling of aqueous systems: package overview and installation guide (version 7.0)[END_REF]) was used to predict the compositions of coexisting solid and aqueous phases that evolved during interaction between representative lithologies from the MoD Mountain area and CO2-bearing fluids. Thermodynamic data for minerals were mostly from [START_REF] Berman | Internally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2[END_REF]. Data for pyrite and pyrrhotite were from [START_REF] Helgeson | Summary and critique of the thermodynamic properties of rock-forming minerals[END_REF]. For aqueous species, thermodynamic data used in the simulations were calculated using the Deep Earth Water (DEW) model [START_REF] Huang | Extended Deep Earth Water Model for predicting major element mantle metasomatism[END_REF][START_REF] Sverjensky | Water in the deep Earth: the dielectric constant and the solubilities of quartz and corundum to 60 kb and 1200 C. Geochim[END_REF] which uses recent experimental and theoretical advances [START_REF] Facq | Carbon speciation in saline solutions in equilibrium with aragonite at high pressure[END_REF][START_REF] Pan | Dielectric properties of water under extreme conditions and transport of carbonates in the deep Earth[END_REF] to expand the extended Helgeson-Kirkham-Flowers (HKF) aqueous equation of state [START_REF] Shock | Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures: Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 C and 5 kbar[END_REF][START_REF] Shock | Inorganic species in geologic fluids: correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes[END_REF] to pressures up to 6.0 GPa.

The composition of 5 wt% aqueous fluid in equilibrium with a pelitic lithology from the Oman Hawasina Formation at 400 -600°C and 0.5 to 2.0 GPa at low water/rock ratios was used.

Specifically, a dilute fluid was equilibrated with the rock composition of sample OM20-17 (de Obeso et al 2021a), containing 0.06 wt% total carbon (Supplementary Table 2). The CaO content of OM20-17 was below detection. For the model calculations it was assumed to 0.1 wt%. In addition, the S content of this sample has not been measured. For this calculation it was assumed 100 ppm. At these high temperatures and low carbon contents, carbonate minerals are unstable and all carbon in the rock will be mobilized into the fluid phase as dissolved CO2.

We calculated the outcome of cooling and decompression of the CO2-rich fluid from OM20-17, to 100 -300 °C and 0.5 to 2.0 GPa. This had no significant effect on its composition. We then calculated the products of reaction between this fluid and average Oman harzburgite (Supplementary Table 2, calculated from [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF][START_REF] Hanghoj | Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; Major and trace element geochemistry, and Os isotope and PGE systematics[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF] at 100 -300 °C and 0.5 -1.0 GPa, at water:rock ratios ranging from 100 to 1.

As a result of data limitations, solid solutions of precipitating minerals were not considered, as the Berman database lacks properties for most Fe-endmembers of minerals commonly observed in listvenites and associated rocks. Thus, for example, the model predicts co-precipitation of pure, endmember magnesite, dolomite and siderite, whereas in listvenite samples we observe Fe-bearing magnesite and dolomite. Among the serpentine polytypes, only chrysotile precipitation can be modeled. We did not include goethite. There are no thermodynamic data for a chromian muscovite component, so this was not included, despite the fact that solid solutions ranging from fuchsite to chromian muscovite are observed in MoD Mountain listvenites (e.g., [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF].

Phase equilibrium calculations constraining the conditions for co-existing graphite (± amorphous carbon compounds) and hematite, and updated calculations for co-existing antigorite and quartz, used ( 1) Thermocalc [START_REF] Powell | Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC[END_REF], ( 2) Perple_X (https://www.perplex.ethz.ch/) [START_REF] Connolly | Multivariable phase-diagrams -an algorithm based on generalized thermodynamics[END_REF][START_REF] Connolly | Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation[END_REF][START_REF] Connolly | The geodynamic equation of state: what and how[END_REF]), with the Holland and Powell thermodynamic data for minerals (2003,1998), and the default equations of state for H2O-CO2 fluids (modified versions of Redlich-Kwong), and (3) SUPCRT [START_REF] Johnson | SUPCRT92 -A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1-bar to 5000-bar and 0C to 1000[END_REF][START_REF] Zimmer | SUPCRTBL: A revised and extended thermodynamic dataset and software package of SUPCRT92[END_REF] with thermodynamic data for minerals from Helgeson et al. [START_REF] Helgeson | Errata II. Thermodynamics of minerals, reactions, and aqueous solutions at high pressures and temperature[END_REF][START_REF] Helgeson | Summary and critique of the thermodynamic properties of rock-forming minerals[END_REF] or Berman (1988, plus [START_REF] Shock | Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures: Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 C and 5 kbar[END_REF][START_REF] Shock | Inorganic species in geologic fluids: correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes[END_REF] do not predict equilibrium coexistence of antigorite + quartz above 15°C.

(U,Th)/He ratio measurements and cooling age calculation

All (U-Th)/He analyses were completed at the UTChron facility at the University of Texas at Austin, using aliquots of zircon separates from the metamorphic sole and lower crustal gabbros, previously analyzed for U/Pb ages by Rioux et al. (in prep.), following procedures of Wolf and Stockli (2010).

Individual zircon grains were morphometrically characterized to determine alpha ejection correction (Ft, Farley et al., 1996;Cooperdock et al., 2020), equivalent spherical radius (ESR), and estimated mass assuming a tetragonal prism. Single-grain zircon sample aliquots were loaded into Pt tubes for in-vacuo laser He heating for 10 min at ~1200°C by diode laser and 4He concentrations were measured by isotope dilution, using a 3 He tracer, on a Blazers Prisma QMS-200 quadrupole mass spectrometer, after cryogenic purification. Blanks and 4 He gas standards were run between unknowns to monitor and quantify the procedural baseline during analytical runs. Aliquot laser reheating was repeated (2-5x) until 4He gas yields dropped <1% total extracted gas.

After degassing, individual zircon grains were removed from the Pt packets and dissolved using a two-step HF-HNO3 and HCl pressure vessel dissolution technique and measured on a Thermo

Element2 HR-ICP-MS following the procedure outlined in Wolf and Stockli (2010). U-Th-Sm concentrations were calculated using isotope dilution with an isotopically enriched, mixed U-Th-Sm spike calibrated against a 1 ppb U-Th-Sm gravimetric standard solution and blank-corrected using the average of multiple procedural blanks.

Final (U-Th)/He ages were calculated using blank corrected U, Th, Sm and He measurements for each aliquot. Reported concentrations were determined using the morphometrically determined mass of each aliquot. The reported error for individual (U-Th)/He ages represents standard error (8%)

based on long-term intra-laboratory reproducibility of Fish Canyon tuff zircon standard, following the approach of Farley et al. (2001). The reported mean sample ages reflect the arithmetic mean of the aliquot ages and their standard deviations.

3.5 Calibration of XRF core scanner data XRF core scanner data were collected onboard DV Chikyu, as described in Kelemen et al. (2020b).

We used the core scanner to analyze nine listvenite samples from BT1B core, and 14 gabbro samples from Hole GT1A core that had known bulk compositions based on XRF analysis at the University of St. Andrews. While onboard DV Chikyu, we used the St. Andrews data to calibrate the XRF data, as follows: wt% SiO2 = 0.89 x (scanner wt% SiO2); wt% MgO = 2.57 + 1.18 x (scanner wt% MgO); wt% FeO T = 1.048 + (scanner wt% FeO T )^0.848; and wt% CaO = 0.878 x (scanner wt% CaO),

where FeO T indicates all Fe is treated as FeO. Fits are illustrated in Supplementary Figure 2. [START_REF] Okazaki | Major mineral fraction and physical properties of carbonated peridotite (listvenite) from ICDP Oman Drilling Project Hole BT1B inferred from X-ray CT core images[END_REF] present a comprehensive analysis of the XRF scanner data, together with X-Ray tomography data for BT1B core.

Calculation of mineral volume proportions

Volume proportions of quartz, magnesite and dolomite were estimated from bulk rock compositions and XRF scanner data as follows. Weight fractions of SiO2, MgO, FeO T and CaO were converted to moles in 100 grams of rock using their molecular weights. (For all data reported in this paper, the sum of wt% SiO2, MgO, FeO T and CaO was greater than 90% of the volatile free, bulk rock composition).

The number of moles of dolomite were taken to be equal to moles of CaO, moles of magnesite were calculated as moles MgO -moles CaO, and moles of quartz were taken to be equal to moles of SiO2.

All Fe was inferred to be in Fe-oxides and hydroxides. If the small amounts of Fe in carbonate minerals were included in such a calculation, this would slightly increase the proportions of magnesite and dolomite, relative to quartz. Volumes of each mineral in 100 grams of rock were calculated using their molar volumes. The data were "projected" from Fe oxy-hydroxides by normalizing the volumes of quartz, magnesite and dolomite to 100%.

Results

Drilling operations

Site BT1 is on the north side of the broad channel of Wadi Mansah, which drains mountainous regions to the east and southeast. Hole BT1A penetrated 1.90 meters of gravels in Wadi Mansah, south of listvenite outcrops flanking the Wadi. After we became concerned that a steep hole there might intersect tens of meters of gravel before reaching bedrock, and/or that the gravel might overlie a major, steep fault along Wadi Mansah that postdates ophiolite emplacement, we moved the drill and inclined the Hole. Hole BT1B is three or four meters closer to the listvenite outcrop. The well head is intact, marked and protected from floods by a concrete monument. Drilling of the fine-grained, silicarich listvenites was challenging, because this lithology has a hardness similar to fine-grained chert or flint (~ 7 on the Mohs' scale), but with patience and expert drilling, we obtained ~ 100% recovery of all lithologies intersected by the borehole. Details of drilling operations are in Kelemen et al. (2020b).

Mapping and structural observations

Geological mapping of the area around MoD Mountain, together with measurement of the orientation of lithological contacts, began in 2009, and continued via brief visits through 2019. As noted in Section 2.5, and reported by [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF], the mountain is underlain by a southeast to northwest striking anticline, with its faulted hinge approximately coincident with the ridge along the summit. Exposed contacts between listvenite and serpentinized harzburgite consistently dip to the southwest on the southwest side of the mountain, and to the east and northeast on the northeast side.

In the valley north of the mountain, listvenite-harzburgite, listvenite-metamorphic sole, and harzburgite-sole contacts form a broad, shallow syncline. A new, detailed geological map and crosssection, incorporating our lithological and structural observations, is presented in Figure 4.

As shown on the map, in outcrops extending for more than 4 km north of Hole BT1B there is a regular tectonic "stratigraphy", with variably altered peridotite overlying the metamorphic sole, in turn overlying Hawasina Formation sediments, all with low angle fault contacts that have been deformed by a series of gentle, broad open folds evident in the map and cross-section.

We noted normal and reverse shear sense indicators on both sides of the anticline. As a result of the folding, it is difficult to interpret the sense of shear recorded in various outcrops. Some shear sense indicators may predate the folding, while others may record deformation during or after folding. In contrast, to the northwest and southeast, outside our map area, the outcrop patterns become much more complex, the sole outcrop thins, and there are some vertical fault contacts where all of these older units are juxtaposed with Late Cretaceous Al Khod conglomerates and younger, shallow marine limestones (Stanger 1985, Villey et 1 and Supplementary Figure 1).

Lithology

As illustrated in Figure 5 together with a few cm of hard, aphanitic, black ultracataclasite. Beneath these fault lithologies, the core sampled ~ 102 meters of the metamorphic sole, grading from dominantly fine-grained, finelybanded, muscovite-bearing metasediments at the top ("greenschists" in Figure 5) to coarser, more massive-appearing, foliated "greenstones", interpreted as metavolcanic rocks, at the bottom.

Serpentinite bands in the core have gradational contacts with host listvenites over 10's of centimeters to ~ 1 m thick. Serpentinites contain antitaxial veins of magnesite with a median line composed of hematite and other Fe-oxides. There are prismatic terminations of magnesite crystals away from vein centers, toward the host serpentine (Figure 6). Antitaxial veins record growth of minerals outward from the vein center. They are commonly interpreted to open due to the "pressure of crystallization" [START_REF] Durney | Incremental strain measured by syntectonic crystallization growths[END_REF][START_REF] Urai | Kinematics of crystal growth in syntectonic fibrous veins[END_REF]. However, this interpretation is less clear in the serpentinites and listvenites of Hole BT1B, where the Mg in the carbonate is derived from the host rocks. To some degree these veins may replace, rather than displace, the host.

Away from the veins, serpentinites also contain up to 10% magnesite ovoids 10 to 100 microns in diameter, unevenly dispersed within a massive serpentine matrix. These magnesite vein and ovoid textures are abundant in the listvenites as well. Thus, the Shipboard Scientific Party suggested that they are indicative of incipient replacement of serpentinite by listvenite, grading from < 10% carbonate (and no quartz) in veins and ovoids to 100% fine-grained carbonate + quartz across sharp reaction fronts (Kelemen et al 2020b). Another notable feature is that some of the serpentinites are optically isotropic in thin section, probably indicative of low temperature formation of poorly ordered or amorphous material with serpentine stoichometry (e.g., [START_REF] Andreani | A microstructural study of a "crackseal" type serpentine vein using SEM and TEM techniques[END_REF]. since antigorite is more SiO2-rich than the other polytypes.

However, the serpentinites in core from Hole BT1B are distinct from the serpentinized zone flanking listvenite on the ridge transect, and from the surrounding, partially serpentinized Banded Horizon harzburgites. Although quartz veins cut the serpentinite in the core, antigorite was not observed.

Moreover, despite the presence of some orthopyroxene pseudomorphs ("bastites") in serpentinites, and a concerted effort to find relict mantle minerals, no olivine or pyroxene were detected in drill core.

Taken together, field and core observations suggest that the contact between serpentinites and partially serpentinized peridotites is gradational over a few meters at most, approximately as sharp as the contact between listvenites and serpentinites. More on this in Sections 5.7 and 5.8, below.

Essentially, two types of listvenite were recovered, magnesite + quartz + iron oxide lithologies, and volumetrically less abundant, dolomite + quartz + iron oxide rocks, previously termed magnesite-lisvenites and dolomite-listvenites [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]. Much of the core contains 0.5 to 3% relict chromian spinel, partially or fully altered to Fe-oxides. Instead or in addition, some samples contain minor amounts of Cr-rich white mica (fuchsite-muscovite solid solutions, supplementary Figure 7 in [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF], in mm to cm scale, ovoid, microscopic intergrowths with quartz. These intergrowths are macroscopically evident in outcrop and core as cm-scale green spots, though in fact Cr-rich mica composes only a few percent of such spots, apparently has undergone alteration to clays in some samples, and was significantly damaged or removed during thin section preparation. As a result, fuchsite crystals recovered from BT1B core are not large enough for 40 Ar/ 39 Ar analyses. Figure 5 of [START_REF] Nasir | Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman[END_REF] is a photomicrograph of crystalline fuchsite from another listvenite in Oman.

Macroscopic listvenite textures are characterized by abundant veins (10 to 200 veins more than 1 mm thick per meter of core, typically ~ 1 per cm) in a fine-grained matrix. In massive listvenites, the finegrained matrix surrounding veins commonly contains ovoids of magnesite or quartz (Figure 8).

Though they have similar textures, ovoids of the two different minerals are rarely adjacent to each other. Both commonly have Fe-oxides in their cores and/or in spherical zones. Microprobe analyses

show that magnesite ovoids have low Fe cores, commonly rimmed with relatively Fe-rich magnesite [START_REF] Beinlich | Ultramafic rock carbonation: Constraints from listvenite core BT1B, Oman Drilling Project[END_REF]. They have sizes and shapes similar to the quartz spherulites.

Carbonate ovoids and cross-cutting magnesite-hematite veins are also observed in serpentinite bands in the core (Figure 6). Thus, the Shipboard Science Party considered it likely that many such veins initially formed within serpentinite, followed by formation of ovoids within surrounding serpentine, and then by replacement of the entire serpentinite matrix by carbonate + quartz (Kelemen et al 2020b). If so, in contrast to conventional interpretations of veins as relatively young features, "cross-cutting" their matrices, in this case the fine-grained listvenite matrix may postdate the earliest veins found within the matrix. This hypothesis is discussed further in Section 5.9

The quartz ovoids have the texture of "spherulites", with radiating microscopic crystals producing a false, "uniaxial interference pattern" in cross-polarized light. Spherulites form during devitrification of amorphous opal as well as rhyolite glass, so [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF] and the Shipboard Science Party Vein types cutting this fine-grained matrix generally record a progression from texturally early, antitaxial magnesite veins -some with cores of hematite + other Fe-oxides (Figure 6) -and related, early Fe-oxide veins, to syntaxial dolomite veins and carbonate-quartz veins, and lastly to syntaxial calcite veins. Some of the late, syntaxial veins contain vugs lined with prismatic calcite and/or dolomite. Syntaxial vein textures record inward crystallization of crystals into fractures that opened due to external, tensional stresses [START_REF] Durney | Incremental strain measured by syntectonic crystallization growths[END_REF][START_REF] Urai | Kinematics of crystal growth in syntectonic fibrous veins[END_REF].

(A poorly exposed, weathered, fuchsite vein has been observed in outcrop 200 meters north of Site BT1B, but no such veins were sampled in BT1B core. We mention this simply to emphasize that Cr and Al were mobile at some stage during listvenite formation or later alteration.) Among the Fe-oxide veins, some contain minor sulfide -generally not detected during core description -and amorphous, organic carbon compounds. The carbon compounds were first identified in core at the drill site, in the lowest listvenite band, as elongate lenses within transposed hematite veins parallel to the penetrative foliation, where they were described as "graphite". Soft organic carbon compounds in these features appear to have been largely lost from sample surfaces during washing and handling of the core prior to shipboard observations and analyses, and again during fabrication of thin sections. However, Raman spectroscopy of small, armored relicts, in oxide veins and also in isolated, dark red spots that resemble relict spinel on the core face and rock slabs, reveals the presence of disordered, thermally immature carbonaceous material (Figure 9), some of which may retain a more ordered organic molecular structure. The carbonaceous materials we can still find, on freshly cut surfaces from the core interior, are commonly on the margins of microscopic chalcocite and covellite crystals, in one case also associated with copper sulfate (chalcanthite) 46z_2_10-15_5x10s_532 nm_x50_25%_600 gr_mm_09 46z_2_10-15_20x3s_532 nm_x50 LWD_100%_600 gr_mm_03 46z_2_10-15_20x5s_532 nm_x100_100%_600 gr_mm_01 4B_78_2_8-15_6x10s_532 nm_x50 LWD_100%_600 gr_mm_02 4B_46_02_24-27_2x30s_532 nm_x50 LWD_100%_600 gr_mm_01 4B_46_02_24-27_2x30s_532 nm_x50 LWD_100%_600 gr_mm_02 4B_46_02_24-27_2x30s_532 nm_x50 LWD_100%_600 gr_mm_07 4B_46_02_24-27_2x30s_532 nm_x50 LWD_100%_600 gr_mm_08 4B_46_02_24-27_2x30s_532 nm_x50 LWD_100%_600 gr_mm_09 4B_46_02_24-27_2x30s_532 nm_x50 LWD_100%_600 gr_mm_11

46Z-2_10-15_5x10s_532nm_x50_25%_600gr_mm_09 46Z-2_24-17_20x3s_532nm_x50_LWD_100%_600gr_mm_02 46Z-2_10-15_20x3s_532nm_x50_LWD_100%_600gr_mm_03 46Z-2_24-27_20x3s_532nm_x50_LWD_100%_600gr_mm_07 46Z-2_10-15_20x5s_532nm_x100_100%_600gr_mm_01 46Z-2_24-27_2x30s_532nm_x50_LWD_100%_600gr_mm_08 78Z-2_8-15_6x10s_532nm_x50_LWD_100%_600gr5_mm_02 46Z-2_24-27_2x30s_532nm_x50_LWD_100%_600gr_mm_09 46Z-2_24-27_2x30s_532nm_x50_LWD_100%_600gr_mm_01 46Z-2_24-27_2x30s_532nm_x50_LWD_100%_600gr_mm_11

Crystallographic preferred orientation of minerals

An overall preferred orientation of veins is not evident from structural data on the core scale, possibly due to differential rotation of core pieces. And, systematic measurements have not been made on the qtz lenses within parallel magnesite-hematite veins field of view in all six panels 1.5 mm parallel magnesite &/or dolomite veins serpentinite mylonite outcrop scale. However, at the meter scale many core intervals contain textures indicative of ductile deformation (Figure 10). Some samples show a strong macroscopic foliation, defined by parallel (possibly transposed) veins enclosing elongate lenses of the fine-grained matrix, in textures commonly interpreted to form via boudinage during ductile deformation. In samples with a strong foliation defined by dozens of subparallel, early magnesite veins per 10 mm 2 , intervening patches of fine-grained quartz commonly have a crystallographic preferred orientation (CPO). Some samples with a strong foliation defined by abundant, subparallel (tectonically transposed?) carbonate veins also have an optically evident, crystallographic preferred orientation in magnesite and/or dolomite.

Similarly, some shear zones in serpentinite have an optically evident, strong shape-and crystallographic-preferred orientation of lizardite crystals, and contain deformed serpentine porphyroclasts.

Brittle deformation textures

A broad range of different breccias and cataclasites are observed in listvenites and in the metamorphic sole, in outcrop and in core. In turn these are cut by sharp faults -some associated with planar bands and branching veins of aphanitic ultracataclasite and/or pseudotachylite -and by late calcite veins. The nature and interpretation of cataclasites and faults observed in core from Hole BT1B are discussed in detail by [START_REF] Menzel | Brittle deformation of carbonated peridotite: Insights from listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)[END_REF].

Geochemical data

The bulk composition of core samples was measured in five different ways: 2021), so for brevity we simply refer readers to those other publications.

Discussion

Subduction zone setting of listvenite formation

On the basis of their structural observations west and northwest of the MoD Mountain area, and U/Pb formation or cooling ages from cross-cutting calcite veins, (60 ± 16 and 58 ± 6 Ma) Scharf et al.

(2020) infer that listvenites in the Fanjah area postdate ophiolite emplacement, echoing the qualitative ideas of [START_REF] Stanger | Silicified serpentinite in the Semail nappe of Oman[END_REF] and [START_REF] Wilde | Preliminary study of Tertiary hydrothermal alteration and platinum deposition in the Oman ophiolite[END_REF]. As noted in Sections 2.5 and 4. In this paper, our mapping and structural observations indicate an intact tectonic "stratigraphy" around the basal thrust of the ophiolite, encompassing the allocthanous Hawasina metasediments, overlain by the metamorphic sole, in turn overlain by Banded Unit dunites and harzburgites of the Samail ophiolite mantle section, with essentially horizontal lithologic contacts extending for kilometers. This is the stratigraphy created during subduction beneath the ophiolite, and -together with the isochron -it indicates that these units were juxtaposed during low angle thrusting along the subduction zone. In turn, all the listvenites in the MoD Mountain area are less than a kilometer away from the basal thrust, and have contacts with surrounding mantle peridotites and the metamorphic sole that are parallel to the basal thrust. All these data, taken together, indicate that the listvenites formed along and above the basal thrust of the ophiolite during subduction. Some may postdate ophiolite emplacement, and may be broadly related to Paleocene to Miocene uplift and extension during formation of the nearby Jebel Akdar and Saih Hatat domes. On the other hand, some could be related to deformation during subduction beneath the ophiolite. The long term persistence of temperatures above 180°C in this region renders it difficult to distinguish formation from cooling ages using (U,Th)/He data.

Temperature of listvenite formation

The temperatures of listvenite formation have been previously constrained using metamorphic phase equilibria, conventional oxygen isotope thermometry, and clumped isotope analyses. [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF] noted the presence of intergrown antigorite (serpentine) + quartz. in the reaction zone between listvenite and serpentinite on the west ridge of MoD Mountain (Figure 2). In some cases, these samples also contain talc. They used Thermocalc [START_REF] Powell | Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC[END_REF], with thermodynamic data from Holland andPowell (2003, 1998), and similar methods using the thermodynamic data of [START_REF] Gottschalk | Internally consistent thermodynamic data for rock-forming minerals in the system SiO2-TiO2-Al2O3-Fe2O3-CaO-MgO-FeO-K2O-Na2O-H2O-CO2[END_REF], to estimate that equilibrium coexistence of antigorite, quartz and talc occurs at 80 to 120°C, depending on the choice and uncertainty of thermodynamic parameters for the minerals, and the (poorly known) pressure at which these assemblages crystallized. For this paper, we obtained similar using Perple_X (https://www.perplex.ethz.ch/) [START_REF] Connolly | Multivariable phase-diagrams -an algorithm based on generalized thermodynamics[END_REF][START_REF] Connolly | Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation[END_REF] Here we show that veins of intergrown amorphous carbon compounds and hematite sampled in the core also must have formed below ~ 200°C (Figure 11). The observation of "reduced" hydrocarbon species intergrown with oxidized iron minerals in veins may seem surprising at first, but they are mutually stable at low temperature due to the stronger temperature dependence of the iron oxidation state, compared to the carbon oxidation state. Above ~ 200°C, reduced, organic carbon compounds and oxidized iron minerals such as hematite cannot coexist at the same oxygen fugacity, but they can crystallize together at moderate oxygen fugacities below ~ 200°C. We've found that this result is robust across all available sets of internally consistent thermodynamic data, and in the pressure range from 0.2 to 1.0 GPa.

Fourteen more recent clumped isotope analyses for listvenites and carbonate-bearing serpentinites from BT1B core yield an average temperature of 147±58°C (1σ). Ten of the 14 temperature estimates lie within the 1σ range, whereas two are lower (45±5 and 52±8°C) and the two least precise estimates

Depth of listvenite formation

The depth of listvenite formation is difficult to constrain. As noted in Section 2.4, published work on the metamorphic sole beneath the Samail ophiolite reports peak temperatures up to 700 to 900°C at pressures ranging from 200 to 1400 MPa, indicative of anomalously hot subduction zone conditions.

Common peak pressure estimates, ~ 1200 MPa, indicate depths exceeding the structural thickness of the ophiolite. Thus, either (1) the published range of pressure estimates for the metamorphic sole is the result of uncertainty rather than true variation in depth, and/or (2) the lower few km of the ophiolite mantle section has undergone thinning in some places, and/or (3) the lenses of the metamorphic sole recording the highest pressures migrated upward with respect to the overlying peridotite, and/or (4) some mineral equilibria record tectonic overpressures rather than lithostatic loads. Also, we infer from Sr isotope data that the fluids that formed the listvenites were not derived from the metamorphic sole (Section 5.6, below, and de Obeso et al 2021a). For all these reasons, it is unclear whether the metamorphic pressures inferred for the sole constrain the depth of listvenite formation. However, (2019), and lie within the cold end of the range of PT conditions recorded by subduction-related metamorphic rocks [START_REF] Hacker | Eclogite formation and the rheology, buoyancy, seismicity, and H2O content of oceanic crust[END_REF][START_REF] Hacker | Pressures and temperatures of ultrahigh-pressure metamorphism: Implications for UHP tectonics and H2O in subducting slabs[END_REF][START_REF] Penniston-Dorland | The global range of subduction zone thermal structures from exhumed blueschists and eclogites: Rocks are hotter than models[END_REF]. Such low temperatures at 25 to 40 km are rare or absent in other tectonic environments.

To summarize, it is possible that the sediments and the overlying mantle peridotites at the base of the ophiolite were juxtaposed by subduction at the leading edge of the mantle wedge, at a depth of 25 to 40 km, and that the MoD Mountain listvenites formed at these depths along a cold, steady state subduction geotherm. Alternatively, if the metamorphic sole has migrated updip with respect to the overlying peridotites, then the listvenites could have formed at lower pressures and shallower depths.

Composition of listvenite protolith and geochemical fluid additions

Given the abundance and variety of mineralogically simple veins, many of which are monomineralic, there is substantial compositional variation in listvenites at the millimeter to meter scale. This variability extends to larger scales in some parts of the core. Nevertheless, remarkably enough, average MgO/SiO2/FeO* (all Fe as FeO) ratios in the listvenites are very similar to those in average residual peridotites from the ophiolite (Figure 12), as discussed further by Okazaki et al. [START_REF] Okazaki | Major mineral fraction and physical properties of carbonated peridotite (listvenite) from ICDP Oman Drilling Project Hole BT1B inferred from X-ray CT core images[END_REF]. These oxides comprise more than 90% of the volatile-free bulk composition of the rock. These data suggest that there was little dissolution and export of major elements from the rocks during transformation of peridotite to serpentinite and then to listvenite as discussed further in Section 5.9.

On the other hand, serpentinites and listvenites record addition of tens of weight percent H2O and CO2 to the original bulk composition of mantle peridotite protoliths. In addition to CO2, dolomite listvenites clearly record substantial addition of CaO, and -since CaO and Sr concentrations are strongly correlated -of Sr as well.

Although the shipboard data do not reveal systematic variation in the abundance of magnesite vs dolomite listvenites downhole, there is a clear change in the abundance of both Al and K, together with many other highly incompatible trace elements. Concentrations of these elements are relatively low above the serpentinite band at 80-100 m depth, and much higher below that band [START_REF] Godard | Geochemical profiles across the listvenite-metamorphic transition in the basal megathrust of the Semail Ophiolite: Results from drilling at Oman DP Hole BT1B[END_REF], Kelemen et al 2020b).

Understanding the source for enrichment in these elements is complicated by uncertainty about their concentration in the peridotite protolith. As noted in Sections 2.3, 2.5 and 4.4, a "Banded Unit" of alternating dunite, harzburgite and lherzolite characterizes the base of the ophiolite mantle section in many regions. Some Banded Unit peridotites record high temperature geochemical "refertilization" by reaction of residual mantle peridotites with infiltrating melt or fluid at > 800°C, with addition of calcic pyroxene and Mg-rich hornblende. This produced enrichment in CaO and Al2O3 to levels well above those in typical residual mantle peridotites in the Samail ophiolite [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF][START_REF] Hanghoj | Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; Major and trace element geochemistry, and Os isotope and PGE systematics[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF]. Indeed, four out of six harzburgite samples from the Banded Unit on MoD

Mountain have Ca and Al contents outside the 1σ range of variability in residual mantle peridotite in the ophiolite [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF], Godard et al 2021, Kelemen et al 2020c).

The listvenites below 100 meters depth in core from Hole BT1B have trace element ratios that are distinct from typical peridotites, but characteristic of the Banded Unit (Figure 12 and Godard et al 2021), as shown in Figure 13. In particular, middle to heavy rare earth element ratios in the Banded Unit, and in the listvenites, are high compared to typical Samail peridotites. Such enrichment in middle rare earth elements is commonly associated with the presence of igneous hornblende in peridotites.

Thus, the listvenites probably inherited these characteristics from the enriched, Banded Unit protolith.

On the other hand, it is clear that most listvenites have higher Sr concentrations than typical Samail ophiolite peridotites and the Banded Unit (de Obeso et al 2021a). Instead, Sr and Ca were added during low temperature alteration, along with H2O and CO2. These topics are discussed further by [START_REF] Godard | Geochemical profiles across the listvenite-metamorphic transition in the basal megathrust of the Semail Ophiolite: Results from drilling at Oman DP Hole BT1B[END_REF]. [START_REF] Khedr | Petrology and chemistry of basal lherzolites above the metamorphic sole from Wadi Sarami central Oman ophiolite[END_REF][START_REF] Khedr | Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity[END_REF][START_REF] Takazawa | Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)[END_REF].

Source of fluid for listvenite formation

Present day Sr isotope ratios in listvenites and serpentinites from MoD Mtn range from ~ 0.708 to 0.715 (de Obeso et al 2021a, [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]. Using current Rb/Sr contents in these samples, age corrected 87 Sr/ 86 Sr ratios at 96 Ma range from ~ 0.708 to 0.714, much higher than the range of Sr ratios in Samail ophiolite peridotites (0.703 to 0.707, [START_REF] Benoit | The remelting of hydrothermally altered peridotite at mid-ocean ridges by intruding mantle diapirs[END_REF][START_REF] Gerbert-Gaillard | Caractérisation Géochimique des Péridotites de l'ophiolite d'Oman : processus magmatiques aux limites lithosphère/asthenosphère[END_REF], Gregory & Taylor Jr 1981[START_REF] Lanphere | Sr isotopic tracer study of the Samail ophiolite, Oman[END_REF][START_REF] Mcculloch | A neodymium, strontium, and oxygen isotopic study of the Cretaceous Samail Ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust[END_REF][START_REF] Mcculloch | Sm-Nd, Rb-Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail Ophiolite[END_REF], the range in Late Cretaceous to modern seawater (~ 0.7075 to 0.7081), or the range in peridotite-hosted ground water in the Samail ophiolite (~0.7065 to 0.7092, [START_REF] Weyhenmeyer | Origin and evolution of groundwater in the alluvial aquifer of the Eastern Batinah Coastal Plain, Sultanate of Oman : a hydrogeochemical approach[END_REF]. Thus, the Sr-, Ca-and CO2-rich fluid(s) that modified the mantle overlying the basal thrust of the ophiolite had relatively high 87 Sr/ 86 Sr ratios compared to fresh, residual mantle peridotites.

Initially, we expected that the metamorphic sole, as sampled by core from Hole BT1B, might be the source of fluids that formed listvenites in overlying peridotites, or at least might be analogous to the source of these fluids. However, this is not consistent with the Sr isotope data on the sole in core from Hole BT1B. Measured and age-corrected Sr isotope ratios in the metamorphic sole are consistently lower than corresponding ratios in the listvenites (de Obeso et al 2021a).

Instead, pelagic, clastic units of the underlying Hawasina sedimentary rocks have measured and agecorrected Sr isotope ratios that span the same range as those in the listvenites (de Obeso et al 2021a). Thus, these sedimentary units in the Hawasina may be analogous to subducted sedimentary rocks that produced CO2-bearing fluids which, in turn, formed the MoD Mountain listvenites. This is likely, despite the presence of C-and Sr-rich limestone and dolomite units with lower Sr isotope ratios A complication is that the clastic units in the Hawasina formation have δ 13 C less than -4 per mil, whereas listvenites have δ 13 C > -3 per mil. These differences in carbon isotope ratios can be understood as the result of temperature dependent carbon isotope fractionation. As discussed in more detail in de Obeso et al. (2021a), at temperatures greater than ~ 300°C, dissolved CO2 in aqueous fluids has δ 13 C higher than co-existing calcite and dolomite [START_REF] Deines | The carbon isotope geochemistry of mantle xenoliths[END_REF][START_REF] Horita | Oxygen and carbon isotope fractionation in the system dolomite-water-CO2 to elevated temperatures[END_REF]. At lower temperatures, calcite and dolomite have δ 13 C higher than co-existing fluids. Dolomite and magnesite crystallized at relatively low temperature, from aqueous fluids that acquired their carbon isotope ratios during higher temperature devolatilization of Hawasina clastic sediment compositions, would have δ 13 C in the range of 1.0 to -3.0‰, as observed in the MoD Mountain listvenites. In Based on the data and reasoning described in the previous paragraph, we hypothesize that subduction zone devolatilization at 400 to 600°C produced CO2-rich aqueous fluids that then cooled and decompressed by flow up the subduction zone, and reacted with peridotite at less than 200°C to produce the MoD Mountain listvenites. To quantify this hypothesis, we made thermodynamic calculations with the compositions of solid reactants given in Supplementary Table Closed system decompression and cooling of fluid produced from OM20-17 to 100 to 300°C, and 0.5 to 1 GPa produced no significant change in the composition of this modeled fluid. (Modeling open system, reactive transport of this fluid, updip along a subduction zone geotherm, is beyond the scope of this paper, as is modeling the fate of subduction zone fluids that have more than 10 wt% CO2predicted for mixed siliciclastic-carbonate rocks (Figure 14) -as they cool and decompress). 2 in de Obeso et al 2021a), evolving along cold (550°C at 2.5 GPa) and hot (500°C at 1.5 GPa) subduction geotherms. Solid curves are results for the average ("cold") subduction zone geotherm from [START_REF] Syracuse | The global range of subduction zone thermal models[END_REF]. Dashed curves follow a hot geotherm suggested by [START_REF] Penniston-Dorland | The global range of subduction zone thermal structures from exhumed blueschists and eclogites: Rocks are hotter than models[END_REF]. The initial carbon contents of the rocks (wt%, all carbon as CO2) are given in the legend.

Listvenite formation

Reaction of the model fluid from OM20-17 with peridotite at 100-300°C and 0.5 to 1.0 GPa is predicted to produce mineral assemblages similar to those modeled by [START_REF] Klein | Thermodynamic constraints on mineral carbonation of serpentinized peridotite[END_REF]: (1) Small masses of "birbirite" (silicified peridotite) at high water/rock ratios and/or low temperatures, through (2) moderate masses of listvenite and soapstone (talc-carbonate rocks) at moderate water/rock and temperature, to (3) relatively large masses of carbonate-bearing serpentinite at low water/rock ratios and/or high temperature (Figure 15). Predicted magnesite and quartz proportions correspond closely to observed proportions in MoD Mtn listvenites (Figure 12). Most of the PT conditions we modeled produced small amounts of hematite coexisting with magnesite and quartz, as observed. Predicted magnesite and siderite proportions correspond to a solid solution with ~ 8 wt% FeO, which is a few wt% higher than observed in MoD Mtn listvenites. Most modeled conditions produced dolomite in listvenite assemblages at water/rock ratios at water/rock ratios less than 10 (log water/rock = 1), consistent with the presence of relatively late, cross-cutting dolomite-bearing veins in the listvenites. Most model runs produce small amounts of kaolinite, and very limited proportions of muscovite, in listvenite mineral assemblages, consistent with the widespread presence of minor amounts of chromian white mica in MoD Mtn listvenites, and in listvenites worldwide. It is possible that addition of a thermodynamic model for fuchsite would yield stable white mica, rather than kaolinite, over a wider range of temperature and water/rock ratios. Alternatively, perhaps some of the green sheet silicates in listvenites are chrome-bearing clays rather than true micas.

Thus, thermodynamic modeling suggests that the CO2-bearing aqueous fluids that formed the MoD Mountain listvenites formed by metamorphic devolatilization in a subduction zone at > 400°C. These fluids then migrated updip to react with peridotite at the leading edge of the mantle wedge, probably at a depth less than 40 km. However, this is just a forward model, and there may be other possibilities. log water/rock ratio grams minerals precipitated per kg fluid Updip migration of fluids in a subduction zone has been predicted in some simplified dynamic models of fluid flow in a viscously deforming subduction zone with high permeability [START_REF] Wilson | Fluid flow in subduction zones: The role of solid rheology and compaction pressure[END_REF].

Subhorizontal lenses of listvenite at

However, the tendency of fluid buoyancy to drive vertical fluid flow may often dominate subduction zone fluid fluxes. Thus, formation of relatively shallow listvenites, like those at MoD Mountain, may be localized and unusual. Elsewhere, CO2-bearing fluids may migrate vertically into overlying mantle peridotite at greater depth (e.g., [START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones: What goes down, mostly comes up[END_REF]. It is interesting to ponder how subduction zone CO2 fluxes may be partitioned between these different transport and mineralization processes.

Multiple reaction fronts

The thermodynamic models presented in Sections 5.6 and 5.7 provide a starting point for understanding "chromatographic effects" during transformation of peridotite to serpentinite. As can be seen in Figure 15, our reaction path models predict sharp fronts where serpentinite is replaced by soapstone, and then by listvenite. Talc-bearing, soapstone assemblages are predicted to crystallize in a limited range of conditions, consistent with the fact that talc is rare in core from Hole BT1B, and in hand specimens from MoD Mtn, where it is almost entirely restricted to narrow (~ 1 m scale) transition zones between listvenite and serpentinite. If we were to use different thermodynamic data, talc might be even less abundant, or absent in models at 100°C, because talc is predicted to be unstable below ~ 100°C with respect to antigorite + quartz when using mineral data from Holland and Powell or Gottschalk, together with the Redlich-Kwong equation of state for H2O-CO2 fluids as modified by [START_REF] Kerrick | A modified Redlich-Kwong equation for H2O, CO2, and H2O-CO2 mixtures at elevated pressures and temperatures[END_REF] and by [START_REF] Holland | Activity-composition relations for phases in petrological calculations: an asymmetric multicomponent formulation[END_REF].

In the model results presented in Figure 15, with reaction progress increasing from high fluid/rock ratios on the right to low fluid/rock ratios on the left, magnesite + quartz and magnesite + talc become unstable with respect to serpentine as dissolved carbon in the fluid is exhausted. Because carbon is a minor constituent of aqueous fluid, but a major component of the listvenite assemblage, exhaustion of dissolved carbon is predicted to occur at a fluid/rock ratio much greater than 1 (log fluid/rock > 0). Of course, because aqueous fluid is composed mainly of H2O, the potential for serpentinization of peridotite continues to much lower fluid/rock ratios.

Although we cannot model it, we can also predict that -in the presence of pervasive fluid flow on the grain scale -there could also be a sharp front where serpentine replaces olivine at water/rock ratios less than one. Thermodynamic calculations for simplified olivine serpentinization by Kelemen et al. (2020a) indicate that olivine + H2O would be stable with respect to serpentine at 100°C and a partial pressure of ~ 10 -2 bars, and at 200°C and P(H2O) ~ 1 bar. While these conditions cannot be modeled using EQ3/6, we can anticipate that low partial pressures of H2O -much lower than lithostatic pressures -are produced along grain boundaries and near the tips of incipient fractures and veins, especially where fluid has been almost completely consumed by peridotite hydration reactions. Under these conditions, there could be a sharp front where serpentine (at higher fluid pressures) becomes stable relative to olivine (at lower fluid pressure).

Throughout the mantle section of the Samail ophiolite, residual mantle peridotites commonly contain about 50 to 80% serpentine, as inferred from the fact that bulk rock analyses yield loss on ignition (mostly, H2O) of 8 to 10 wt% as compared to 13 to 16 wt% H2O in completely serpentinized, Mg endmember harzburgite and dunites. These partially serpentinized peridotites commonly show a "mesh texture", with relict olivine and pyroxene "cores" transected by a "mesh" of cross-cutting serpentine veins -typically 10 to 100 microns apart [START_REF] Francis | The serpentinite mass in Glen Urquhart, Inverness-shire, Scotland[END_REF][START_REF] Green | Ultramafic breccias from the Musa Valley, eastern Papua[END_REF][START_REF] Green | The petrogenesis of the high-temperature peridotite intrusion in the Lizard area[END_REF][START_REF] Raleigh | Experimental deformation of serpentinite and its tectonic implications[END_REF]. In some regions within the Samail ophiolite, particularly areas of relatively subdued topography that have undergone extensive, penetrative weathering, relict mantle minerals in the mesh cores are completely replaced by serpentine (e.g., OmanDP Sites BA1, BA2, BA3 and BA4, Kelemen et al 2021, Kelemen et al 2020c). However, along the steep canyons and narrow ridges that are typical of outcrops in the mantle section of the ophiolite, subject to relatively rapid erosion, the pervasive presence of the serpentine vein mesh surrounding relict mantle minerals attests to relatively rapid fluid transport in fractures and veins, compared to slow transport of H2O into the mesh cores by diffusion and/or imbibition.

In contrast, as noted above, the serpentinites sampled in core from Hole BT1B contain no relict olivine or orthopyroxene, though pyroxene pseudomorphs ("bastites") are evident. A zone of 100% serpentinized peridotites a few meters thick was sampled by [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF] in a transect across a listvenite-peridotite contact on the watershed ridge east of the summit MoD Mountain.

Outside this zone, samples of peridotite had compositions and textures typical of partially serpentinized residual mantle peridotites throughout the ophiolite. Based on these observations, we infer that there is a 100% serpentinite zone a few meters thick, with sharp fronts -less than a meter thick, between serpentinte and listvenite on one side, and between partially and completely serpentinized peridotite on the other side.

Volume change during listvenite formation

The likely volume change during fluid-rock reactions has long been debated. Whereas [START_REF] Coleman | Chemical Study of Serpentinization -Burro Mountain, California[END_REF] proposed that serpentinization involved simple addition of H2O to peridotite, with no significant change in the volatile-free solid composition, Carmichael (1987), [START_REF] Nahon | Pseudomorphic replacement in tropical weathering: Evidence, geochemical consequences, and kinetic-rheological origin[END_REF][START_REF] Nahon | Pseudomorphic replacement in tropical weathering: Evidence, geochemical consequences, and kinetic-rheological origin[END_REF][START_REF] Fletcher | Mineral growth in rocks: kinetic-rheological models of replacement, vein formation, and syntectonic crystallization[END_REF][START_REF] Fletcher | Mineral growth in rocks: kinetic-rheological models of replacement, vein formation, and syntectonic crystallization[END_REF] argued that such reactions take place at nearly constant volume, with addition of some components balanced by dissolution and export of others. Fletcher & Merino provided quantitative calculations to support this hypothesis. Where a fluid that is super-saturated in a new mineral phase, A, starts to crystallize A within a host mineral B that is initially in equilibrium with fluid, expansion of B around A leads to an increase in local effective stress. In turn, because chemical potential is proportional to the mean stress, this reduces supersaturation in A and leads to undersaturation in B. This process continues, with very small volume changes, until the rate of crystallization of A becomes equal to the rate of dissolution of B, at a steady state stress.

Using the methodology of [START_REF] Fletcher | Mineral growth in rocks: kinetic-rheological models of replacement, vein formation, and syntectonic crystallization[END_REF] and a saturation index of 2 (as they did), [START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF] calculated that the steady state, effective stress during replacement of olivine with serpentine is ~ 40 MPa. However, because olivine is very far from equilibrium with water at low temperature, the saturation index for water reacting with olivine to form serpentine at 50 to 250°C and P(H2O) > 1 bar is close to 10 7 , and the steady state, effective stress estimated using the method of Fletcher and Merino is ~ 800 MPa [START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF]. Clearly, such a large differential stress cannot be sustained within most rocks, which will deform at a lower stress, before the steady state can be reached, either via ductile mechanisms (if reactions are slow and temperatures are high) or via fracture and frictional deformation (if reactions are fast and temperatures are low). The latter outcome is sometimes termed "reaction-induced cracking" [START_REF] Jamtveit | Reaction induced fracturing during replacement processes[END_REF][START_REF] Rudge | A simple model of reaction-induced cracking applied to serpentinization and carbonation of peridotite[END_REF] or "reaction-driven cracking" [START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF]. Thus, while some workers still disagree over the extent of volume change in specific replacement processes, the approach of Fletcher and Merino nicely explains a continuum between nearly constant volume replacement of one mineral by another at high temperature, close-to-equilibrium conditions, and large volume changes accommodated by fractures and frictional sliding at low temperature, far-from-equilibrium conditions.

As noted in Section 5.5 , despite dramatic, local variability, the average major element composition of listvenites from MoD Mountain -in drill core and surface samples -is strikingly similar to the average composition of residual mantle harzburgites in the Samail ophiolite. Together, the concentrations of MgO, SiO2 and FeO* (all Fe as FeO) account for more than 90% of the volatile-free bulk composition of listvenite samples. Thus, the fact that average Mg/Si/Fe ratios in the listvenites are almost identical to those in Samail peridotites suggests that either (1) large scale dissolution of the peridotite protoliths was nearly congruent, exporting dissolved, major elements in approximately their initial proportions, or (2) there was little dissolution and export of major elements in the protolith during addition of CO2 to form listvenites, and addition of H2O to form serpentinites.

Thermodynamic models, experimental data, and observations of natural rock samples strongly favor the second of these hypotheses. The solubility of silicate and Fe-oxide minerals in rock-buffered aqueous fluids at low temperature is too low to allow for large scale dissolution and export of major elements in open system, fluid-rock reactions. In turn, models of addition of CO2 and/or H2O to peridotites, predict very little removal of other species, togther with large increases in the mass and volume of the solid products of reaction. Birbirite formation may involve net volume and mass loss due to extensive dissolution of peridotite reactants at water/rock ratios greater than ~ 100. However, the models of listvenite formation illustrated in Figure 15 [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF].

Because porosities in peridotite, serpentinite and listvenite rarely if ever exceed 5%, all of these solid volume changes must be accommodated mainly by expansion of the entire rock volume. It is interesting to speculate how much uplift -and/or lateral expansion -in forearc regions is caused by partially serpentinized peridotite serpentinite listvenite hydration and carbonation of the mantle wedge. However, at the plate tectonic scale, the rates of reactions similar to those outlined in equations (1-6) are unknown. It is likely that the strains due to reaction are comparable to, or smaller than, other rates of deformation at convergent plate boundaries, rendering the effect of solid volume expansion difficult to detect at the regional scale.

Based on the considerations outlined in this section, it is likely that large increases in the solid volume occurred during formation of the MoD Mountain listvenites, and were accommodated mainly by reaction-driven cracking, frictional sliding along existing fractures, and/or reaction-assisted granular flow [START_REF] Menzel | Ductile deformation during carbonation of serpentinized peridotite[END_REF]. The presence of abundant, antitaxial magnesite, magnesite-hematite, and Fe-oxide veins in both serpentinites and listvenites is qualitatively consistent with a reaction-driven cracking process. However, we have not identified any obvious strain markers that would allow a quantitative evaluation of this hypothesis using rock textures.

Building upon an idea from [START_REF] Hansen | Carbonated serpentinite (listwanite) at Atlin, British Columbia: A geological analogue to carbon dioxide sequestration[END_REF], the Shipboard Scientific Party developed the hypothesis that large volume increases due to hydration of olivine and pyroxene (reactions 1 & 2) may have initially formed fractures at (and beyond) a serpentinization front -not observed in drill core, but traversed by [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF] -and these fractures were conduits for fluid flow and sites of localized deformation during the smaller volume changes due to carbonation of serpentinites (reactions 5 and 6) as schematically illustrated in Figure 17.

Low temperature ductile deformation in subduction zones

As noted above, the listvenites at MoD Mountain may have formed at depths of 25 to 40 km, at temperatures less 200°C, yielding low temperature "geotherms" of 5 to 8°C/km depth. Such small increases of temperature with depth are characteristic of the forearc above subduction zones [START_REF] Peacock | Thermal and petrologic structure of subduction zones[END_REF]. High fluid pressures at these depths may account for the somewhat surprising indications of low temperature, ductile deformation in core samples from Hole BT1B, based on the observed crystallographic preferred orientation (CPO) in quartz, magnesite and serpentine in core samples (Section 4.4, Figure 10). Some of the fabrics illustrated in Figure 10 could be inherited. For example, in early formed, magnesite-hematite veins, magnesite forms parallel crystals, perpendicular to vein margins, with small misorientations between adjacent crystals. When these veins are parallel (because they form that way, or after they are transposed), this imparts a CPO to the sample, and may also give the impression that sub-grain boundaries are present, even if there was no deformation of magnesite crystals via dislocation creep. Similarly, quartz replacing opal may inherit a CPO, or a CPO may arise due to anisotropic stress during recrystallization, without substantial strain.

On the other hand, the serpentine mylonite illustrated in Figure 10 almost certainly records large strains due to distributed deformation along grain boundaries. Moreover, clear examples of ductile deformation and shear zones with classical indictors of substantial strain indicate that ductile deformation was active during the initial stages of listvenite formation, perhaps assisted by positive feedback between weakening, due to reaction-induced recrystallization, and porosity enhancement due to deformation, as discussed by [START_REF] Menzel | Ductile deformation during carbonation of serpentinized peridotite[END_REF]. An important role for ductile deformation in reaction zones at the top of subducting oceanic crust may help to explain why, in the relatively hot Cascadia and SW Japan subduction zones, there are very few earthquakes at the top of the subducting crust [START_REF] Abers | Imaging the source regionof Cascadia tremor and intermediate-depth earthquakes[END_REF][START_REF] Abers | Thermal-petrological controls on the location of earthquakes within subducting plates[END_REF][START_REF] Hirose | Three-dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by doubledifference tomography[END_REF].

Conclusions

Observations of drill core of listvenite (completely carbonated peridotite), serpentinite, and subduction-related metamorphic rocks from OmanDP Hole BT1B provide constraints on temperature, into the continental mantle lithosphere -may be a globally important reservoir for carbon [START_REF] Foley | An essential role for continental rifts and lithosphere in the deep carbon cycle[END_REF][START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones: What goes down, mostly comes up[END_REF][START_REF] Li | Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China[END_REF][START_REF] Scambelluri | Carbonation of subduction-zone serpentinite (high-pressure ophicarbonate; Ligurian Western Alps) and implications for the deep carbon cycling[END_REF] During BT1B listvenite formation, total solid volume increased by tens of percent due to hydration followed by carbonation. While core and surface samples provide few direct constraints on the mechanism that accommodated this expansion, one hypothesis is that large volume changes during hydration of olivine and pyroxene along a serpentinization front caused large stresses and fractures that accommodated expansion via frictional sliding, and provided secondary porosity for the CO2-rich fluids that transformed serpentinites to listvenites.

Based on observed crystallographic preferred orientation in quartz, magnesite and serpentine in macroscopically identified shear zones, it is inferred that ductile deformation of listvenite and serpentinite occurred under low temperature conditions at the base of the mantle wedge during subduction. Low temperature ductile deformation, coeval with serpentinization and listvenite formation, may have been facilitated by recrystallization associated with the hydration and carbon

Figure 2 :

 2 Figure 2: Photograph of the west end of the MoD Mountain ridge, from the SW, looking NE, showing bands of listvenite (rusty orange) parallel to contacts between partially serpentinized harzburgite (brown) and dunite (tan) comprising the "Banded Unit" at the base of the Samail ophiolite mantle section. Greenish metabasalts and metasediments of the metamorphic sole underlying the ophiolite are exposed along indistinct ridge in lower left. The listvenite band at top right is more than 100 m thick and extends for 1.5 kilometers along the length of the summit ridge. The listvenite band in the center of the photo is 15-20 m thick. Photo taken with a telephoto lens from ~ 23.35°N, 58.17°E, along azimuth ~ 030°.

Figure 1 :

 1 Figure 1: Outcrop area of the Samail ophiolite in Oman and the UAE, based on[START_REF] Nicolas | Accretion of Oman and United Arab Emirates ophiolite: Discussion of a new structural map[END_REF]. Metamorphic sole, in red. Black circles indicate approximate location of listvenites[START_REF] Nasir | Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman[END_REF][START_REF] Stanger | Silicified serpentinite in the Semail nappe of Oman[END_REF][START_REF] Wilde | Preliminary study of Tertiary hydrothermal alteration and platinum deposition in the Oman ophiolite[END_REF]. Red rectangle indicates the approximate location of Figure3, a geologic map of MoD Mtn and vicinity, which was the site of Oman DP Hole BT1B, and the focus of this paper.

Figure 3 :

 3 Figure 3: Map and cross section of the area NNE of Oman Drilling Project Site BT1. Yellow, HS: Hawasina sediments; light green, MS: metamorphic sole; rusty red, L: listvenite; dark green, SP: serpentinized peridotite; blue: inferred gabbro, based on outcrops south of Wadi Mansah; grey: gravels of Wadi Mansah. UTM coordinates in meters. Cross section has no vertical exaggeration. Map and cross-section modified from Figures F1 and F2 inKelemen et al. (2020b)

  graphite fromHelgeson et al.) and various equations of state for H2O-CO2 fluids[START_REF] Shock | Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures: Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 C and 5 kbar[END_REF][START_REF] Shock | Inorganic species in geologic fluids: correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes[END_REF] modified from[START_REF] Helgeson | Theoretical prediction of the thermodynamic behavior of aqueous electrolytes by high pressures and temperatures; IV, Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 600 degrees C and 5kb[END_REF]. All of these different combinations provided consistent results, with antigorite + quartz stable with respect to talc at temperatures up to 80 to 120*C. However, calculations using the Deep Earth Water (DEW) model[START_REF] Huang | Extended Deep Earth Water Model for predicting major element mantle metasomatism[END_REF][START_REF] Sverjensky | Water in the deep Earth: the dielectric constant and the solubilities of quartz and corundum to 60 kb and 1200 C. Geochim[END_REF] and the extended Helgeson-Kirkham-Flowers (HKF) aqueous equation of state

Figure 4 :

 4 Figure 4: Geologic map and cross section of MoD Mountain and vicinity. Complex map pattern arises from intersection of topography with broadly folded, gently dipping units, as is more evident in the cross section (no vertical exaggeration) illustrating the antiform coinciding with MoD Mountain, and the syncline to its north. Dips are all measured on lithologic contacts, including dunite/harzburgite contacts within the Samail ophiolite mantle section.

  , the top of the Hole sampled ~ 200 meters of listvenite interlayered with two serpentinite bands from 80 to 100 m depth, and 181 to 185 m depth. Below 185 m, the listvenite is ubiquitously deformed, with visual core descriptions indicating a mixture of brittle and ductile deformation. At about 197 m, core was composed of a few tens of cm of soft, clay-rich fault gouge,

Figure 5 :

 5 Figure 5: Lithologic and mineralogical column for core from Hole BT1B, modified from Godard et al. (2021).

Figure 6 :

 6 Figure 6: Antitaxial magnesite-hematite veins, and magnesite spheroids in serpentinites and listvenites in core from Hole BT1B. Top panels: plane light (left) and crossed-polarized images (right, quartz plate (+1l) inserted) of magnesite-hematite veins near the lower contact of the upper serpentinite band, TS_BT1B_44Z-3_9-11.5, ~100 m depth, from Figure F47 in Kelemen et al.(2020b). Middle panels: tiny magnesite spheroids in serpentinite, TS_BT1B_44Z-3_9-11.5, ~100 m depth, from FigureF29inKelemen et al. (2020b). F. Bottom panels: Cross-polarized images, right one with quartz plate (+1l) inserted, of texturally similar, "antitaxial" magnesite-hematite veins and magnesite ovoids in quartz-rich, listvenite matrix, TS_BT1B_47Z-3_15-19 at about 110 m depth, from FigureF35inKelemen et al. (2020b).

(

  Kelemen et al 2020b) interpreted these as replacing opal, which would have been among the earliest SiO2 minerals to form in many of the listvenites. Importantly, opal is commonly found in other listvenites and serpentine-magnesite associations worldwide (Abu-Jaber & Kimberley 1992, Aftabi & Zarrinkoub 2013, Akbulut et al 2006, Arisi Rota et al 1971, Barnes et al 1973, Beinlich et al 2010, Borojević Šoštarić et al 2014, Boschi et al 2009, Ece et al 2005, Jurković et al 2012, Lacinska & Styles 2013, Lapham 1961, Oskierski et al 2013a, Oskierski et al 2013b, Posukhova et al 2013, Quesnel et al 2016, Searston 1998, Ulrich et al 2014, Zarrinkoub et al 2005).

Figure 8 :

 8 Figure 8: Plane light photomicrographs illustrating magnesite ovoids in a matrix composed of magnesite, quartz, and subordinate hematite and Fe-oxyhydroxides, TS_BT1B_20Z-1_42-46, ~ 40 m depth (left top) and TS_BT1B_27Z-2_6-8.5, ~ 59 m depth (left bottom), and quartz spherulites with carbonate and hematite inclusions, in matrix of fine-grained quartz and hematite with subordinate, microscopic carbonates, TS_BT1B_60Z-1_12-17, ~140 m depth (right).

Figure 9 :

 9 Figure 9: Carbonaceous material in drill core. A: Drill site photo of 1 to 2 mm vein of shiny, grey, soft material described as graphite on the drill site, in a vein rimmed with hematite. Field of view 5 cm wide, core 74Z-01, ~ 195 m depth. B: Shipboard XRD spectra of soft "graphite" powder extracted from veins with "graphite" + hematite, replacing Figure F43 in Kelemen et al. (2020b). Interpretation of these data is complicated by the similarity of the quartz and graphite peaks at ~ 26° 2Θ, but quartz also has a prominent peak at ~ 21°C which is absent from the blue spectrum for 78Z-02_16-24. C: Raman spectra of black material in samples BT1B_77Z-03_30-38 (blue), 78Z-02_8-15 (red) and 78Z-02_50-56 (green, ~ 192-198 m depth. Broad double peaks at wavenumbers of ~1350/cm and 1600/cm are indicative of disordered carbon compounds; no Raman spectra diagnostic of graphite were obtained. Many microscopic, soft, black domains contained hematite, with a single broad peak at ~1350/cm, instead of, or in addition to, disordered carbon compounds. D: Raman spectra of black material in samples BT1_44Z-02_10-15, BT1_78Z-02_8-15, and BT1_44Z-02_24-27, showing broad, double peaks indicative of disordered carbon compounds, some associated with covellite. Core 44Z ~ 98 m depth. E: dark grey copper sulfate "cow pies", spatially associated with brightly reflecting covellite, near amorphous carbon material, field of view ~ 100 microns, sample BT1_44Z-02_24-27. Data and images from core 44Z are from a 1 mm diameter black spot with a red rim in the interior of the core, exposed by the rock saw during sample preparation.

Figure 10 :

 10 Figure 10: Examples of crystallographic preferred orientation (CPO) in localized zones in core from Hole BT1B. All images in cross-polarized light with quartz plate inserted. Images on right are of the same area as on left, but rotated 90° with respect to the polarizers. Areas showing optical continuity have a crystallographic preferred orientation. Top, TS_BT1B_31Z-4_12-14, ~ 65 m depth, quartz lenses within sub-parallel, anastomosing magnesite-hematite veins. Middle, TS_BT1B_78Z-2_34-38, ~ 195 m, thin section composed almost entirely of parallel magnesite and/or dolomite veins, with a CPO in the carbonates. Bottom, TS_BT1B_74Z-1_59-62, ~ 183 m, serpentinite mylonite, with a strong CPO in the fine-grained matrix, and visible, internal deformation in serpentine porphyroclasts.

( 1 )

 1 Major and minor element compositions of nine samples were measured by XRF at St. Andrews University. (2) Major and minor element compositions of 74 samples, including those previously analyzed at St. Andrews, were measured via XRF (both fusion and pressed pellets) onboard the Drilling Vessel Chikyu. (3) Major element compositions of the cut face of selected core sections were analyzed onboard using an XRF core scanner. (4) Trace element compositions of a few samples were analyzed onboard via ICP-MS. (5) Trace element compositions of 61 samples were analyzed via ICP-MS at the Université de Montpellier. These data, and subsequent analyses, are tabulated and described in Kelemen et al. (2020b) and Godard et al. (

5. 2

 2 Faulting, deformation and veining after listvenite formation (U,Th)/He cooling ages (Section 4.3) indicate that the region around MoD Mountain remained above the closure temperature for He diffusion in zircon, ~ 180°C[START_REF] Reiners | Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating[END_REF], or were reheated above this temperature, during 30 to 60 million years after formation of the igneous crust in the ophiolite and peak metamorphism of the sole. Based on textural observations, it is clear that the breccias and cataclasites sampled in BT1B drill core postdate listvenite formation[START_REF] Menzel | Brittle deformation of carbonated peridotite: Insights from listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)[END_REF].

  are higher (227±52 and 247±52°C)[START_REF] Beinlich | Ultramafic rock carbonation: Constraints from listvenite core BT1B, Oman Drilling Project[END_REF]. Though a few MoD Mtn listvenites might have formed at temperatures greater than 200°C, such temperatures are too high for crystallization of intergrown amorphous carbon + hematite, they are too high for crystallization of intergrown antigorite + quartz, and they are too high for crystallization of opal. Also, carbonation of peridotite above ~150°C would form abundant talc + magnesite, whereas talc is absent in most MoD Mountain listvenites, and rare in serpentinite-listvenite contact zones that are gradational over a few meters(Falk & Kelemen 2015, Kelemen et al 2020b). The range in temperature estimates based on phase equilibrium and clumped isotope ratios from the MoD Mountain listvenites may indicate that mineral assemblages in the listvenites and surrounding serpentinites formed gradually over a range of times and temperatures. In addition, some of the clumped isotope data may record closure temperatures during cooling, rather than the peak temperature at which the MoD Mountain listvenites first crystallized, as proposed for clumped isotope data from fine-grained 10-meter scale magnesite veins in California (Garcia del Real et al 2016). Alternatively, since the highest clumped isotope temperatures from Beinlich et al. are also the most imprecise estimates, perhaps they result from analytical uncertainties or disequilibrium effects. When the 14 clumped isotope temperatures from Beinlich et al. are combined with the 31 older clumped isotope temperatures from Falk and Kelemen, the full data set yields an average of 100 ± 46°C (1σ), with the highest temperatures from Beinlich et al. falling outside the 2.5σ (99% probability) range.

  temperatures and pressures recorded by the sole do provide a window into the thermal evolution of subduction beneath the ophiolite before and during listvenite formation.Initiation of subduction, during formation of the metamorphic sole, involved thrusting of a hot mantle wedge over newly formed, hot basaltic crust. Over time, subduction of progressively older oceanic crust and -eventually -the pelagic sediments of the Hawasina Formation would have caused cooling of the subduction zone as it evolved toward a steady state geotherm. A few clumped isotope analyses on calcite in sediments beneath the ophiolite and the metamorphic sole on MoD Mountain yield temperatures of 150 to 200°C[START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]. It is possible that these were peak temperatures during diagenesis of the sediments along a late, steady state subduction geotherm at the pressure and depth recorded by the metamorphic sole in core from Hole BT1B, 800 -1200 MPa, or about 25 to 40 km.In turn, even lower temperatures for most listvenite samples may record continued, isobaric cooling of rocks flanking the subduction zone. Temperatures of 100 to 200°C at depths of 25 to 40 km are inferred for fore-arc regions above subduction zones from heat flow data (reviewed in[START_REF] Peacock | Thermal and petrologic structure of subduction zones[END_REF] predicted for steady state, oceanic subduction geotherms in numerical models (e.g.,[START_REF] Peacock | Thermal and petrologic structure of subduction zones[END_REF][START_REF] Peacock | Thermal structure of the Costa Rica -Nicaragua subduction zone[END_REF][START_REF] Syracuse | The global range of subduction zone thermal models[END_REF], including those recently modeled by Van Keken et al.

Figure 12 :

 12 Figure 12: Ternary diagram illustrating volume proportions of quartz, magnesite and dolomite, projected from hematite, calculated from whole rock compositions, for Samail mantle peridotites (average in grey triangle, data as large, grey open circles, barely visible beneath the averages and the 10 2 contour area, Godard et al. 2000;Monnier et al. 2006;Hanghoj et al. 2010}, Banded Unit peridotites near the base of the Samail ophiolite mantle section (small, green open circles, barely visible, Falk & Kelemen 2015, Khedr et al 2013, Khedr et al 2014, Takazawa et al 2003), and listvenites from Hole BT1B and MoD Mtn (open squares, Falk & Kelemen 2015, Kelemen et al 2020b), superimposed on contoured histogram of mineral proportions from shipboard XRF scanner data. Contour interval 10 ½ . Okazaki et al. (2021) provide more thorough interpretation of shipboard XRF scanner data plus X-Ray Computed Tomography data on the whole core. Average listvenite, Banded Unit and Samail mantle peridotite compositions are provided in Supplementary Table2.

  in the Hawasina Formation, because devolatilization of clay and mica bearing, clastic metasediments produces abundant, CO2-rich aqueous fluids, while limestone and marble remain relatively refractory at low to moderate temperature, subduction zone conditions (e.g.,[START_REF] Kerrick | Metamorphic devolatilization of subducted oceanic metabasalts: Implications for seismicity, arc magmatism and volatile recycling[END_REF][START_REF] Stewart | Pervasive subduction zone devolatilization recycles CO2 into the forearc[END_REF]. Indeed, there is evidence for a deeply subducted component with terrigenous isotope characteristics -like those of the Hawasina clastic sedimentary rocks -elsewhere in the Samail ophiolite. A series of felsic intrusions in the sole, mantle and lower crust along the length of the ophiolite have low, agecorrected 143 Nd/ 144 Nd (and thus, presumably, high 87 Sr/ 86 Sr(t)), attributed to melting of high-grade metasediment in the subduction zone below the ophiolite[START_REF] Amri | Genesis of granitoids by interaction between mantle peridotites and hydrothermal fluids in oceanic spreading setting in the Oman ophiolite[END_REF][START_REF] Briqueu | Sr, Nd and Pb isotopic constraints on the genesis of a calc-alkaline plutonic suite in the Oman mountains related to the obduction process[END_REF], Cox et al 1999[START_REF] Haase | Constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the Oman ophiolite[END_REF][START_REF] Haase | Melts of sediments in the mantle wedge of the Oman ophiolite[END_REF][START_REF] Lippard | The Ophiolite of Northern Oman[END_REF], Rioux et al 2021a[START_REF] Rioux | Tectonic development of the Samail ophiolite: High precision U-Pb zircon geochronology of crustal growth and ophiolite emplacement[END_REF], Rioux et al 2021b[START_REF] Rollinson | Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite[END_REF][START_REF] Spencer | Evidence for melting mud in Earth's mantle from extreme oxygen isotope signatures in zircon[END_REF].

  addition, low carbon solubilities in low temperature, low pressure aqueous fluids saturated in carbonate minerals in mineral assemblages similar to those in the Hawasina clastic sedimentary rocks (Figure 22 and associated text in Chapter 4, Falk 2014, Kelemen & Manning 2015) may be insufficient to produce the MoD Mountain listvenites (Falk & Kelemen 2015)de Obeso, 2021 #300}.

  [START_REF] Dhabi | [END_REF]

  , methods described in Section 3, and results outlined in Figures14 and 15. As the source of fluid, we chose sample OM20-17, a pelitic end-member from among the Hawasina clastic sedimentary rocks analyzed by[START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF] and deObeso et al. (2021a). As the peridotite reactant, we used an average Samail harzburgite composition calculated from published studies[START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF][START_REF] Hanghoj | Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; Major and trace element geochemistry, and Os isotope and PGE systematics[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF].Devolatilization of clastic Hawasina sediments, including OM20-17, is predicted to produce fluids with ~ 20,000 ppm dissolved C at ~ 400°C to ~ 600°C along subduction zone geotherms (Figure14).

Figure 14 :

 14 Figure 14: Calculated dissolved carbon concentrations at a water/rock ratio of 5% (a) and extent of carbon removal (b) for samples of Hawasina sediments in the MoD Mountain area (Supplementary Table2in de Obeso et al 2021a), evolving along cold (550°C at 2.5 GPa) and hot (500°C at1.5 GPa)

  MoD Mountain contain a cumulative mass of about 2 billion tons of CO2 over a strike length of 2 km NS, and 5 km EW, corresponding to 1 million to 400,000 tons of CO2 per m along strike. The allochthonous sedimentary units below the ophiolite are about 3 km thick.Within these, clastic units comprise at least half the section, and contain about 2300 ppm C, or 0.84 wt% CO2 on average (de Obeso et al 2021a), yielding a total of about 35 tons CO2 per m along strike, per m subducted. (As noted above, rocks composed mainly of calcite and/or dolomite in the subducting sedimentary section would be unlikely to contribute significant amounts of CO2 to subduction fluids at temperatures less than 800°C). At subduction velocities of 0.05 to 0.1 m/year, 90% decarbonation of the clastic units in the Hawasina Formations with a density of 2.75 tons/m 3 , at ~ 400-500°C would produce at least 1.7 to 3.5 tons of CO2 per year per m of strike length. If most of this CO2 reacted with peridotite at the depth and temperature of MoD Mtn listvenite formation, this could supply the observed mass of CO2 at MoD Mountain in less than 600,000 years.Most of the model results at different conditions predict precipitation of a few weight percent magnesite (and some dolomite) in the serpentinization domain. This is consistent with observation of carbonate veins in serpentinites from Hole BT1B, and with the hypothesis that formation of carbonate veins in the serpentinite zone preceded transformation of the serpentinite host rocks to listvenite (also see Figure6and associated text in Section 4.4, The fluid temperatures, compositions and fluxes used in these calculations are different from the constraints used in some calculations by[START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]. In that previous work, we explored the possibility that CO2 to form the listvenites was supplied over tens of millions of years, carried in 100 to 200°C fluids containing a few hundred ppm dissolved carbon, derived from pore fluids and/or dewatering of opal and clay minerals, from the Hawasina sedimentary rocks immediately below the site of listvenite formation at MoD Mountain. However, one of us (Falk!) insisted on mentioning the possibility that the CO2 to form listvenites was derived from fluids formed deeper in the subduction zone, that migrated updip. We now prefer this latter hypothesis, for the reasons outlined above.

Figure 15 :

 15 Figure 15: Results of thermodynamic reaction path models of reaction between fluids derived from devolatilization of Hawasina pelitic sedimentary rock sample OM20-17 (Section 5.6) and average Oman harzburgite. Mineral end-member abbreviations Qz quartz, Sd siderite, Mgs magnesite, Kln kaolinite, Ms muscovite, Dol dolomite, Chl chlorite, Py pyrite, Tlc talc, Srp serpentine (chrysotile), Pyr pyrrhotite. Red, vertical dashed line indicates where magnesite/quartz molar and volume proportions are ~ 2:1, as observed in magnesite listvenites from MoD Mtn (e.g., Figure 12).

  yield predicted mass and volume increases of 10 to 55 percent relative to an anhydrous, peridotite reactant, as illustrated in Figure 16. Similar results have been produced by thermodynamic models of serpentinization (volume increase of 40-60%, de Obeso & Kelemen 2018, Malvoisin 2015), experimental observation of closed system serpentinization (30-60%, Klein & Le Roux 2020) and analysis of microstructures in partially serpentinized peridotites formed in an open system (59±30 to 74±36%, Malvoisin et al 2020).

Figure 16 :

 16 Figure 16: Calculated mass and volume change relative to an anhydrous peridotite, for the reaction path models illustrated in Figure 15. Dashed, vertical red lines indicate where molar and volume proportions of magnesite/quartz reach 2:1, as onserved in MoD Mtn listvenites (e.g., Figure 12). Dashed, horizontal line highlights the minimum increase in solid volume calculated for listvenite formation.

  These modeled and observed volume changes approximate those resulting from simplified, Fe-free, stoichiometric reactions. Thus, hydration of olivine to form serpentine and brucite, and 63% increases in the solid volume, respectively. (Volume change calculated as 100% (product volume -reactant volume)/(reactant volume)). Direct carbonation of 85% and 74% increases in the solid volume, respectively. And, carbonation of serpentine plus brucite Mg3Si2O5(OH)4 + Mg(OH)2 + 4CO2 = 4MgCO3 + 2SiO2 + 3H2O, (5) and serpentine alone Mg3Si2O5(OH)4 + 3CO2 = 3MgCO3 + 2SiO2 + 2H2O, (6) both produce solid volume increases ~ 22%.

Figure 17 :

 17 Figure 17: Schematic illustration of sequential volume changes during replacement of partially serpentinized peridotite by serpentinites and then replacement of serpentinites by listvenites. The height of the rectangles corresponds to the relative volumes, produced by reaction of CO2-bearing fluid with an initial, anhydrous peridotite. The tan and brown stripes in the "listvenite" represent alternations of different listvenite compositional bands. Stars on photo of the SW side of MoD Mtn illustrate position of listvenite, serpentinite and partially serpentinized peridotite samples(Falk &

  depth, and deformation during mass transfer of H2O and CO2 from subducted sediments into overlying mantle peridotites at the leading edge of the mantle wedge. Listvenites, and a surrounding zone of serpentinite, formed at temperatures less than ~ 200°C and poorly constrained depths of 25 to 40 km. Serpentinization and carbonation involved reaction of partially serpentinized, residual mantle peridotite with CO2-rich, aqueous fluids produced by devolatilization of subducting, clastic sediments analogous to the Hawasina formation, probably at 400 to 600°C. These fluids cooled as they were transported up the subduction zone to the site of listvenite formation.Such processes could play a significant role in the global carbon cycle. Not all listvenites form above subduction zones. However, listvenites are found at and near the basal thrust in other ophiolites, worldwide[START_REF] Akbulut | The genesis of the carbonatized and silicified ultramafics known as listvenites: A case study from the Mihaliccik region (Eskisehir), NW Turkey[END_REF][START_REF] Šoštarić | The origin and age of the metamorphic sole from the Rogozna Mts., Western Vardar Belt: New evidence for the one-ocean model for the Balkan ophiolites[END_REF][START_REF] Escayola | The Point Rousse listvenites, Baie Verte, Newfoundland: Altered ultramafic rocks with potential for gold mineralization? Current Research[END_REF][START_REF] Menzel | Carbonation of mantle peridotite by CO2-rich fluids: the formation of listvenites in the Advocate ophiolite complex (Newfoundland, Canada)[END_REF][START_REF] Quesnel | Paired stable isotopes (O, C) and clumped isotope thermometry of magnesite and silica veins in the New Caledonia Peridotite Nappe[END_REF][START_REF] Quesnel | Syn-tectonic, meteoric water-derived carbonation of the New Caledonia peridotite nappe[END_REF][START_REF] Scarsi | Lawsonite-bearing eclogite from a tectonic mélange in the Ligurian Alps: New constraints for the subduction plate-interface evolution[END_REF][START_REF] Sofiya | Relict chromian spinels in Tulu Dimtu serpentinites and listvenite, Western Ethiopia: implications for the timing of listvenite formation[END_REF][START_REF] Ulrich | Dissolution-precipitation processes governing the carbonation and silicification of the serpentinite sole of the New Caledonia ophiolite[END_REF]. The leading edge of the mantle wedge -and subduction modified mantle that has later been incorporated

  al 1986[START_REF] Wilde | Preliminary study of Tertiary hydrothermal alteration and platinum deposition in the Oman ophiolite[END_REF]. He cooling ages on samples from the metamorphic sole southeast of Fanjah, near MoD Mountain, are 38.7 ± 7.7 and 44.4 ± 8.0 Ma, cooling ages of zircons from the metamorphic sole at the base of the Wadi Tayin massif to the east are 54.5 ± 7.4 and 61.8 ± 2.6 Ma, and the cooling age of zircons from the lower crust of the Samail massif is 46.4 ± 3.9 Ma (Supplementary Table

	4.3 Zircon (U,Th)/He cooling ages
	Zircon (U,Th)/

  2, our geochronological and field observations are inconsistent with this interpretation. The listvenite mineral isochron age of 97 ± 29 Ma (2σ)[START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]) yields a 93% chance (1.5σ) that the listvenites formed before 75 Ma, while subduction beneath the ophiolite was still active, and a 97% chance that the listvenites formed before 68 Ma. Thus, the isochron data support listvenite formation during subduction, and show that the calcite veins cutting listvenite, dated by Scharf et al., postdate listvenite formation.

  [START_REF] Connolly | The geodynamic equation of state: what and how[END_REF]) and mineral properties fromHolland and Powell.

	In contrast, calculations using the Deep Earth Water (DEW) model (Huang & Sverjensky 2019,
	Sverjensky et al 2014) and the extended Helgeson-Kirkham-Flowers (HKF) aqueous equation of state
	(Shock et al 1992, Shock et al 1997) do not predict equilibrium coexistence of antigorite + quartz
	above 15°C. Since antigorite-quartz intergrowths indicate coeval crystallization of both phases, in
	rocks that formed at > 80°C, we presume that these latter results stem from incorrect extrapolation of
	thermodynamic data to low temperature.
		-30
		-35				hm
								cco
		-40						Figure 11: Diagram illustrating
								temperature and oxygen fugacity for
	2 ) log (fO	-50 -45						the well-known oxygen buffers, hematite-magnetite (hm, 6Fe2O3 = 4Fe3O4 + O2) and graphite-carbon dioxide (cco, C + O2 = CO2). Graphite and amorphous carbon
			cco					compounds are stable below the cco curves, while hematite is stable
		-55						above the hm curve. hm curves are solid red (1 GPa) and black (0.2
								GPa). cc curves are solid green (1
		-60		hm				GPa) and blue (0.2 GPa). Dashed blue curves are for cco at 0.2 GPa
								total pressure, with P(CO2) = 0.1,
		-65						0.01, 0.001 and 0.0001 GPa from left to right. Dotted blue curves are
		0	50	100	150	200	250	300 for cco at 0.2 GPa total pressure = 350 400
					temperature (°C)	P(CO2), with a reduced activity of graphite (i.e., amorphous carbon
								compounds) 0.01 and 0.1 from left
								to right.

Leaving the DEW results aside, the other temperature estimates based on co-existing antigorite + quartz are broadly consistent with temperature estimates based on δ 18 O in quartz and carbonate minerals in the listvenite, assuming that fluid δ 18 O was similar to seawater, and with clumped isotope analyses of magnesite and dolomite yielding temperatures of 80 to 130°C for listvenite samples

[START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]

. Observation of quartz spherulites in listvenite, possibly reflecting devitrification of opal, also suggests that listvenite formation occurred in this "low temperature" regime, at ~ 150°C or less, in the presence of aqueous fluid.

mineralization processes, as discussed in more detail by [START_REF] Menzel | Ductile deformation during carbonation of serpentinized peridotite[END_REF]. Such a process could be active in subduction zones where the interface between subducting oceanic crust, sediments, and hanging wall peridotites is aseismic.

Author contributions

Kelemen and de Obeso conducted field mapping at MoD Mountain. de Obeso made new Sr and C isotopic analyses of listvenites, the metamorphic sole and the underlying sedimentary rocks, updating work by [START_REF] Falk | Geochemistry and petrology of listvenite in the Oman Ophiolite: Complete carbonation of peridotite during ophiolite emplacement[END_REF]. Stockli carried out the reconnaissance (U,Th)/He analyses of zircons from the metamorphic sole and lower curst, using zircon separates samples provided by Rioux.

Godard expertly led the BT1B geochemistry team onboard Drilling Vessel Chikyu, and then heroically continued with analyses at the Université de Montpellier on behalf of the Shipboard Scientific Party.

Okazaki labored with Kelemen on the triangular histogram of shipboard XRF scanner data. Leong and de Obeso conducted the EQ3/6 thermodynamic modeling, in consultation with Kelemen. Working with his computer in 46°C weather on the drill site, Manning provided key insights into conditions for coexisting hematite and organic carbon compounds. Ellison supervised Kelemen's Raman spectroscopy analyses at the University of Colorado, Boulder, and offered essential advice and data interpretation. Kotowski led analysis of core from the metamorphic sole and shared her results. Urai Suggestions from Othmar Müntener, an anonymous reviewer, and Associate Editor John Lassiter led to substantial improvements of this paper. 1 and2, and Supplementary Tables 1 and2 constitute data that are original with this paper. When this manuscript is accepted, the tabulated data will uploaded to the Zenodo repository (https://zenodo.org).

Data Availability: Samples in Supplementary