Benjamin Malvoisin
email: benjamin.malvoisin@unil.ch.

Håkon Austrheim

György Hetényi

Julien Reynes

Jörg Hermann

Lukas P Baumgartner

Yury Y Podladchikov

Sustainable densification of the deep crust

The densification of the lower crust in collision and subduction zones plays a key role in shaping the Earth by modifying the buoyancy forces acting at convergent boundaries. It takes place through mineralogical reactions, which are kinetically favored by the presence of fluids.

Earthquakes may generate faults serving as fluid pathways but the influence of reactions on the generation of seismicity at depth is still poorly constrained. Here we present new petrological data and numerical models to show that, in the presence of fluids, densification reactions can occur very fast, in the order of weeks, and consume fluids injected during an earthquake, which leads to porosity formation and fluid pressure drop by several hundreds of megapascals. This generates a mechanically highly unstable system subject to collapse and further seismic wave emission during aftershocks. This mechanism creates new pathways for subsequently arriving fluids, and thus provides a route for self-sustained densification of the lower crust.

INTRODUCTION

Mineralogical reactions modify the density and the rheology of the entire lithosphere with important implications for geodynamics in collision and subduction zones [START_REF] Austrheim | Eclogitization of lower crustal granulites by fluid migration through shear zones[END_REF][START_REF] Hetényi | Density distribution of the India plate beneath the Tibetan plateau: Geophysical and Version de l'auteur[END_REF]. The presence of fluids at grain boundaries increases element transport and reaction rates by several orders of magnitude [START_REF] Rubie | The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism[END_REF][START_REF] Caine | Oxygen diffusion in a fine-grained quartz aggregate with wetted and nonwetted microstructures[END_REF].

Permeability estimates of the dry crust (10 -20 -10 -16 m 2 ; [START_REF] Ingebritsen | Modeling the Formation of Porphyry-Copper Ores: Science[END_REF] preclude pervasive flow as a fluid transport mechanism, thus requiring the formation of preferential fluid pathways for metamorphic reactions to proceed [START_REF] Plümper | Fluid Version de l'auteur[END_REF][START_REF] Omlin | Pore Fluid Extraction by Reactive Solitary Waves in 3-D[END_REF]. Seismic events generate a high-permeability damage zone able to transport fluids and induce reactions in the seismogenic zone [START_REF] Caine | Oxygen diffusion in a fine-grained quartz aggregate with wetted and nonwetted microstructures[END_REF][START_REF] Reches | Gouge formation by dynamic pulverization during earthquake rupture[END_REF]; typically 0-30 km depth on continents). Earthquakes are also recorded at deep-crustal and intermediate-depth levels in collision and subduction zones [START_REF] Frohlich | Deep Earthquakes[END_REF][START_REF] Halpaap | Earthquakes track subduction fluids from slab source to mantle wedge sink[END_REF]. Thermodynamic calculations predict metamorphic reactions at pressure-temperature conditions consistent with the locations where these earthquakes are recorded [START_REF] Hacker | Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions[END_REF][START_REF] Hetényi | Density distribution of the India plate beneath the Tibetan plateau: Geophysical and Version de l'auteur[END_REF][START_REF] Nakajima | Intermediate-depth earthquakes facilitated by eclogitization-related stresses[END_REF], and some laboratory experiments of deformation lead to acoustic emissions due to fracturing during densification reactions [START_REF] Shi | Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite[END_REF][START_REF] Incel | Reaction-induced embrittlement of the lower continental crust[END_REF].

There is thus a feedback loop between earthquakes and metamorphic reactions at convergent margins but the underlying mineralogical and mechanical processes of this loop are still enigmatic.

Rocks exposed in the Bergen Arcs (Norway) provide detailed insights into this problem. Their mineralogical assemblages experienced Grenvillian granulite to eclogite metamorphic facies transformation in the root of the Caledonian orogeny [START_REF] Austrheim | Eclogitization of lower crustal granulites by fluid migration through shear zones[END_REF] at 2.0-2.2 GPa and 670-(pseudotachylite) during seismic slip [START_REF] Austrheim | Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust[END_REF]. Previous studies reported coseismic damage in the vicinity of pseudotachylites [START_REF] Austrheim | Garnets recording deep crustal earthquakes[END_REF][START_REF] Austrheim | Fragmentation of wall rock garnets during deep crustal earthquakes[END_REF][START_REF] Putnis | Metamorphic processes and seismicity: The Bergen Arcs as a natural laboratory[END_REF][START_REF] Petley-Ragan | Microstructural Records of Earthquakes in the Lower Crust and Associated Fluid-Driven Metamorphism in Plagioclase-Rich Granulites[END_REF]. The eclogite facies minerals form in and along the pseudotachylites, suggesting that the reaction occurs immediately after the earthquake [START_REF] Pollok | Complex replacement patterns in garnets from Bergen Arcs eclogites: A combined EBSD and analytical TEM study[END_REF][START_REF] Austrheim | Fragmentation of wall rock garnets during deep crustal earthquakes[END_REF]. Failure may rely on stress-pulses linked to earthquakes in the seismogenic zone [START_REF] Jamtveit | Earthquake-induced transformation of the lower crust[END_REF], or on weakening by shear heating [START_REF] Braeck | Spontaneous thermal runaway as an ultimate failure mechanism of materials[END_REF]. However, reactions can also be the cause for the onset of deformation rather than a response as suggested by cracking due to density change associated with reaction [START_REF] Kirby | Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF]. Here we acquire constraints on the interplay between reaction and deformation in the orogenic roots to predict the couplings between lower crust's densification, fluid flow at depth and the related seismicity.

REACTION IN A CLOSED SYSTEM

A granulite from Holsnøy Island (Bergen Arcs; 60°35.561'N, 5°4.374'E) is locally eclogitized and cross-cut by a completely recrystallized, ~ 5 mm wide pseudotachylite (Figure 1). The wall rock to the pseudotachylite contains two different generations of garnet.

Garnets G1 are up to 5 mm wide with compositions in major elements (XPrp = 0.50; XGrs = 0.17) similar to the single generation of garnets found in the unreacted granulite at ~1 m away from the pseudotachylite, suggesting crystallization in granulite facies conditions. They are fragmented by micro-cracks parallel to the pseudotachylite and aligned inclusions of eclogitic minerals (Figure 1B), indicating coseismic deformation as already proposed by [START_REF] Austrheim | Fragmentation of wall rock garnets during deep crustal earthquakes[END_REF]. The dominant phase in the matrix is fine-grained plagioclase with a jigsaw puzzle texture interpreted as a result of cataclasis during slip along the pseudotachylite [START_REF] Putnis | Metamorphic processes and seismicity: The Bergen Arcs as a natural laboratory[END_REF][START_REF] Petley-Ragan | Microstructural Records of Earthquakes in the Lower Crust and Associated Fluid-Driven Metamorphism in Plagioclase-Rich Granulites[END_REF]. Garnets G2 are < 30 µm wide, and Ca-richer and Mg-poorer (XPrp = 0.30; XGrs = 0.38) than garnets G1. They are part of the eclogitic assemblage consisting of omphacitic pyroxene (XJad = 0.27-0.39), kyanite, quartz and carbonate in patches surrounding garnets G1, and zoisite in the plagioclase-rich matrix. The eclogitic patches are surrounded by a symplectic rim composed of an intergrowth of plagioclase and diopsidic pyroxene (Figure 1A).

To constrain the fluid input phase, we measured the O-H absorption of nominally anhydrous minerals with Fourier transform infrared spectroscopy (see Supplemental information).

Concentrations are given as H2O with uncertainties corresponding to the standard deviation estimated on n measurements for garnet and to the propagation of the uncertainty on the absorption coefficient of [START_REF] Katayama | Water content in natural eclogite and implication for water transport into the deep upper mantle[END_REF] for clinopyroxene. At less than 1 cm from the pseudotachylite, clinopyroxene contains 210 ± 37 µg/g H2O (n = 34) and garnet G1 contains 178 ± 32 µg/g H2O (n = 25) (sample BA7). Garnets G2 are too small to be analyzed. Garnets G1 are composed of different types of domains: i) inclusion-rich domains with bulk H2O contents above 300 µg/g probably formed by fragmentation followed by local recrystallization and fluid inclusions trapping as described in [START_REF] Austrheim | Fragmentation of wall rock garnets during deep crustal earthquakes[END_REF]Supplementary Figure S1), indicating the presence of a free aqueous fluid; ii) inclusion-free domains containing 150 µg/g H2O in their core and less than 50 µg/g at their rim (Figure 2A andB). These latter H2O concentrations are higher than the minimum concentration of 7 µg/g H2O measured in garnets found in a fresh granulite at ~ 50 cm from the pseudotachylite (sample BA3), and having major element composition similar to garnets G1 (Supplemental table S1). This suggests H2O incorporation by diffusion but the zoning is opposite to the one expected for diffusion from micro-cracks into garnet G1. H uptake and loss by diffusion in garnets G1 is likely governed by a proton-polaron reaction [START_REF] Reynes | Experimental constraints on hydrogen diffusion in garnet[END_REF], whereby charge balance is maintained by conversion of Fe 3+ to Fe 2+ during RedOx reactions in the matrix. For the metamorphic temperature of 750°C, experimentally determined diffusivities for this type of defect range from D = 10 -15 -10 -13 m 2 .s -1 [START_REF] Blanchard | Hydrogen diffusion in Dora Maira pyrope[END_REF][START_REF] Kurka | Kinetics of hydrogen extraction and deuteration in grossular[END_REF][START_REF] Reynes | Experimental constraints on hydrogen diffusion in garnet[END_REF]. For such diffusivities, the timescale for diffusion and reaction are similar for eclogitization conditions. We therefore modelled water zoning in garnet with a 2D numerical model considering both H2O diffusion in garnet and hydration reaction in the plagioclase matrix (see Supplemental Materials).

The model is able to reproduce the measured H2O zoning in garnet if we consider the system is closed after fluid injection (Figure 2 and Supplementary Figure S3). The free water initially present in the matrix both reacts to form hydrous minerals and diffuses into garnet up to the point where the consumption of H2O by the hydration of the matrix leads to a lowering of aH2O. Garnet then releases its stored H leading to a reversal of the H2O diffusion profile (Figure 2C to E). This produces a higher amount of reaction products around the garnet (Supplementary Figure S4), similar to the eclogitization patches observed in the natural samples. This reversal of the diffusion profile occurs for the whole range of diffusivities measured experimentally (10 -15 m 2 .s -1 < D < 10 -13 m 2 .s -1 ) and for reaction rates up to two orders of magnitude smaller than the one measured experimentally for plagioclase hydration [START_REF] Schramke | The reaction muscovite + quartz andalusite + K-feldspar + water; Part 1, Growth kinetics and mechanism: American Journal Version de l'auteur[END_REF]; Supplementary Figure S2).

We found in the natural sample, at the transition from eclogite to symplectite (Figure 1B), ~ 10 µm wide amphibole inclusions in omphacitic pyroxene (Supplementary Figure S5). Chlorine and potassium increase from 0 to ~ 3 wt.% and 0 to ~ 4 wt.% (Supplementary Figure S5), respectively, from the clinopyroxene core to its rim in contact with the symplectite. The uptakes of H2O by hydration reaction and CO2 by carbonate formation lead to an increase in soluble element activity in the fluid inducing the progressive enrichment in Cl and K in the reaction Version de l'auteur. Publié par Geology, 48, 673-677: 10.1130/G47201.1 6 products [START_REF] Kullerud | Chlorine-rich amphiboles: Interplay between amphibole composition and an evolving fluid[END_REF]. This desiccation process also requires reaction in a closed system after a fluid pulse. Such a desiccation process was already reported in the context of salt precipitation during lower crust hydration [START_REF] Markl | Composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks[END_REF].

REACTION-INDUCED WEAKENING

We study the impact of fluid consumption by reaction on the evolution of fluid pressure and porosity with a model coupling reaction, fluid flow and viscoelastic deformation [START_REF] Malvoisin | Coupling changes in densities and porosity to fluid pressure variations in reactive porous fluid flow: Local thermodynamic equilibrium[END_REF][START_REF] Omlin | Pore Fluid Extraction by Reactive Solitary Waves in 3-D[END_REF]. The 1 x 1 m-sized model starts with a granulite reacting in eclogite facies conditions towards thermodynamic equilibrium at 750°C. The timescale of reaction depends on a first-order kinetic law incorporating the effect of the distance from the equilibrium and the dependency of reactive surface area on porosity. The initial fluid pressure is 2.0 GPa but it evolves as a result of eclogitization and viscous compaction. The above scenario of earthquake-induced fluid pulse followed by reaction translates here as an initial high-porosity zone in the center of the model representing the damage zone surrounding the pseudotachylite (up to 0.5 % of porosity) and no fluid flux at the boundaries (Supplementary Movie S1). The results discussed in the following are obtained with r0 = 5.10 -6 s -1 , a permeability of k = 10 -19 m 2 and a viscosity of ηϕ = 10 17 Pa.s [START_REF] Schramke | The reaction muscovite + quartz andalusite + K-feldspar + water; Part 1, Growth kinetics and mechanism: American Journal Version de l'auteur[END_REF][START_REF] Bürgmann | Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics[END_REF][START_REF] Ingebritsen | Modeling the Formation of Porphyry-Copper Ores: Science[END_REF].

The formation of a ~ 18 % denser assemblage leads to a fluid pressure decrease from 2.0 to 1.5 GPa in the numerical model. In the meantime, porosity increases by one order of magnitude (Figure 3A andB) and the new mineralogical assemblage evolves from eclogite to amphibolite facies in the center of the model (Figure 3C). Such a sequence of mineralogical assemblages reproduces the observation in the eclogitic patches at Holsnøy, with eclogitic garnets in the core of the patches and amphibole further out, followed by plagioclase-bearing symplectite at the rim (Supplementary Figure S5). At larger scale, newly formed Ca-rich plagioclase preferentially occurs in the pseudotachylite and at its rim (Supplementary Figure S6). Thermodynamic calculations indicate that Ca content in plagioclase increases when pressure decreases (Supplementary Figure S6B). This suggests a larger fluid pressure decrease in the highly permeable zones near the pseudotachylite as predicted by the numerical model (Figure 3A).

After one week, the model indicates a decrease in porosity to its initial value due to viscous deformation favored by high effective pressure and porosity [START_REF] Yarushina | De)compaction of porous viscoelastoplastic media: Model formulation[END_REF]. This estimate of reaction duration is consistent with the results of [START_REF] Taetz | Fast intraslab fluid-flow events linked to pulses of high pore fluid pressure at the subducted plate interface[END_REF] even though it strongly depends on r0 which is poorly constrained.

A sensitivity analysis reveals that k has no effect on the results due to the no fluid-flux boundary condition and that the model results discussed above are valid for r0 > 2.10 -6 s -1 at ηϕ = 10 17 Pa.s and r0 > 2.10 -6 s -1 at ηϕ = 10 18 Pa.s (Supplementary Figure S9). Low ηϕ values favor limited fluid pressure and porosity variations if r0 is at least three times smaller than the experimental estimates from [START_REF] Schramke | The reaction muscovite + quartz andalusite + K-feldspar + water; Part 1, Growth kinetics and mechanism: American Journal Version de l'auteur[END_REF]. Together with the uncertainty on the kinetic data, a low fluid availability is the main candidate for a lowering of r0. However, the observations of hydrous mineral precipitation and water diffusion demonstrate the presence of an aqueous fluid during eclogitization. Moreover, a low fluid availability implies a higher viscosity (10 19 Pa.s for dry plagioclase at 700°C; [START_REF] Bürgmann | Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics[END_REF] for which r0 has to be more than 100 times smaller than the experimental estimates to produce limited fluid pressure and porosity variations. Alternatively, plastic and brittle deformations, not modelled here, may contribute to compaction.

In nature, the loading is not isotropic as assumed here. Under differential stresses, compaction is associated with strain localization [START_REF] Wong | The brittle-ductile transition in porous rock: A review[END_REF]. At high pressure, the connection of Version de l'auteur. Publié par Geology, 48, 673-677: 10.1130/G47201.1 8 densification sites (anticracks) forms shear instabilities (transformational faulting, [START_REF] Kirby | Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere[END_REF] which have been observed in experiments [START_REF] Incel | Reaction-induced embrittlement of the lower continental crust[END_REF] and in natural settings [START_REF] Nakajima | Intermediate-depth earthquakes facilitated by eclogitization-related stresses[END_REF]. The pore collapse modelled here is likely to generate other earthquakes (aftershocks) possibly connecting fluid reservoirs to not yet reacted granulites and preparing pathways for subsequent fluid injection phases. This mechanism is probably not the only one responsible for the eclogitisation of ~ 40 % of the Holsnøy Island [START_REF] Austrheim | Garnets recording deep crustal earthquakes[END_REF].

Nevertheless, it self-sustains and continuously induces fluid-assisted seismicity as long as there is a fluid source available.

A current analogy to lower-crustal metamorphic earthquakes could be the cluster of events beneath southernmost Tibet [START_REF] Monsalve | Seismicity and one-dimensional velocity structure of the Himalayan collision zone: Earthquakes in the crust and upper mantle[END_REF] where eclogitization occurs [START_REF] Hetényi | Density distribution of the India plate beneath the Tibetan plateau: Geophysical and Version de l'auteur[END_REF]. The range of magnitudes spans from 1 to 4, and the largest event is not purely doublecouple but includes an isotropic component [START_REF] Alvizuri | Source mechanism of a lower crust earthquake beneath the Himalayas and its possible relation to metamorphism: Tectonophysics, v. 769[END_REF]. On Holsnøy Island, pseudotachylites are dispersed along a roughly linear structure extending over 5 km, with a thickness of at least 150 m. Faulting-related displacement on individual ruptures can locally reach 1.7 m [START_REF] Jamtveit | Earthquake-induced transformation of the lower crust[END_REF] but is generally smaller, around 0.1 m on average. If rupture occurred simultaneously along this structure (5 km long, 150 m wide, 10 cm average slip and assuming a shear modulus of 50 GPa), it would yield a Mw 4.3 earthquake, comparable to the largest Himalayan event. This is a maximum estimate for Holsnøy since the pseudotachylites are not aligned along a single fault plane [START_REF] Austrheim | Fluid and deformation induced metamorphic processes around Moho beneath continent collision zones: Examples from the exposed root zone of the Caledonian mountain belt[END_REF], nevertheless much smaller events would also compare to the Himalayan cluster with detections down to magnitude 1.

Alternatively, rupture on a non-planar fault could have also occurred in Holsnøy.

CONCLUSION

Version de l'auteur. Publié par Geology, 48, 673-677: 10.1130/G47201.1 9 We show here that earthquakes may lead to a fluid pulse followed by hydrothermal reactions (Figure 4A andB) similar to seismic pumping occurring at shallow depth [START_REF] Sibson | Seismic pumping-a hydrothermal fluid transport mechanism[END_REF].

Reactions subsequent to seismicity progressively consume the fluid in a closed system (desiccation), leading to porosity generation and fluid pressure decrease by up to several hundreds of megapascals (Figure 4C). This significantly weakens the lower crust mechanically, leading to aftershocks that generate new fracture networks by a process similar to transformational faulting (Figure 4D). These networks may be isolated and the reactions shut down as in the sample studied here. Alternatively, they may be connected to a (subsequent) fluid source allowing for the reaction to continue. The reactions lead to extremely short residence time for free fluids in the deep crust, which is otherwise non-reactive over the majority of geological time (Figure 4E). Porosity. C: Mineral modes. In A and B, the evolution with time of the parameters is displayed at four points aligned along the initial Gaussian distribution in porosity: in the center of the model (red), at 8 cm from the center (blue), at 15 cm from the center (green) and at 25 cm from the center (black). These points are also displayed in Supplementary Movie S1.

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS

Figure 2 .

 2 Figure 2. H2O distribution in granulitic garnets found in the vicinity of the pseudotachylite. A

Figure 3 .

 3 Figure 3. Numerical modelling of the couplings between reaction, deformation and fluid flow

Figure 4 .Figure 1 Figure 2

 412 Figure 4. Sequence of processes associated with reaction-induced aftershocks in the lower crust.

ACKNOWLEDGMENTS

We thank K. Chanard and C. Alvizuri for help during sampling in the field. L. Nicod is thanked for preparing thick sections for Fourier Transform Infrared analyses. M. Robyr is thanked for help with the acquisition of X-ray maps with the microprobe. B.M., G.H. and J.H. acknowledge support from the Swiss National Science Foundation (Grant No. PZ00P2_168083, PP00P2_157627, 200021_169062 and 206021_163995). H.A. acknowledges support from the Norwegian Research Council (SWaMMIS project, Grant no. 231354). Insightful reviews by three anonymous reviewers and editorial handling by Dennis Brown significantly improved the manuscript.


Benjamin Malvoisin
email: benjamin.malvoisin@unil.ch

Chang Zhang

Othmar Müntener

Lukas P Baumgartner

Peter B Kelemen

Oman Drilling

Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project

whether they are published or not. The documents may come

Introduction

Ultramafic rocks react with water at slow spreading ridges, at ocean-continent transitions, in subduction zones and in ophiolites. This process of "serpentinisation" raises the interest of the scientific community not only because it is ubiquitous but also because it has a first-order impact in geodynamics and in biology [START_REF] Kelley | An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 N. Nature[END_REF][START_REF] Sleep | H2-rich fluids from serpentinization: Geochemical and biotic implications[END_REF][START_REF] Delescluse | Serpentinization pulse in the actively deforming Central Indian Basin[END_REF][START_REF] Wada | Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization[END_REF][START_REF] Müntener | Serpentine and serpentinization: A link between planet formation and life[END_REF][START_REF] Russell | Serpentinization as a source of energy at the origin of life[END_REF][START_REF] Reynard | Serpentine in active subduction zones[END_REF] since it produces serpentine ((Mg, F e) 3 Si 2 O 5 (OH) 4 ), brucite ((Mg, F e) OH 2 ), magnetite (F e 3 O 4 ) and hydrogen (H 2 ) with physico-chemical properties (i.e. composition, density, magnetism, rheology and energetic potential) strongly different from the primary olivine and pyroxene [START_REF] Moody | Serpentinization: a review[END_REF]Escartín et al., 2001;[START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF]McCollom & Bach, 2009;Malvoisin, Carlut, & Brunet, 2012). [START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15N on the Mid-Atlantic Ridge[END_REF][START_REF] Klein | Compositional controls on hydrogen generation during serpentinization of ultramafic rocks[END_REF] developed thermodynamic models predicting the abundance and composition of the products of serpentinisation as a function of temperature and reactant compositions. They predict that magnetite formation mainly occurs above 200 • C whereas Fe-brucite and Fe-serpentine are the dominant iron-bearing phase below this temperature [START_REF] Klein | Magnetite in seafloor serpentiniteSome like it hot[END_REF]. Experiments on powders provide serpentinisation rates that are dependent on temperature, pressure, grain size and protolith composition [START_REF] Martin | Some experimental and theoretical observations on the kinetics of hydration reactions with particular reference to serpentinization[END_REF][START_REF] Wegner | Experimentally determined hydration and dehydration reaction rates in the system MgO-SiO2-H2O[END_REF]Malvoisin, Brunet, et al., 2012;[START_REF] Andreani | Aluminum speeds up the hydrothermal alteration of olivine[END_REF]. However, these constraints are not sufficient to quantitatively model serpentinisation in natural systems. Experiments on rock aggregates indeed revealed orders of magnitude slower kinetics compared to rates measured on powders [START_REF] Malvoisin | Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization[END_REF][START_REF] Klein | Experimental constraints on fluidrock reactions during incipient serpentinization of harzburgite[END_REF]. Water transport thus plays a key role on reaction rate and local thermodynamic equilibrium. Some peridotites interacting with near-surface fluids have undergone rapid cooling. This generated fluid pathways through thermal cracking (Demartin et al., 2004;[START_REF] Boudier | Does Anisotropy of Thermal Contraction Control Hydrothermal Circulation at the Moho Level below Fast Spreading Oceanic Ridges?[END_REF] and tectonic deformation [START_REF] Rouméjon | Serpentinization of mantle-derived peridotites at mid-ocean ridges: Mesh texture development in the context of tectonic exhumation[END_REF]. Understanding the evolution of these pathways and the generation of new pathways during serpentinisation requires understanding of the couplings between reaction and deformation. Serpentine and brucite have a lower density than olivine and pyroxene. Serpentinisation in a closed system is thus expected to induce an increase in solid volume ( ΔV V ) of approximately 50 % [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF][START_REF] O'hanley | Solution to the volume problem in serpentinization[END_REF]. This change in volume has two opposite consequences for fluid pathway evolution during serpentinisation. It leads to both decrease and increase in permeability through porosity filling by reaction products [START_REF] Farough | Evolution of fracture permeability of ultramafic rocks undergoing serpentinization at hydrothermal conditions : An experimental study[END_REF][START_REF] Godard | Incipient hydration of mantle lithosphere at ridges: A reactive-percolation experiment[END_REF] and stress build-up leading to cracking [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF][START_REF] O'hanley | Solution to the volume problem in serpentinization[END_REF]B. W. Evans, 2004;[START_REF] Jamtveit | Reaction induced fracturing during replacement processes[END_REF], respectively. This latter process of reaction-induced cracking is constrained with observations in natural systems [START_REF] Coleman | A chemical study of serpentinization-burro mountain, California[END_REF][START_REF] Loney | Structure and petrology of the alpine-type peridotite at burro mountain, California, U.S.A[END_REF][START_REF] Plümper | The interface-scale mechanism of reaction-induced fracturing during serpentinization[END_REF][START_REF] Rouméjon | Serpentinization of mantle-derived peridotites at mid-ocean ridges: Mesh texture development in the context of tectonic exhumation[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF], thermodynamic theory [START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF], thermodynamic models of water rock reaction in natural and simplified systems [START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15N on the Mid-Atlantic Ridge[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]de Obeso et al., 2017), physical and numerical models [START_REF] Rudge | A simple model of reaction-induced cracking applied to serpentinization and carbonation of peridotite[END_REF][START_REF] Ulven | Reaction-driven fracturing of porous rock[END_REF][START_REF] Shimizu | The roles of fluid transport and surface reaction in reaction-induced fracturing, with implications for the development of mesh textures in serpentinites[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF]O. Evans et al., 2018, n.d.) and experiments [START_REF] Zhu | Experimental evidence of reaction-induced fracturing during olivine carbonation[END_REF][START_REF] Zheng | Effects of Confinement on Reaction-Induced Fracturing During Hydration of Periclase[END_REF][START_REF] Lafay | Intracrystalline Reaction-Induced Cracking in Olivine Evidenced by Hydration and Carbonation Experiments[END_REF]. Despite the key role of solid volume change during serpentinisation, this parameter is not well constrained since it has not been measured in natural samples yet and it is closely related to mass transfer during reaction.

The transport of aqueous species during reaction can modify solid volume change [START_REF] Thayer | Serpentinization considered as a constant-volume metasomatic process[END_REF][START_REF] Gresens | Composition-volume relationships of metasomatism[END_REF][START_REF] Carmichael | Induced Stress and Secondary Mass Transfer: Thermodynamic Basis for the Tendency toward Constant-Volume Constraint in Diffusion Metasomatism[END_REF][START_REF] Fletcher | Mineral growth in rocks: Kineticrheological models of replacement, vein formation, and syntectonic crystallization[END_REF][START_REF] Putnis | Fluid-induced processes: metasomatism and metamorphism[END_REF]. For example, pseudomorphic replacement and microtexture preservation are interpreted as evidence for isochoric replacement thanks to mass transfer [START_REF] Merino | Implications of replacement for reactiontransport modeling[END_REF][START_REF] Centrella | Coupled mass transfer through a fluid phase and volume preservation during the hydration of granulite: An example from the Bergen Arcs, Norway[END_REF]. Some serpentinised peridotites have a different composition than mantle rocks. In addition to water incorporation, the main change in major elements measured in abyssal serpentinised peridotites consists in a decrease of the MgO/SiO 2 ratio by 5 % on average [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]de Obeso & Kelemen, 2018). This decrease can be explained by Si gain or Mg loss during reaction [START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF][START_REF] Jöns | Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography[END_REF]de Obeso & Kelemen, 2018). Enrichment in fluorine, boron and chlorine are also measured in abyssal peridotites [START_REF] Thompson | Boron contents of serpentinites and metabasalts in the oceanic crust: Implications for the boron cycle in the oceans[END_REF][START_REF] Bonatti | Serpentinization of oceanic peridotites: temperature dependence of mineralogy and boron content[END_REF][START_REF] Sharp | Water-soluble chlorides in massive seafloor serpentinites: A source of chloride in subduction zones[END_REF][START_REF] Vils | The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere[END_REF][START_REF] Bonifacie | Chlorine isotopic composition in seafloor serpentinites and high-pressure metaperidotites. Insights into oceanic serpentinization and subduction processes[END_REF][START_REF] Debret | F, Cl and S input via serpentinite in subduction zones: Implications for the nature of the fluid released at depth[END_REF][START_REF] Anselmi | Chlorine in the Elba, Monti Livornesi and Murlo serpentines: evidence for sea-water interaction[END_REF]. Previous studies on trace elements in abyssal serpentinised peridotites are limited to the hand-specimen scale without in-situ investigations. The bulk geochemistry shows patterns in rare earth elements (REE) highly related to those of their protoliths. Al, V, Sc and Cr are considered as 'immobile' during hydrothermal alteration [START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 1520'N, ODP Leg 209): Implications for fluid/rock interaction in slow spreading environments[END_REF]. The high field strength elements (HFSEs) show low solubility and mobility in aqueous fluids during hydrothermal alteration relative to REEs. It was thus proposed to use REE and compatible elements to identify the protoliths of serpentinites and to recover the processes experienced by these protoliths (Deschamps et al., 2013). In contrast, U, Pb and Sr contents are characterized by strong enrichments associated with alteration on the seafloor, and in particular with the precipitation of carbonates [START_REF] Seifert | Geochemistry of serpentinized mantle peridotite from Site 897 in the Iberia Abyssal Plain[END_REF][START_REF] Co Olivier | Rare earth and trace elements of microbialites in Upper Jurassic coral-and sponge-microbialite reefs[END_REF].

These changes in composition can influence the geochemical budgets at large scale [START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 1520'N, ODP Leg 209): Implications for fluid/rock interaction in slow spreading environments[END_REF]. However, it is difficult to determine the respective contribution of serpentinisation and alteration on the seafloor to mass transfer and porosity evolution in these samples.

A way to focus on the mineralogical processes associated with serpentinisation is to work on samples collected by drilling. [START_REF] Tutolo | Nanoscale constraints on porosity generation and fluid flow during serpentinization[END_REF] measured several percent of porosity in serpentinised peridotites drilled in the Atlantis massif. The observed pore diameters are orders of magnitude smaller than the pores formed during alteration on the seafloor (10 μm (Jöns et al., 2017)). Nanoscale porosity probably plays a key role for fluid transport during replacement reactions but its mechanism of formation remains poorly understood due to a lack of constraints on volume change and mass transfer during reaction.

We measure here solid volume change associated with serpentinisation in a sample drilled during the International Continental Scientific Drilling Program (ICDP)

supported Oman Drilling Project. We couple these results with measurements of major and trace element composition, thermodynamic modelling, and nanotomography.

The combination of these tools provides new constraints for determining the mineralogical processes at play during serpentinisation.

Sampling and methods

We study a core sample from Oman Drilling Project Hole BA4A (sample BA4A-81-1-1-17), drilled into serpentinised peridotites of the Wadi Tayi massif of the Samail Ophiolite, the largest sub-aerial exposure of oceanic lithosphere on Earth. The Samail Ophiolite was formed by igneous accretion at a fast-spreading ridge and then obducted onto the Arabian Continental margin [START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF]. Serpentinisation may have occurred throughout the history of the ophiolite and is still ongoing [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Kelemen | In situ carbonation of peridotite for CO2 storage[END_REF]. The extent of hydration (serpentinisation) of core from Hole BA4A well is generally more than 95 %, with only minor relicts of primary Serpentine and brucite minerals were identified by Raman microspectrometry at the University of Lausanne with a Horiba LabRAM HR800 equipped with a 532.1 nm laser.

X-ray microtomography

A 8 mm and 23 mm high core is scanned with a Skyscan-1173 at the University of Lausanne. 2200 X-ray images are acquired with a 360 • rotation of the samples at a step size of 0.225 • with a time of acquisition of 800 ms per frame. The acquisition is performed at 70 kV and 114 μA by accumulating 33 frames with a voxel size of 11.07

μm. The 3D images reconstructed from the transmission images are segmented and analysed with Blob3D [START_REF] Ketcham | Computational methods for quantitative analysis of threedimensional features in geological specimens[END_REF]. We develop a Matlab code to display the result of the image analysis and to extract the extent of reaction from the segmented 3D image. The extent of reaction is calculated based on the measured volumes of olivine, spinel and brucite/serpentine in a 350 μm wide sliding window parallel to the orientation of the vein cross-cutting the sample. The similar attenuation of olivine and Fe-brucite (identified with Scanning Electron Microscopy) makes segmentation difficult. These phases can be distinguished based on their aspect ratio (e) calculated as the length ratio of the minimum and maximum axes of an ellipsoid fitted to each segmented grain (Figure S1 in Supplementary materials). Fe-brucite has a lower e than olivine due to its platy crystal form. Therefore, the olivine volume used to calculate the extent of reaction displayed in Figure 1 is determined by only considering grains with e > 0.35. We also exclude from olivine grain segmentation groups of voxels with a volume smaller than 1.5.10 5 μm 3 . The radius of a sphere of this latter volume corresponds to three times the voxel size. The extent of reaction estimated with X- ray microtomography is a maximum due to these two constraints on olivine grain identification.

Electron microprobe analysis and X-ray maps

Point analyses and X-ray maps were acquired with a JEOL JXA-8530F Hyper-Probe at the University of Lausanne. All the data were acquired at 15 keV. The X-ray maps were acquired at 40 nA with a dwell time of 50 ms and a resolution of 8.5 μm.

Spot analyses of olivine, serpentine/brucite mixture and magnetite were acquired in the same region as the maps to produce internal standards for converting the count number of the X-ray maps into composition in oxide mass and then in mole fraction.

A mineral phase was then attributed to each pixel of the map based on composition criteria (Figure 2). Point analyses reveal that serpentine and brucite are intermixed at a scale of less than 1 μm. The proportion of serpentine and brucite at each pixel identified as a serpentine/brucite mixture was determined assuming (Mg, F e) 3 Si 2 O 5 (OH) 4 and (Mg, F e) (OH) 2 as structural formulas for serpentine and brucite, respectively:

X Br = n Mg + n F e -3 2 n Si n Mg + n F e -n Si (1)
where n Mg , n F e and n Si denote the molar content of Mg, Fe and Si, respectively, and X Br is the mole fraction of brucite.

The Mg number of olivine (X Mg Ol ) is determined by averaging the composition of the pixels attributed to olivine. The Mg numbers of serpentine and brucite (X Mg S and X Mg B , respectively) are determined by fitting for the serpentine/brucite mixture, the Mg number (X Mg ) as a function of X Br at each pixel with the following relationship:

X Mg = X Br 3 -2X Br X Mg B + 3 (1 -X Br ) 3 -2X Br X Mg S (2)
Finally, the Mg numbers calculated for each phase are used to determine their molar volume with a linear combination of the end-member molar volumes taking the fol- lowing values: 43.79, 46.39, 24.63, 26.43, 108.5, and 115 cm 3 .mol -1 for forsterite, fayalite, Mg-brucite, Fe-brucite, Mg-serpentine and Fe 2+ -serpentine, respectively [START_REF] Chichagov | MINCRYST: A crystallographic database for minerals, local and network (WWW) versions[END_REF].

To track changes in composition during reaction (export or import of aqueous species), we calculate the mean composition of the serpentine/brucite mixture (Xm Br ) by weighting the mole contents in brucite and serpentine at each pixel with their molar volume. Averages are calculated for all the pixels identified as serpentine/brucite mixtures in the maps. We also estimate Xm Br with a smaller sampling size to determine the scale at which compositional changes occur (scale of homogenization). The side length of the square sampling box (sl) varies from the pixel size (8.5 μm) to the map width (5.6 mm; Figure 3). We generate 3,000 randomly located sampling boxes for each sl. We then sort these boxes as a function of the distance to the main vein network (l) to also determine the variation of the standard deviation of Xm Br with sl and l.

Estimate of volume change during reaction

In the sample studied here, primary olivine contains thin mineral platelets of [START_REF] Moseley | Symplectic exsolution in olivine[END_REF][START_REF] Putnis | Electron petrography of high-temperature oxidation in olivine from the Rhum Layered Intrusion[END_REF]). They display a topotaxial intergrowth with olivine leading to parallel oriented platelets along a preferred olivine crystallographic orientation [START_REF] Zhang | Magnetite lamellae in olivine and clinohumite from Dabie UHP ultramafic rocks[END_REF][START_REF] Ashworth | Symplectic Reaction in Olivine and the Controls of Intergrowth Spacing in Symplectites[END_REF]. Baronnet and Boudier (pers. comm.) first proposed use of such non-reactive and oriented markers to quantify solid volume change during serpentinisation (Figure 4). Their calculation is based on the measurement of the angle change between the olivine surface and an initially straight marker.

Such markers are also expected to rotate and to space out as a result of volume change during reaction (Figure 4). Here we extend the approach of Baronnet and Boudier (pers. comm.) by considering these latter effects to estimate solid volume change by measuring the displacement of several platelets of magnetite and clinopyroxene.

First, we determine the deformation and composition of these platelets in three dimensions with focused ion beam tomography at the CIME-EPFL laboratory (Lausanne). We use a Zeiss Crossbeam 540 apparatus equipped with a field emission scanning electron miscroscope. We erode layer by layer with an ion beam a 18x15x19 μm cuboid containing a deformed platelet and located at the surface of an olivine grain.

After each milling of a 10 nm thick layer, a back-scattered electron image is acquired at an acceleration voltage of 1.7 kV with a current of 2.5 nA, a dwell time of 15 μs per pixel and a pixel size of 10x10 nm. The stack of images is then aligned with the FIJI image processing software by running the MultiStackReg plug-in [START_REF] Thévenaz | A pyramid approach to subpixel registration based on intensity[END_REF]. We segment the images based on pixel intensity with a home-made Matlab code by first applying a median filter and then using dilatation/erosion techniques.

As the inclusions are parallel in each olivine grain, we can compare orientations in olivine and in fully serpentinised domains around each olivine. We therefore estimate solid volume change in two dimensions by integrating the displacement of several inclusions. We acquire optical photomicrographs of primary olivine surrounded by serpentine + brucite. We select olivine grains with surface(s) oblique to the inclusion orientations and with inclusions both inside the grain and in the secondary phases (Figure 5 A). The olivine rim is mapped in two dimensions with a set of segments.

We then compute regularly spaced lines parallel to these segments (spacing d ini = d between the lines) to generate a mesh (Figure 5 grains are projected on the mesh which can be deformed by modifying d. The leastsquares method is finally applied to determine the value of d, d best , for which the orientation of the needles outside the olivine grain best fits the orientation of the needles inside the olivine grain (Figure 5D). The surface area of the mesh at d best (S best ) can finally be compared to the surface area of the mesh before deformation (S ini ) to estimate the change in surface area during reaction ( ΔS S = Sini-S best

S best

). Thin sections obtained from perpendicular sections of the sample display the same mesh textures with reaction zones of approximately uniform size around the grains. We therefore assume homogeneous volume change for 3D calculations in the following.

For spherical grains, ΔS S does not depend on the orientation of the 2D thin section used for measurement. The change in surface area can be converted in a change in solid volume ( ΔV V ) with the following equation:

ΔV V = ΔS S 1 -(1 -ξ) 2/3 + 1 3/2 -1 ξ (3)
with ξ the local extent of reaction. ξ can be estimated from 2D images as:

ξ = 1 - S ol S best 3/2 (4)
with S ol the surface area of the olivine grain. For spherical grains, Equation 4only provides the actual extent of reaction if the grains are cut through their centers.

It overestimates ξ otherwise. Therefore, ΔV V determined with ξ from equation 4 is a maximum. The limit of equation 3 as ξ tends towards zero is ΔV V = ΔS S , which provides a minimum estimate for the change in volume.

Laser-ablation inductively coupled plasma mass spectrometry (LA-

ICPMS)

The chemical data are acquired with a sector-field Element XR inductively coupled plasma mass spectrometer (ICP-MS) coupled with an Atlex 193 nm ArF excimer laser housed inside an Australian Scientific Instruments (ASI) RESOlution system (including an S155 dual volume sample cell) at the University of Lausanne. We use two strategies to acquire the data: spot mode and continuous scanning. Both datasets are acquired along the direction perpendicular to the main vein network. For each element, one isotope is measured in a peak-hopping mode. The on-sample laser energy densities are approximately 10 J.cm -2 with repetition rates at 10 Hz. The laser beam shape of the spot mode is rectangular with a size of 100 x 25 μm. For continuous analyses, the laser beam is a disk with a diameter of 100 μm and the scanning rate is 10 μm.s -1 .

To reduce the results, glass standard BCR-2G was analyzed along with the samples to be used as an external calibration standard, while silica (SiO 2 = 33 wt.%) is used as an internal standard using LAMTRACE [START_REF] Jackson | LAMTRACE data reduction software for LA-ICP-MS[END_REF] and Iolite (Version 3.11, The University of Melbourne) [START_REF] Paton | Iolite: Freeware for the visualisation and processing of mass spectrometric data[END_REF] software. An interval of 1.58 s is selected in the data reduction for continuous scanning. For smoothing and clarity, we use a moving average on three points for plotting the results. The raw geochemical data are given in the Supplementary Materials.

Results

Mineralogical distribution

The sample is cross-cut by a ∼ 5mm wide anastomosing network of serpentine veins (referred to as "black veins" in the core description (Kelemen et al., n.d.)). Despite complete serpentinisation in the vicinity of these veins, the typical mesh texture of serpentinised peridotites is preserved (Figure 2 A). Approximately 10 μm wide serpentine veins surround mesh cores composed of a concentric alternation of brucite-rich and serpentine-rich layers. Raman spectroscopy reveals that the serpentine minerals are both chrysotile (main peak at 3699 cm -1 and shoulder at 3689 cm -1 ) and lizardite

(main peak at 3684 cm -1 and secondary peak at 3705 cm -1 ) in the vein network. In the mesh cores, lizardite is intermixed on the sub-micrometer scale with brucite (main peak at 3636 cm -1 and secondary peak at 3652 cm -1 ). This latter peak corresponds to the peak expected for pure brucite (Mg (OH) 2 ; [START_REF] Speziale | Vibrational spectroscopy of Fe (OH)2 at high pressure: Behavior of the O-H bond[END_REF]). The shifting towards lower frequencies of the main brucite peak suggests the presence of F e (OH) 2 [START_REF] Speziale | Vibrational spectroscopy of Fe (OH)2 at high pressure: Behavior of the O-H bond[END_REF]. Magnetite is only found as sub-micron sized grains disseminated in serpentine and brucite. It mainly occurs in the wider serpentine veins of the anastomosing network. Up to ∼ 2 mm wide chromites are found throughout the samples. They can react at their margin with the development of a 2 μm wide reaction zone mainly composed of magnetite. We also observe minor ∼ 50 μm wide clinopyroxenes in the rock matrix. They do not display evidence for reaction in agreement with thermodynamic calculations [START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15N on the Mid-Atlantic Ridge[END_REF].

≥ 3 mm away from the anastomosing vein network, olivine grains are found in the mesh core (Figure 2). The transition from olivine-free to olivine-bearing serpentinised peridotites occurs at approximately 4 mm from the main serpentine vein (Figure 1).

The extent of reaction is generally high with a minimum estimated at 97 % with X-ray microtomography and at 80 % with X-ray maps acquired with the microprobe.

Olivine grains commonly contain ∼ 1μm wide, parallel inclusions (Figure 6).

We use these platelets to estimate solid volume change during serpentinisation in section 3.4. Energy dispersive spectroscopy reveals that the inclusions are intergrowths of clinopyroxene and magnetite, containing minor concentrations of Cr. FIB-SEM nanotomography shows that the platelets have a symplectic texture (Figure 7 A). Its composition is similar when it is included in the olivine or in the serpentine/brucite mixture and thus the composition of both clinopyroxene and magnetite appear to be largely unaffected by serpentinisation. The only effect of reaction appears to be the generation of discontinuities in the platelet separating ∼ 1 μm long segments of clinopyroxene. The presence of traces of magnetite in the discontinuities as well as their orientation in parallel to other magnetite grains suggest that the discontinuities were formerly filled with magnetite which reacted during serpentinisation.

Mineralogical composition

Point EPMA analyses and X-ray maps indicate homogeneous iron distribution in olivine with a Mg number ( Mg Mg+F e mole fraction) of 0.895 ± 0.011. NiO content in olivine ranges from 0.25 to 0.33 wt.%.

For the brucite/serpentine mixture, X Mg varies as a function of X Br along a mixing line defined by equation 2 (Figure 8). The fit of the data leads to X Mg S = 0.956 ± 0.022 and X Mg B = 0.724 ± 0.022. X Br averaged over the entire mapped area is 0.515 ± 0.002. The mean value of X Mg in the serpentine/brucite mixture is 0.896. deviation highly depends on l and is 2 to 3 times higher in the first 2 mm near the main serpentine vein than further away where olivine also occurs in the matrix (Figure 3).

Chemical composition

Compositional profiles, measured from both X-ray maps and LA-ICPMS, display constant contents of FeO, MgO and SiO 2 at a scale larger than several hundreds of micrometers which is also the scale at which constant brucite contents are measured.

Figure 2 A reveals variations of more than 15 % in the Mg/(Mg+Fe) ratio at a smaller scale and near the main serpentine vein. This is associated with brucite and serpentine segregation in the mesh cores. Spatial variations in Al 2 O 3 and Cr 2 O 3 are mainly associated with the primary distribution of chromite in the sample. From a trace element point of view, there are three distinct correlations between the abundances and the distances from the main vein. Concentrations of fluid-mobile elements such as Na, Ca, Rb, Sr and Ba gradually decrease with increasing perpendicular distance away from the main vein. Some spikes in Ca and Na profiles may be due to the preservation of fine clinopyroxene grains or inclusions (Figure 9). Some elements display constant contents, such as B, Co, Ni, Cu, Zn and Yb (Figure 10). Most of them are transition metal or heavy rare earth elements (HREEs).

The light-REEs, Nb, Pb and U are very low and their variation is difficult to quantify (for example, the contents of Ce mostly cluster around 1 ppb, ignoring the spikes possibly related to clinopyroxene). U is generally less than 1 ppb and provides a possible line of evidence for the absence of carbonate formation. Finally, Al, Sc, Ti, V and Sr show periodic increases at a length scale of 5 mm (Figure 11). The bulges formed by these increases are rather symmetric with a flat plateau at the highest concentration surrounded by sharp flanks where the concentration decreases. Ol relics appear

Solid volume change

We used the misorientation of clinopyroxene/magnetite intergrowths initially included in olivine to quantify volume change during serpentinisation. The platelet analysed with FIB-SEM tomography is planar when it is included in olivine with less than 5 • of misorientation (Figure 7 B). It progressively bends when it is included in the serpentine/brucite mixture (Figure 7 A andB) with misorientation values of more than 15 • the further away from the olivine surface. Bending preferentially occurs along small cracks that are mostly located at clinopyroxene/magnetite contacts. The acute angle between the platelet and the olivine surface increases with this bending. This indicates a positive change in solid volume during reaction. Systematic differences in the orientation of ensembles of inclusions, between those hosted in olivine and those hosted in surrounding zones of serpentine + brucite replacing olivine, are also observed in scanning electron images (Figure 6).

To extract quantitative estimates of solid volume change during reaction, we use the procedure described in section 2.3. We analyse 64 optical photomicrographs of serpentinised olivine. The results of inclusion reorientation are summarized in Figure 12 and provided for each photomicrograph in the Supplementary materials. We measure positive ΔV V from 8 to 168 %. ΔV V minimum and maximum bonds have mean values of 59 ± 29 and 74 ± 36 %, respectively (see section 2.3 for the definition of these bounds).

The sample studied here appears to be ideal for studying mass transfer and volume change during reaction. It comes from depth (215 m) and has thus experienced relatively limited surface alteration; for example, we do not observe carbonates or evidence for carbonation in U, Sr or Ca datasets consistent with the results of shipboard core description (Kelemen et al., n.d.). Moreover, the NiO content of the remaining olivine is below 0.32 wt. % indicating that the protolith is probably a dunite [START_REF] Suhr | Melt Migration under Oceanic Ridges: Inferences from Reactive Transport Modelling of Upper Mantle Hosted Dunites[END_REF].

Serpentinisation requires transport of ∼ 150 g of water per kilogram into an initially impermeable peridotite. The sample is crossed by an anastomosing vein perpendicular to which a gradient in extent of reaction is measured (Figures 2 and1). This suggests that water migrated from the main serpentine vein into the peridotite matrix.

The observed mesh texture on the wall of the main vein indicates that a fracture network with a typical spacing of 100 μm promoted fluid transport inside the peridotite.

Such an organised crack network may have formed by reaction-induced fracturing [START_REF] Jamtveit | Reaction induced fracturing during replacement processes[END_REF][START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF][START_REF] Plümper | The interface-scale mechanism of reaction-induced fracturing during serpentinization[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF] and/or thermal cracking (Demartin et al., 2004;[START_REF] Boudier | Does Anisotropy of Thermal Contraction Control Hydrothermal Circulation at the Moho Level below Fast Spreading Oceanic Ridges?[END_REF]. The origin of the main serpentine vein is more difficult to constrain since tectonic processes could also promote fracturing [START_REF] Rouméjon | Serpentinization of mantle-derived peridotites at mid-ocean ridges: Mesh texture development in the context of tectonic exhumation[END_REF]. The fluid transport direction is confirmed with compositional profiles in trace elements. A decrease in fluid-mobile elements (Na, Ca, Sr, Rb and Ba) is indeed observed from and perpendicular to the main vein. The history of the Oman ophiolite is complex with four main events during which serpentinisation probably occurred: hydrothermal circulation at mid-ocean ridges, interaction with metamorphic fluids during obduction [START_REF] Searle | Structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction[END_REF],

Cretaceous weathering of the ophiolite, followed by a marine transgression (de Obeso & Kelemen, 2018), and modern ongoing low temperature serpentinisation through interaction with rainwater [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Kelemen | In situ carbonation of peridotite for CO2 storage[END_REF][START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO2 Capture and Storage[END_REF]. Fluids collected at surface seeps or in wells in the Oman ophiolite generally display high Na, Cl, Sr and Ca concentrations [START_REF] Chavagnac | Characterization of hyperalkaline fluids produced by low-temperature serpentinization of mantle peridotites in the Oman and Ligurian ophiolites[END_REF][START_REF] Paukert | Reaction path modeling of enhanced in situ CO2 mineralization for carbon sequestration in the peridotite of the Samail Ophiolite, Sultanate of Oman[END_REF][START_REF] Canovas | Geochemical bioenergetics during low-temperature serpentinization: An example from the Samail ophiolite, Sultanate of Oman[END_REF][START_REF] Paukert Vankeuren | Multitracer determination of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman[END_REF], the same as the elements that display a progressive decrease from the main vein in the sample studied here.

Modern water sources and seawater both contain all these components at relatively high concentration [START_REF] Millero | The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale[END_REF][START_REF] Paukert Vankeuren | Multitracer determination of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman[END_REF]. The time of serpentinisation can thus not be determined with the data presented here. The enrichment in fluid-mobile components in the Oman ophiolite fluids could be secondary and acquired through dissolution of fluid-mobile elements previously trapped in gabbro or in peridotites serpentinised on the seafloor [START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF]. Meanwhile, the Sr isotope ratios in peridotite-hosted carbonates, water, and leachates are substantially more radiogenic than Cretaceous seawater [START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO2 Capture and Storage[END_REF]. Several sources may thus contribute to the modern composition of the fluids collected in Oman.

The standard deviation of the Mg/(Mg+Fe) ratio in the serpentine/brucite mixture is three times higher for a sampling size below 100-500 μm than above (Figure 3). This indicates that major elements are redistributed at a scale below 100-500 μm corresponding to the mesh scale. This redistribution increases near the main vein (Figure 3) indicating a larger scale segregation of brucite and serpentine. There is thus mass transport probably associated with further fluid-rock interaction at a scale of less than several hundreds of micrometers. This local mass transfer is probably the incipient stage of the mass transport responsible for the change in chemical composition observed in serpentinised peridotites with a decrease of the MgO/SiO 2 ratio during more extensive alteration [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF]de Obeso & Kelemen, 2018).

We do not measure spatial changes in major element composition at the centimeter scale in microprobe and LA-ICPMS data. The Mg/(Mg+Fe) ratio in the serpentine/brucite mixture is 0.896, similar to the one measured in the remaining olivine grains (0.895 ± 0.011). Brucite and serpentine display an equimolar distribution in the serpentine/brucite mixture (X Br = 0.515 ± 0.002) and magnetite is rare in the investigated area (Figure 2 B). This suggests that the initial (Mg+Fe)/Si ratios of olivine are conserved during reaction. Excluding trace elements and at a scale higher than several hundreds of micrometers, serpentinisation in the studied sample corresponds to the isochemical hydration of olivine: where the compositions of serpentine and brucite are retrieved from microprobe data with equation 2. The Mg numbers of brucite and serpentine can theoretically provide constraints on temperature and water to rock ratio. However, the uncertainties on the thermodynamic parameters of Fe-brucite and Fe-serpentine preclude precise quantitative estimates. The X Mg B measured here is at the low end of the range for thermodynamic predictions [START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15N on the Mid-Atlantic Ridge[END_REF][START_REF] Klein | Compositional controls on hydrogen generation during serpentinization of ultramafic rocks[END_REF]. The thermodynamic calculations indicate that more iron is incorporated in brucite at low temperature (T < 100

(
• C) and low water to rock ratio (< 0.5). The scarcity in magnetite in the sample is also compatible with such conditions.

Al, Ti, V, Sc and Cr concentrations periodically vary at a typical length scale of several millimetres without relationship to the extent of serpentinisation (Figure 11). Measured compositional profiles recall the diffusion profiles for the same elements measured in unaltered olivine-bearing xenoliths at a length scale typical of the olivine crystal size in unaltered peridotites [START_REF] Tollan | Frozen melt-rock reaction in a peridotite xenolith from sub-arc mantle recorded by diffusion of trace elements and water in olivine[END_REF]. Therefore, we interpret the measured composition as reflecting the primary olivine composition acquired through diffusion or growth at high temperature. Preserving olivine primary composition requires that Al, Ti, V, Sc and Cr are immobile during the whole serpentinisation process.

Our study of major and trace elements behaviour during serpentinisation indicates very limited mass transfer except for elements associated with the fluid (Na, Ca, Sr, Rb and Ba). In contrast, several studies have reported a decrease in MgO/SiO 2 in serpentinised peridotites, compared to their mantle peridotite protoliths [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]de Obeso & Kelemen, 2018).

This decrease is either related to Mg loss during alteration on the seafloor [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF] or to Si metasomatism [START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 1520'N, ODP Leg 209): Implications for fluid/rock interaction in slow spreading environments[END_REF]de Obeso & Kelemen, 2018).

Thermodynamic calculations predict a significant decrease in the MgO/SiO 2 ratio only when considering SiO 2 gain. Mg loss is not predicted at equilibrium even at high water to rock ratio (10 5 ) [START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]. However, high-resolution tomography indicates brucite dissolution and porosity formation in abyssal serpentinised peridotites exposed on the seafloor [START_REF] Jöns | Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography[END_REF]. Mg loss would then be associated with kinetic effects limiting the precipitation of Mg-bearing phases (e.g. carbonates) during fluid flow. Brucite is not commonly observed in the samples dredged on the seafloor and studying its fate in the cores drilled in Oman can thus provide insights into the decrease in the MgO/SiO 2 ratio measured in abyssal peridotite. As brucite contains approximately 30 % iron, its reaction is expected to produce iron-bearing phases such as magnetite while magnesium is either leached from the system with the fluid [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF] or combined with dissolved silica to form serpentine [START_REF] Beard | Onset and progression of serpentinization and magnetite formation in Olivine-rich troctolite from IODP hole U1309D[END_REF].

In our study, we observe tiny magnetite grains (< 1 μm) that could be associated with incipient brucite alteration at low temperature (Figure 2). Magnetite production is also expected to produce hydrogen according to experiments on samples from Oman [START_REF] Miller | Low temperature hydrogen production during experimental hydration of partially-serpentinized dunite[END_REF].

FIB-SEM tomography shows that clinopyroxene/magnetite inclusions are mainly deformed through local displacement along cracks leading to platelet curvatures at angles higher than 10 • (Figure 7). Deformation is controlled by the local environment and in particular by the relative orientation of the inclusion and the olivine surface (Figure 6). Several processes such as exhumation at a mid-ocean ridge, obduction, depressurization during uplift, and drilling-induced damaging of the rock have probably deformed the studied samples. However, these large scale deformation events are not expected to lead to the observed locally-controlled deformation and in particular to deformation sensitive to the olivine surface orientation. Moreover, the measured changes in volume based on platelet displacement are all positive. This strongly suggests that the deformation studied here is induced by the precipitation of the serpentine/brucite mixture. High stresses can build-up during mineral growth (i.e. crystallisation pressure) [START_REF] Steiger | Crystal growth in porous materials -I: The crystallization pressure of large crystals[END_REF]. Displacement along cracks in the platelets indicates the generation of stresses sufficient for brittle deformation during serpentinisation. This is consistent with the theoretical predictions and observations for reaction-induced fracturing during serpentinisation [START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF][START_REF] Lafay | Intracrystalline Reaction-Induced Cracking in Olivine Evidenced by Hydration and Carbonation Experiments[END_REF]. The measured changes in volume are between 59 ± 29 and 74 ± 36 %, with average values that appear to be slightly higher than the 52 % calculated for reaction 5 in a closed system. Volume increase during reaction is thus a local process probably only affected by local mass transfer at the mesh scale as indicated by the scattering of the volume change data. Such an interpretation is consistent with the preserved trace element zoning in primary olivine (Figure 11), and with recent data collected in serpentinisation experiments [START_REF] Klein | Quantifying the volume increase and chemical exchange during serpentinization[END_REF]. This indicates that serpentinisation is a replacement reaction with coupled dissolution-precipitation at the olivine surface (A. [START_REF] Putnis | Mineral Replacement Reactions[END_REF]. Such a process requires a constant connection between the fluid source and the reactive surface. Experiments on salts show that porosity is formed and spontaneously evolves in a network of channels during replacement (C. V. [START_REF] Putnis | Direct observations of pseudomorphism: Compositional and textural evolution at a fluid-solid interface[END_REF][START_REF] Putnis | An experimental study of the replacement of leucite by analcime[END_REF][START_REF] Raufaste | The mechanism of porosity formation during solvent-mediated phase transformations[END_REF]. Regarding serpentinisation, tectonic deformation and reaction-induced fracturing can promote fluid transport down to the hundreds of micrometer scale [START_REF] Rouméjon | Serpentinization of mantle-derived peridotites at mid-ocean ridges: Mesh texture development in the context of tectonic exhumation[END_REF].

However, they do not explain fluid transport to the olivine surface at the micron scale.

Reaction kinetics is fast in the presence of water (Malvoisin, Brunet, et al., 2012) and serpentinisation is thus controlled by fluid transport from the mesh borders to the olivine surface [START_REF] Malvoisin | Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization[END_REF]. The low Mg number of brucite measured here indicates reaction at low water to rock ratio [START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15N on the Mid-Atlantic Ridge[END_REF]. Fluid transport in the serpentine/brucite mixture is thus pervasive and controlled by the pore space structure. [START_REF] Tutolo | Nanoscale constraints on porosity generation and fluid flow during serpentinization[END_REF] identified a nanoscale porosity of several percent in serpentinised peridotites. The measurements of solid volume changes several percent higher than the one expected in a closed system could reflect the generation of such a porosity in the serpentine/brucite mixture. It remains unclear from mineralogical and mechanical points of view how this porosity can be generated while the solid volume is increasing in a stressed environment.

The protolith of the sample studied here is a dunite as suggested by the low NiO content in olivine (< 0.32 wt. %), the Ni profiles in the serpentinised peridotite (Figure 10), and the absence of pyroxene pseudomorphs in this section of the drillcore (Kelemen et al., n.d.). Applying the conclusions obtained here to serpentinised peridotites in general (thus including harzburgite) requires consideration of the influence of mineralogy on ΔV V . Dunite serpentinisation consists in the formation of serpentine from olivine. As the (Mg + Fe)/Si ratio is 2 in olivine whereas it is 1.5 in lizardite, the system contains divalent cations in excess and brucite is also formed. Harzburgite serpentinisation does not only involve the reaction of olivine but also of orthopyroxene with a (Mg + Fe)/Si ratio of 1. The silica activity is higher than for dunite serpentinisation, and the brucite formed in the vicinity of olivine can further react to form serpentine and magnetite [START_REF] Frost | On silica activity and serpentinization[END_REF][START_REF] Beard | Onset and progression of serpentinization and magnetite formation in Olivine-rich troctolite from IODP hole U1309D[END_REF]. The overall serpentinisation process can be summarised in this case with the following reaction: olivine (Mg 2 SiO 4 ) + orthopyroxene (MgSiO 3 ) + 2 water = lizardite (Mg 3 Si 2 O 5 (OH) 4 ) for which ΔV V is 45 %. This value is very close to the value of 52 % calculated here for dunite serpentinisation. Moreover, the modal amount of orthopyroxene in harzburgite is in general not sufficient to convert all the brucite into serpentine. The protolith is thus expected to play a minor role on solid volume change.

Expansion during serpentinisation has consequences not only for mineralogical processes but also for tectonics and geodynamics. [START_REF] Germanovich | Deformation and surface uplift associated with serpentinization at mid-ocean ridges and subduction zones[END_REF] use an elastic model to predict surface deformation associated with volume increase in an elliptic serpentinised domain located at depth. They estimate the influence of the extent of reaction and the depth and the shape of the domain on surface topography. The calculated uplifts are up to ∼ 150 m and consistent with the topography measured at slow-spreading mid-ocean ridges and above subduction zones where serpentinisation is expected to occur. [START_REF] Palmer | Hydration and uplift of the oceanic crust on the Mid-Atlantic Ridge associated with hydrothermal activity: Evidence from boron isotopes[END_REF] calculates similar values for seafloor uplift without accounting for elastic deformation. [START_REF] Francis | Serpentinization faults and their role in the tectonics of slow spreading ridges[END_REF] proposes a model of stress change and faulting induced by serpentinisation. [START_REF] Horning | Local Seismicity of the Rainbow Massif on the Mid-Atlantic Ridge[END_REF] find microearthquakes in an oceanic core complex (Rainbow massif) which they attribute to serpentinisationinduced deformation. In all these studies, a large ΔV V value is used because the authors implicitly assume reaction in a closed system. Our measurements justify such an assumption and, thus, strengthen the proposed impact of serpentinisation in geodynamics. Moreover, they support the use of surface deformation monitoring as a tool to determine where and at which rate serpentinisation is active. For example, this could be used on the seafloor near hydrothermal sites or at active on land serpentinisation sites.

  mantle minerals. The sample we study comes from one of the freshest portions of the core at a depth of 215 m. It is a dunite, principally composed of serpentine, brucite and olivine in a mesh texture, which is cross-cut by a serpentine vein surrounded by a reaction front. It thus seems ideal for quantifying mass transfer and volume change during reaction, since it allows for comparison between fully and partly reacted rock domains. The variations of the measured values are given with errors corresponding to their standard deviation. The composition and the microtexture of the samples were first characterized at the University of Lausanne with a field-emission scanning electron microscope (Tescan Mira II LMU) operating at 10 kV and 23 mm working distance and equipped with an Energy Dispersive Spectrometer from Oxford Instruments.

Figure 1 .

 1 Figure 1. Extent of reaction as a function of distance from the serpentine vein center. The extent of reaction illustrated in this figure is a maximum; it is calculated based on X-ray microtomography (see section 2.1 for the computational method).

Figure 2 .

 2 Figure 2. Compositional maps of sample BA4A-81-1-1-17 obtained from X-ray maps acquired with the microprobe. A: Mg/(Mg+Fe) ratio. The white arrow points towards the main serpentine vein. The black arrows point towards the core of a mesh preserved after complete serpentinisation. B: phases. The serpentine/brucite mixture is displayed in red, olivine in orange, spinel in blue and magnetite in green. The scale bar is 1 mm.

Figure 3 .

 3 Figure 3. Standard deviation of XBr as a function of the logarithm of the sampling size (sl) and the distance from the main vein (l).

Figure 4 .

 4 Figure 4. Sketchs illustrating the effect on inert markers displacement of solid volume change during the formation of a serpentine/brucite mixture (Serp/Brc). A: initial distribution of straight and parallel inert markers in olivine. B and C: predicted displacement of the inert markers for changes in volume of 10 % and 50 %, respectively. The three main effects of volume change during reaction for inert markers are: bending at the olivine/reaction products contact (red arrows), rotation (white arrows) and increase of the spacing (blue arrows).

Figure 5 .

 5 Figure 5. Steps for determining solid volume change during serpentinisation. We measure the misorientation of inclusions contained in olivine (Ol) initially and in the serpentine/brucite mixture (Serp/Brc) after reaction. A: an olivine grain surrounded by serpentinisation reactionproducts is first selected. The grain surface must be oblique to the inclusions. B: the inclusions are mapped (red lines outside the olivine grain and blue lines inside the olivine grain) and projected on a 2-D mesh generated from the olivine surface. C: the inclusions are deformed by modifying the mesh spacing in the direction normal to the olivine grain surface. D: the best model (green lines) is selected from a set of models with different mesh spacing by minimizing the angle between the needles inside and outside the olivine grain using the least-squares method.Comparing initial mesh spacing to the spacing in the best model provides an estimate of solid volume change during reaction (see text for details).

Figure 3

 3 Figure 3 gives the standard deviation of X Br as a function of the sampling size (sl) and the distance to the main serpentine vein (l). It thus provides an estimate of the scale and extent of mass transfer in the sample. The standard deviation decreases with sl and is high for sl below approximately 500 μm. Below this sl, the standard

Figure 6 .

 6 Figure 6. Back-scattered electron image of an olivine grain (Ol) reacted at its border to form a serpentine/brucite mixture (Serp/Brc). Olivine contains parallel inclusions of magnetite(Mag)clinopyroxene(Cpx) intergrowths which are displaced in the serpentine/brucite mixture.

Figure 7 .Figure 8 .Figure 9 .

 789 Figure 7. FIB-SEM nanotomography of a platelet which has partly experienced serpentinisation. A: result of segmentation. The contact between olivine (Ol) at the bottom and the serpentine/brucite mixture (Serp/Brc) at the top is displayed with the yellow surface. The platelet is composed of clinopyroxene (Cpx; green) and magnetite (Mag; gray). It is displaced along cracks at some magnetite/clinopyroxene contacts in the serpentine/brucite mixture (red arrow). It is also discontinuous at its top (white arrows). B: angle between a reference orientation and the normal to the platelet surface. The reference orientation is chosen as the normal to the platelet when it is still included in the olivine grain (bottom part).

Figure 10 .Figure 11 .

 1011 Figure 10. Concentration of B, Co, Ni and Cr as a function of the distance to the main vein network. The profiles are acquired perpendicular to the main vein network. The first appearance of olivine is located with a red vertical line.-16-

Figure 12 .

 12 Figure 12. Change in solid volume ( ΔV V ) measured in sample BA4A-81-1-1-17. Minimum (blue bars) and maximum (orange bars) values of ΔV V are estimated for each photomicrograph. The procedure for calculating these minimum and maximum bonds is described in the Method section. The mean (bold lines) ± the standard deviation (thin lines) are displayed with dashed and plain lines for the minimum and maximum values of ΔV V , respectively. The bold dotted line corresponds to the change in volume calculated when considering no mass transfer during reaction.

  Mg 0.90 F e 0.10 ) 2 SiO 4 +3H 2 O = (Mg 0.96 F e 0.04 ) 3 Si 2 O 5 (OH) 4 +(Mg 0.72 F e 0.28 ) (OH) 2 (5)

  Demartin, B., Hirth, G., & Evans, B. (2004). Experimental Constraints on Thermal Cracking of Peridotite at Oceanic Spreading Centers. In Lithosphere and oceans geophysical monograph series (Vol. 148, pp. 167-185). Deschamps, F., Godard, M., Guillot, S., & Hattori, K. (2013). Geochemistry of subduction zone serpentinites: A review. Lithos, 178 , 96-127. doi: 10.1016/j.lithos .2013.05.019 Escartín, J., Hirth, G., & Evans, B. (2001). Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere. Geology, 29 (11), 1023-1026. doi: 10.1130/0091-7613(2001)029 1023:SOSSPI 2.0.CO Journal of Geophysical Research: Solid Earth.

				doi:
	10.1029/148GM07
	de Obeso, J. C., & Kelemen, P. B.	(2018).	Fluid rock interactions on residual
	mantle peridotites overlain by shallow oceanic limestones: Insights from
	Wadi Fins, Sultanate of Oman.	Chemical Geology, 498 , 139-149.	doi:
	10.1016/j.chemgeo.2018.09.022
	de Obeso, J. C., Kelemen, P. B., Manning, C. E., Michibayashi, K., & Harris, M.
	(2017).	Listvenite formation from peridotite: Insights from Oman Drilling
	Project hole BT1B and preliminary reaction path model approach. In Ameri-
	can geophysical union, fall meeting abstracts.
	Evans, B. W.	(2004).	The Serpentinite Multisystem Revisited: Chrysotile
	Is Metastable.	International Geology Review , 46 (6), 479-506.	doi:
	10.2747/0020-6814.46.6.479
	Evans, O., Spiegelman, M., & Kelemen, P. B.	(n.d.).	A continuum model of
	reaction-driven cracking.

Acknowledgments

Alain Baronnet is thanked for sharing his ideas for measuring volume change during reaction. We thank Lucie Navratilova at EPFL for help during the acquisition of FIB-SEM tomography. We thank the Associate Editor, J.S. Beard and an anonymous reviewer for their comments on our manuscript. B.M. acknowledges support from the Swiss National Science Foundation (Grant No. PZ00P2 168083). C.Z. acknowledges a financial support from the China Scholarship Council (No. 201804910292). P.B.K.s effort on this paper and the underlying research has been supported by Alfred P. Sloan Foundation Grant 2014-3-01, NSF Research Grants EAR-1520732 and EAR-1516300, the International Continental Scientific Drilling Program (ICDP; Kelemen, Matter, Teagle Lead PIs) Oman Drilling Project and P.B.K.s Arthur D. Storke Chair at Columbia University. The Oman Drilling Project was also funded by NASA Astrobiology Institute (NNA15BB02A, Templeton PI), the German Research Foundation (DFG: KO 1723/21-1, Koepke PI), the Japanese Society for the Promotion of Science (JSPS no:16H06347, Michibayashi PI; and KAKENHI 16H02742, Takazawa PI), the European Research Council (Adv: no.669972; Jamtveit PI), the Swiss National Science Foundation (SNF: 20FI21 163073, Frh-Green PI), JAMSTEC, the TAMU-JR Science Operator, and contributions from the Sultanate of Oman Ministry of Regional Municipalities and Water Resources, the Oman Public Authority of Mining, Sultan Qaboos University, CNRS-Univ. Montpellier, Columbia University of New York, and the University of Southampton. Supplementary materials including results of the change in volume analysis and geochemical data are available at https://doi.org/10.5281/zenodo.3706790. manuscript submitted to JGR: Solid Earth