Martin Erdmann
email: m.erdmann@mineralogie.uni-hannover.de

Lennart A Fischer

Lydéric France

Chao Zhang

Marguerite Godard

Jürgen Koepke

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): An experimental study

Keywords:

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally-altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, twopyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program expeditions 309, 312, and 335 at Site 1256 sampled such a fossilized oceanic magma chamber in the eastern equatorial Pacific. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly-to strongly-altered basalts to partially-or fully-recrystallized granoblastic hornfels.

Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately-altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH 2 O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH 2 O. High silica melt at the expected temperature (1000-1050°C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH 2 O=0.1.

INTRODUCTION

Anatectic processes associated with the dynamic behavior of crystal mush zones with an overlying axial melt lens (AML) at fast-spreading mid-ocean ridges have major influences on the composition of the oceanic crust. Based on geophysical studies (e.g., [START_REF] Caress | Tomographic image of the axial low-velocity zone at 12°50′N on the East Pacific Rise[END_REF][START_REF] Crawford | Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements[END_REF][START_REF] Gl | Modelling the seismic properties of fast-spreading ridge crustal Low-Velocity Zones: insights from Oman gabbro textures[END_REF][START_REF] Dunn | Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30[END_REF] and field observations in ophiolites (e.g., [START_REF] Nicolas | Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge[END_REF][START_REF] Nicolas | Accretion of Oman and United Arab Emirates ophiolite -Discussion of a new structural map[END_REF][START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Umino | Along-strike variation of the sheeted dike complex in the Oman Ophiolite: Insights into subaxial ridge segment structures and the magma plumbing system[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF], the crystal mush zone at fast-spreading mid-ocean ridges has a melt fraction of less than 20% (e.g., [START_REF] Caress | Tomographic image of the axial low-velocity zone at 12°50′N on the East Pacific Rise[END_REF][START_REF] Crawford | Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements[END_REF][START_REF] Gl | Modelling the seismic properties of fast-spreading ridge crustal Low-Velocity Zones: insights from Oman gabbro textures[END_REF][START_REF] Dunn | Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30[END_REF], and is located between the Moho and the sheeted dikes. At the top of the crystal mush zone, directly beneath or within the sheeted dike root zone, a small (~50 -100 m thick and ~1000 m wide) melt lens is commonly present (e.g, [START_REF] Detrick | Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise[END_REF]. This AML, locally filled with nearly-pure basaltic melt (e.g., [START_REF] Detrick | Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise[END_REF][START_REF] Dunn | Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30[END_REF], feeds the basalts of the upper crust and is assumed to be the place where the main stage of midocean ridge basalt (MORB) differentiation occurs. The AML roof is a very complex horizon, where magmatic processes interact with metamorphic overprint and hydrothermal influence [START_REF] Coogan | Roof assimilation at fast spreading ridges: An investigation combining geophysical, geochemical, and field evidence[END_REF][START_REF] Nicolas | Root zone of the sheeted dike complex in the Oman ophiolite[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] France | Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (northern Oman ophiolite)[END_REF]. Below this horizon the AML is filled with a basaltic magma at a temperature of ~1200°C, and above it the seawaterderived hydrothermal cells operate at temperatures <600°C [START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF]. The two convective systems are thought to be separated by a thin conductive boundary layer (~100 m thick; e.g., [START_REF] Lister | On the Penetration of Water into Hot Rock[END_REF]) that has a maximum thermal gradient of approximately 3°C/m [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF], by far the most extreme quasi-steady-state thermal boundary on Earth.

This boundary layer consists of granoblastic hornfels and former hydrothermally-altered sheeted dikes, which were contact-metamorphosed by the heat of the underlying dynamic AML.

The combination of water influx due to the dehydration of the previously hydrothermally altered dikes and heat supply from an underlying AML [START_REF] Detrick | Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise[END_REF]) may result in anatexis of magma chamber roofs (e.g., [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Coogan | Roof assimilation at fast spreading ridges: An investigation combining geophysical, geochemical, and field evidence[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF][START_REF] France | Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (northern Oman ophiolite)[END_REF][START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF]. This is an important process at fast-spreading ridges, since the generated anatectic melts have the potential to skew the composition of fresh MORB [START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF]. Evidence for MORB contamination by assimilation of altered lithologies at crustal levels is widespread at fast-spreading ridges (e.g., [START_REF] Perfit | Geochemical Studies of Abyssal Lavas Recovered by DSRV Alvin From Eastern Galapagos Rift, Inca Transform, and Ecuador Rift 3. Trace Element Abundances and Petrogenesis[END_REF][START_REF] Michael | The influence of primary magma composition, H2O and pressure on mid-ocean ridge basalt differentiation[END_REF][START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF][START_REF] Coogan | Roof assimilation at fast spreading ridges: An investigation combining geophysical, geochemical, and field evidence[END_REF][START_REF] Wanless | Dacite Petrogenesis on Mid-Ocean Ridges: Evidence for Oceanic Crustal Melting and Assimilation[END_REF][START_REF] Wanless | Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers[END_REF][START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF].

Apart from MORB contamination, anatexis at the AML roof is recognized to be an important process that generates felsic melt in the sheeted dike root-zone of oceanic crust (e.g., [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Koepke | Petrogenesis of oceanic plagiogranites by partial melting of gabbros: an experimental study[END_REF][START_REF] Brophy | La-SiO[END_REF][START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF][START_REF] Wanless | Dacite Petrogenesis on Mid-Ocean Ridges: Evidence for Oceanic Crustal Melting and Assimilation[END_REF][START_REF] Grimes | Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon[END_REF], where cross-cutting felsic dikes and veins are often observed (e.g., [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF]. The origins of these dikes and veins have been debated for decades (see below) yet are still poorly understood. [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] pointed out that if dehydration melting of hydrated basaltic rocks is a primary mechanism for the formation of silicic melts, large volumes of anhydrous, pyroxene-and plagioclase-rich restites would be predicted. It is verified that such rocks exist as granoblastic hornfels, sandwiched between the AML and the sheeted dikes, and they are well known both from modern oceanic crust (e.g., [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF][START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF] and from the Oman and Troodos ophiolites (e.g. [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF].

The aim of the present study is to experimentally reproduce the complex anatectic processes occurring at the roof of fast-spreading oceanic ridge magma chambers by using six natural analogues recovered from Site 1256 of the Integrated Ocean Drilling Program (IODP) expeditions 309, 312, and 335 as starting materials. These materials span a lithological range from poorly-to strongly-altered dike basalt to partially-or fully-recrystallized granoblastic hornfels. The samples are well suited for characterizing the solidus temperatures for the different starting materials as well as for evaluating the compositional spectrum of the experimental melts and the residual minerals, and for addressing the question of whether water-saturated conditions are required for partial melting. Since the compositions of felsic veins as well as of the granoblastic hornfels from the drilled dike/gabbro transition at Site 1256 are available, a direct comparison of the experimental products with the corresponding natural counterparts is possible, enabling a very straightforward approach for unraveling the complex magmatic-metamorphic processes ongoing at the dike/gabbro transitions at fastspreading mid-oceanic ridges.

BACKGROUND Oceanic plagiogranites and granoblastic hornfels

Oceanic crustal plagiogranites (see [START_REF] Koepke | The formation of SiO2-rich melts within the deep oceanic crust by hydrous partial melting of gabbros[END_REF] for a review and definition) are typically found, although not in large volumes, in the upper oceanic crust. They have been described as vein networks and/or discrete intrusive bodies in ophiolites [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF] and as small felsic veins in the relatively young oceanic crust (e.g., [START_REF] Dick | A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge[END_REF][START_REF] Blackman | Oceanic core complex formation, Atlantis Massif-oceanic core complex formation, Atlantis Massif, Mid-Atlantic Ridge: drilling into the footwall and hanging wall of a tectonic exposure of deep, young oceanic lithosphere to study deformation, alteration, and melt generation[END_REF][START_REF] Teagle | the Expedition 309/312 Scientists[END_REF]). The dike/gabbro transition of present day oceanic crust has been sampled at only three locations, all related to crust formed at the East Pacific Rise (EPR; IODP Site 1256, Pito Deep, and Hess Deep). Especially at Site 1256, oceanic plagiogranites have been typically recognized as small felsic veins [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF][START_REF] Koepke | The formation of SiO2-rich melts within the deep oceanic crust by hydrous partial melting of gabbros[END_REF], often in association with granoblastic hornfels. As the origin of SiO 2 -enriched rocks is still under controversial debate (extreme fractionation vs. partial melting vs. liquid immiscibility; e.g., [START_REF] Koepke | The formation of SiO2-rich melts within the deep oceanic crust by hydrous partial melting of gabbros[END_REF]), these rock associations are key to our understanding of felsic melt generation in oceanic crust. Focusing on partial melting, as this study does, the general survey of protoliths, products, and residues can shed light on this complex process.

Rocks from the conducting boundary layer were first described as pyroxene and/or hornblende hornfels (e.g., [START_REF] Gillis | Cracking at the magma-hydrothermal transition: evidence from the Troodos Ophiolite, Cyprus[END_REF], which are very-fine-grained to microcrystalline granoblastic rocks corresponding to partially-to-completely re-crystallized, hydrothermally-altered dike rocks. The typical mineral assemblage of the pyroxene hornfels is clinopyroxene, orthopyroxene, magnetite, ilmenite, and plagioclase (±amphibole). The presence of primary amphibole divides these rocks into "dry" and "wet" granoblastic subtypes. A detailed review of granoblastic hornfels evolution and characteristics for a present day oceanic crust is given by [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF] and [START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF]. Granoblastic hornfels were initially observed in ophiolites in intimate association with uppermost gabbros typically located within the dike/gabbro transition zone [START_REF] Rothery | The base of a sheeted dyke complex, Oman ophiolite: implications for magma chambers at oceanic spreading axes[END_REF][START_REF] Nicolas | Rooting of the Sheeted Dike Complex in the Oman Ophiolite[END_REF]. Other studies from the Troodos and Oman ophiolites focused on processes occurring at the roof of a fossil AML and arrived at the general conclusion that the hornfels are the residues of a partial melting event above the AML, while the felsic veins correspond to the related anatectic melts [START_REF] Gillis | Cracking at the magma-hydrothermal transition: evidence from the Troodos Ophiolite, Cyprus[END_REF][START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF]. Granoblastic hornfels are also common in modern oceanic fast-spreading crust and can be directly linked to magma chamber roof processes, because they are sampled within the top of a frozen AML [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF]Koepke et al. 2008;[START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF]. Moreover, because the conductive boundary layer above an AML is mainly composed of granoblastic hornfels [START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF]), these rocks are crucial for understanding the heat flux in the hydrothermally-influenced oceanic crust and the role of this altered crust as potential contaminator of fresh MORB melts.

IODP Site 1256 stratigraphy

IODP Site 1256, located in the eastern Pacific Ocean in ~ 15 Ma old crust, is the only place on Earth where the dike-gabbro transition has been penetrated in a continuous drill section (Fig. 1; [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF]) and is, thus, perfectly suitable as a source of samples in this study.

The gabbro sequence, interpreted as part of the frozen melt lens [START_REF] Koepke | Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges[END_REF], was reached within seismic layer 2 at 1407 m below seafloor (mbsf). This shallow level for the gabbros may be related to the high magmatic budget associated with the superfast EPR spreading rate (up to 220 mm/year; [START_REF] Wilson | Fastest known spreading on the Miocene Cocos-Pacific Plate Boundary[END_REF] 19 to 12 Ma ago. The extrusive part of the crust at Site 1256 is composed of a ~100 m thick sequence of lava primarily comprised of a >74 m thick ponded flow, followed by ~700 m of sheet flows and massive lavas, and a relatively thin (~350 m) sequence of sheeted dikes. The lower ~60 m of the sheeted dikes are partially to completely re-crystallized to granoblastic hornfels, named "granoblastic dikes" by IODP Expedition 312 scientists [START_REF] Teagle | the Expedition 309/312 Scientists[END_REF][START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF]. One hundred meters below the granoblastic hornfels two intrusive bodies of gabbro (52 m and 24 m thick) are present, separated by 24 m of granoblastic hornfels [START_REF] Teagle | the Expedition 309/312 Scientists[END_REF][START_REF] Koepke | Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges[END_REF]. The uppermost gabbros are strongly hydrothermally altered, indicating a deep influence of hydrothermal circulation. These gabbros, interpreted as part of an in-situ frozen AML [START_REF] Koepke | Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges[END_REF], bear xenoliths of granoblastic hornfels, considered as relics of former dikes, indicating that contamination of a melt lens by overlying roof rocks played a significant role in the evolution of the magmatic system [START_REF] Teagle | the Expedition 309/312 Scientists[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] Koepke | Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges[END_REF].

Previous experiments testing an anatectic origin of felsic melt in oceanic crust

Numerous experiments on the dehydration melting of amphibolites and greenstones have been performed in the last decades, in order to investigate the formation of Archean TTG rocks (tonalites, trondhjemites, and granodiorites ;[START_REF] Barker | Trondhjemites, dacites and related rocks[END_REF]; see summarizing chapter on "Dehydration Melting" in [START_REF] Johannes | Petrogenesis and Experimetal Petrology of Granitic Rocks[END_REF]. Additionally, some partial melting experiments conducted under hydrous conditions using basaltic compositions as starting material showed that felsic melt production is possible by this process [START_REF] Holloway | Melting relations of basalt with equilibrium water pressure less than total pressure[END_REF][START_REF] Helz | Phase Relations of Basalts in their Melting Range at PH2O = 5 kb as a Function of Oxygen Fugacity: Part I. Mafic Phases[END_REF][START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF][START_REF] Kawamoto | Experimental constraints on differentiation and H2O abundance of calc-alkaline magmas[END_REF]. However, results of these studies are, for the most part, not directly applicable to the formation of felsic melt at the roof of a mid-ocean ridge AML, as most of the corresponding experiments were performed under elevated pressures (≥300 MPa) in order to simulate typical dehydration melting processes related to subduction zones or deep continental basements. Such conditions are not relevant for anatectic processes occurring at the AML roof; these processes are characterized by low lithostatic pressures (≤ 100 MPa) under potentially high water activity (aH 2 O) due to the low water solubility in silicate melts under such shallow pressures. Moreover, these studies used natural starting materials which are different from the lithologies present at the roof of oceanic magma chambers. One study that included experiments at low pressure (100 MPa) was performed by [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] who systematically investigated the effects of pressure and water on different metamorphosed protoliths. For simulating anatectic processes operating in the deep oceanic crust (e.g., Koepke et al. 2005a,b), partial melting experiments were performed using oceanic gabbro as starting material [START_REF] Koepke | Petrogenesis of oceanic plagiogranites by partial melting of gabbros: an experimental study[END_REF][START_REF] Wolff | The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust[END_REF][START_REF] Koepke | Partial melting of oceanic gabbro triggered by migrating water-rich fluids: a prime example from the Oman Ophiolite[END_REF]. However, these studies used fresh, rather primitive cumulate gabbro as starting material, and did not address the special compositional nature of the AML roof rocks which consist of a rock spectrum ranging from hydrated, altered basalts to dry granoblastic hornfels.

The first experimental study using hydrothermally-altered basalt from the lower sheeted dikes as starting material was performed by [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF]. The starting material for that study, however, was sampled in the Oman ophiolite, which may not be the best proxy for fast spreading ridges, because typical basalts from Oman are relatively enriched in SiO and lacked any primary magmatic mineral, which is uncommon at Site 1256, where the roof rocks are basalts bearing either primary phases or partially-to fully-recrystallized granoblastic hornfels. Thus, a direct application of [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] experimental results to Hole 1256D should be performed with caution.

EXPERIMENTAL AND ANALYTICAL TECHNIQUES

Starting material

Six different samples from IODP Site 1256 were used as starting material for hydrous partial melting experiments, representative of the different basaltic lithologies of the drilled section.

For petrographic details, modal amounts, and texture type classification indicating the grade of contact-metamorphic overprint according to [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF] see Table A1 (electronic supplementary). The rocks were sampled at the lower part of the sheeted dike complex, close to the AML. Two of the samples are basalts showing different grades of typical hydrothermal alteration (D10: 27 % alteration, D11: 11 %), containing low-grade hydrous alteration phases (e.g. actinolite, chlorite, secondary oxides) and preserved primary magmatic phases (plagioclase, clinopyroxene, oxides). Sample GD12 is a basalt with an initial granoblastic overprint (type 5 according to the classification of [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF]; GD12 is the sample with the highest grade of secondary low-grade alteration (43%). Three samples show a strong imprint of contact-metamorphism expressed by granoblastic, hornfelsic textures composed of newly-formed plagioclase, clinopyroxene, orthopyroxene, and oxides. Sample GD14 shows a "wet", amphibole-rich paragenesis equilibrated in the amphibolite facies, while GF36 and GF38 are "dry" two-pyroxene hornfelses with amphibole only present as a secondary alteration phase in very small amounts (Table A1). Two of the six samples are significantly enriched in amphibole: GD12 has 36% actinolitic amphibole as a result of hydrothermal alteration, while GD14 has 20% hornblendic amphibole regarded as part of the prograde contact-metamorphism leading to the granoblastic overprint (see also microphotographs in Fig. 1).

Major and trace element data of the starting materials were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) and ICP-mass spectrometry (MS), respectively, at "ACTLABS" (Activation Laboratories Ltd., Ancaster, Ontario, Canada).

While the major element compositions of the starting materials vary only slightly (e.g., SiO 2 48.6-51.5 wt%; Al 2 O 3 12.1-15.0 wt%; see Table 1), the trace element pattern of the whole rocks varies significantly from slightly enriched to more depleted compositions in comparison to typical N-MORB (see Fig. A1 in the electronic supplementary material). The starting materials with anhydrous granoblastic textures (two-pyroxene hornfels GF36 and GF45) are strongly depleted in light rare earth elements (LREEs) displaying marked Sr and Eu positive anomalies, and negative Zr and Hf anomalies as is typical of residues from a former partial melting event [START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF].

Felsic rocks from IODP Site 1256

A characteristic feature of the granoblastic hornfelses at IODP Site 1256 is that they contain intrusions of felsic, evolved plutonic rocks. These are small, irregular patches, veins (~1-2 mm wide), and dikelets (<1.5 cm wide) consisting of oxide gabbro, diorite, tonalite, and trondhjemite [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF][START_REF] Teagle | the Expedition 335 scientists[END_REF]) with a fine-to medium-grained granular texture. Here, for the comparison to experimental anatectic melts, we focus only on those felsic veins and dikelets of the Hole 1256D drill core characterized as tonalites based on the mineral mode. They are the most evolved samples of the sample suite and are minor secondary altered. Thus, they are best suited for a comparison with the experimental equivalents. The chemical bulk rock analyses of the tonalites show SiO 2 contents varying between 62 and 70 wt% with contents of MgO varying between 1.1 and 2.1 wt% (Tab. 1).

High amounts of P 2 O 5 (0.19 to 0.60 wt%) demonstrate the evolved character of the tonalites. N-MORB normalized rare earth element (REE) patterns of the tonalites are roughly flat with pronounced negative Ba, Rb, and Sr anomalies (Fig. A1). The positive Zr anomaly is strong and gradually enhanced by decreasing total REEs. Major and trace element compositions of the tonalitic veins from IODP Site 1256 also match those of typical oceanic plagiogranites from the Oman and Troodos ophiolites that form small-scale intrusions within the gabbrodike transition [START_REF] Rollinson | New models for the genesis of plagiogranites in the Oman ophiolite[END_REF][START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF].

Experiments

For the experiments, the natural rocks were crushed and sieved. [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] investigated the influence of starting material grain size on the composition of the experimental phases, and showed that the experimental products up to a grain size of 250 µm exhibit identical phase compositions. Thus, a starting material grain size of 125 to 200 µm was used here in order to obtain larger melt pools in low-temperature experiments, under virtually equilibrium conditions. Additionally, for low-temperature experiments close to the solidus we used "micro-rocks" as starting material (mm-sized fragments of the whole rocks; see [START_REF] Wolff | The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust[END_REF]) in order to obtain melt pools large enough for analysis. To ensure comparability among starting compositions, deionized water was added to the majority of the experiments in order to guarantee water-saturated conditions. Since all runs were performed at temperatures ≤1030°C, Au capsules were used which afforded the advantage of avoiding Fe loss to the capsule material.

To simulate conditions prevailing directly above the AML, an H 2 -controlled internally-heated pressure vessel (IHPV) at the Institut für Mineralogie in Hannover, Germany was used.

Temperature varied from 910°C to 1030°C in steps of 30°. The vessel was pressurized with argon gas to 100 MPa. This corresponds to a lithostatic pressure found at ~2-3 km depth in oceanic crust. Even though this is slightly deeper than the environment where the samples used were obtained at the EPR, this pressure enables a direct comparison to other experimental studies (e.g., [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF][START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF]. Moreover, at lower pressure it is difficult to ensure stable conditions in the vessel, especially with regard to a defined oxygen fugacity (fO 2 ). The fO 2 corresponds to QFM +1 (QFM corresponds to the fO 2 of the quartz-fayalite-magnetite oxygen buffer) and was set with a defined P H2 , depending on the experimental temperature, controlled by a Shaw membrane (e.g., [START_REF] Scaillet | Improvements of the Shaw membrane technique for measurement and control of f H2 at high temperatures and pressures[END_REF]).

While the fO 2 for primitive, dry MORB melts is suggested to be slightly below (e.g. [START_REF] Bézos | The Fe3+/[Sigma]Fe ratios of MORB glasses and their implications for mantle melting[END_REF] or slightly above QFM (Cottrell and Kelly 2011), the supposed hydrothermal influence in nature and the accompanying mostly water-saturated conditions in our experiments justify this higher fO 2 . A lower fO 2 was simulated in one experimental series at T=1000°C where CO 2 was added to the fluid phase in order to lower the aH 2 O and, thus, the fO 2 (e.g., [START_REF] Scaillet | Experimental Crystallization of Leucogranite Magmas[END_REF]. These experiments aim to answer the question of whether extensive hydrothermal influence and, thus, water-saturated conditions are required to produce anatectic melts at the AML roof. Additionally, two experimental series were performed at higher pressure (i.e. 200 MPa) at T=1000 and 1030°C to test the pressure influence on phase stabilities (corresponding to ~ 6 km crustal depth). The pressure was monitored with a strain gauge manometer with an uncertainty of ±5 MPa. The temperature range over the sample was ±10°C. Experimental run products (six capsules with the six different starting materials were run at the same time) were isobarically quenched, using a rapid quench unit. All experimental conditions are listed in Table 2 and the detailed setup of the rapid-quench IHPV that was used is described in [START_REF] Berndt | A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts[END_REF].

Electron-probe micro analyzer (EPMA)

Experimental products (glass and minerals) were analyzed with the Cameca SX 100 electron microprobe in Hannover. For minerals, we used 15 kV high voltage, 15 nA beam current, 10 seconds counting time on peak and 10 seconds on background, and a focused beam. For analysis of water-bearing glasses, which is problematic due to "alkali-loss" effects (e.g., [START_REF] Morgan | Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses[END_REF], a special procedure was applied following [START_REF] Koepke | Petrogenesis of oceanic plagiogranites by partial melting of gabbros: an experimental study[END_REF].

The beam current was set to 4 nA and the counting time varied for the different elements; 4 s for K, 8 s for Si, Al, Ti, and Fe, 10 s for Na and Zr, 12 s for Mg, 16 s for Ca, and 30 s for Ba, P, S, Cl, and Mn. Alkalis were measured first. Background counting time was always the same as peak counting time. Whenever possible, a defocused beam with a spot size of 5, 10, or 20 μm was used, and Na loss was checked by comparing the results of measurements with different beam sizes on identical glasses of an experimental sample with larger melt pools.

Standard glasses with compositions similar to the experimental glasses and with known water contents have been used for estimating water contents in experimental glasses by applying the "by-difference" method (e.g., [START_REF] Devine | Comparison of microanalytical methods for estimating H 2 O contents of silicic volcanic glasses[END_REF]. Even though the uncertainties are relatively high (± 0.5-0.7 wt% according to [START_REF] Parat | Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C-O-H-S) on Phase Equilibria and Mineral Composition[END_REF]), the application of the "by-difference" method controls the problem of small melt pools precluding water determination with a direct method such as infrared spectroscopy. With known melt composition and water content the aH 2 O was calculated following the thermodynamic model of [START_REF] Burnham | The nature of multicomponent aluminosilicate melts[END_REF] for a given water solubility in a melt subjected to known pressure (major effect) and temperature (minor effect).

RESULTS

Attainment of equilibrium

Attainment of global equilibrium in partial-melting experiments is strongly dependent on the grain size (see review of [START_REF] Johannes | Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite[END_REF]. The use of fine-grained starting material (i.e. ≤10 µm) enhances the chance of achieving overall equilibrium. However, in-situ analyses of newly-formed experimental phases are virtually impossible due to tiny crystals and melt pools (e.g., [START_REF] Koepke | An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands[END_REF]. Consequently, here we use a relatively large grain size of 125-200 µm. As expected, in some experiments cores of unaffected starting material are still present after the experiment (e.g. clinopyroxene at D11 and T ≤970°C, Fig. 2).

However, longer run durations (i.e. several weeks) did not guarantee that residual mineral cores in the starting material would dissolve (e.g., [START_REF] Wolf | Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time[END_REF][START_REF] Johannes | Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite[END_REF]). The partial melting experiments of [START_REF] Johannes | Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite[END_REF] used a starting material with a grain size of ~ 10 µm; even after 36 days, unreacted cores of the starting material were observed. Thus, the possibility of disequilibrium melting and the subsequent effect on interpretation of results should be considered. [START_REF] Wolff | The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust[END_REF] addressed the potentially unsystematic effects caused by unreacted starting material cores which do not take part in the general melting reaction on both phase relations and phase compositions. Because the effect is strongest at low temperatures, this should be considered when comparing low-temperature runs with those performed at higher temperature. However, the achievement of at least local equilibrium in our experiments is supported by several observations: (1) Compositions of newly-formed mineral phases, both as isolated crystals and as rim growth, are homogeneous within a sample, i.e., no zonation in newly-formed phases;

(2) newly-formed crystals at higher temperature (e.g., olivine at ≥1000°C) are commonly euhedral; at lower temperatures plagioclase forms sponge-like grains that are homogenous in composition and widely infiltrated by melts (e.g., D10 at 970°C in Fig. 2); (3) all phase compositions vary systematically with temperature, and compositional trends are consistent with trends expected from the literature; and (4) melt compositions vary systematically with temperature and are homogeneous within each experiment.

Experiments close to the solidus using altered dikes as starting materials (D10 and D11) were duplicated with mm-sized pieces of the starting material in order to obtain melt pools large enough for de-focused EPMA measurements. It is obvious that the conditions of these experiments are far away from global equilibrium conditions. However, when comparing phase compositions (melt and minerals) of these experiments with those performed with a fine-grained powder under the same conditions, it can be stated that they are identical within analytical error. Such an experimental procedure has previously been shown to be suitable [START_REF] Wolff | The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust[END_REF].

Phase Relations

Phase relations of the experiments are illustrated in Figure 3a and highlighted with backscattered electron (BSE) images for starting material D11 at different temperatures and for all starting compositions at 970°C in Figure 2 (see also experimental conditions in Table 2). Each experiment was analyzed with ImageJ image processing and analysis software [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF]) which determined solid phase and melt proportions using BSE image grey scales. In most experiments, the residual assemblage is composed of clinopyroxene + plagioclase (except for GD14 at T=1030°C) + titanomagnetite (except for D10, D11, and GF45 at T=1030°C). Ilmenite is stable in all runs at T≤970°C. Olivine occurs only at higher temperatures (variable from 940°C to 1000°C) and replaces orthopyroxene. Orthopyroxene and olivine only coexist in the GF36 composition at T=970°C. The reaction from orthopyroxene to olivine with increasing temperature is crucial because this relation can be used for constraining the temperature of granoblastic hornfels formation, since the natural rocks are in general orthopyroxene-bearing and practically free of olivine. Olivine is present in most of our experiments at T≥970°C except for in sample D11 (only present at T=1030°C) and for GF45 (only present at T≥1000°C), implying that the formation temperatures of the natural granoblastic lithologies were generally below 1000°C.

Although most experiments were water saturated, amphibole is absent in all experimental products. This is in contrast to [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] who produced strongly peraluminous water-saturated melts that coexisted with an amphibole-rich residual assemblage. Such a scenario is also observed in granoblastic rocks from the Troodos ophiolite where amphiboles occur as interstitial crystals and are interpreted to be magmatic [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF]. However, magmatic amphiboles are not observed in granoblastic rocks from Site 1256. Moreover, results of our study show that for the boundary conditions applied here, amphibole is not a required phase for the residue, implying that the absence of amphibole in natural granoblastic hornfels does not necessarily indicate that these rocks were formed under low aH 2 O. Phase relations and reactions for the starting materials that were used are formulated in Table 3 in order to emphasize the temperature at which the anhydrous granoblastic mineral assemblage could be reproduced.

Additionally, for one experimental series at T=1000°C and 100 MPa, beside water, a second fluid phase (CO 2 ) was added in order to test the effect of lower aH 2 0 (<0.5) on phase relations and composition. As expected, a reduced aH 2 0 shifted the solidus temperature to significantly higher values. The phase relations and modal fractions of this experiment (~20 vol% melt, ~40 vol% plagioclase, ~30 vol% pyroxenes, ~10 vol% Fe-Ti oxides; Fig. 3a) is very similar to the corresponding run performed under water saturation at T=940°C. Hence, runs with "dry" granoblastic hornfels as starting material (GF36, GF45) are below the solidus under a reduced aH 2 0. Crystallization of Fe-Ti oxides proceeds at significantly higher temperatures in waterundersaturated experiments compared to the corresponding experiments at water saturation, which is probably due to the more reduced redox conditions at lower aH 2 0 (e.g., [START_REF] Berndt | An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200MPa[END_REF]) which destabilizes Fe-Ti oxides.

In addition to the 100 MPa experiments, two runs (T=1000°C and 1030°C) were performed at 200 MPa and water-saturated conditions in order to study the influence of pressure on phase assemblages. Pressure is expected to influence phase relations because fluid and water solubilities are strongly dependent on pressure (e.g., [START_REF] Holtz | Effects of H 2 O on liquidus phase relations in the haplogranite system at 2 and 5 kbar[END_REF][START_REF] Holtz | Maximum and minimum water contents of granitic melts generated in the crust: a reevaluation and implications[END_REF][START_REF] Berndt | A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts[END_REF]. Previous crystallization experiments have shown that the main pressure effect is the shift of plagioclase saturation to lower temperatures (e.g., [START_REF] Feig | Effect of water on tholeiitic basalt phase equilibria: an experimental study under oxidizing conditions[END_REF]). Accordingly, plagioclase crystallization was suppressed in runs performed at 200 MPa, while remaining stable in all comparable experiments at 100 MPa (Fig. 3a). The 200 MPa pressure (equivalent to ~6 km depth) represents an unrealistic depth for the AML roof at the fast-spreading EPR, but may be suitable for slower spreading-rates (e.g. 5-7 km depth at the Mid-Atlantic Ridge; [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF][START_REF] Mccaig | Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading[END_REF]. Thus, experiments at higher pressure provide important preliminary data on the phases occurring in this geological setting, where coupled magmatic-hydrothermal interactions have already been recognized [START_REF] Mccaig | Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading[END_REF].

Composition of the anatectic melts

The degree of partial melting (melt fraction F; see Figure 3b) is, as expected, strongly temperature dependent but also varies significantly with the starting material. The first melts of D10 and D11, analyzable by EPMA, occur at T=910°C (F ≈ 10-20 vol%). First melts of GD12 and GD14 occur at T=940°C (F ≈ 10-20 vol%), and of GF36 and GF45 at T=970°C (F ≈ 10 vol%). This trend is in agreement with the increasingly refractory character of the starting material, varying from altered sheeted dikes (D10, D11) to partially re-crystallized (GD12, GD14) to fully re-crystallized granoblastic hornfels (GF36, GF45). The first melts of D10 and D11 are high in SiO 2 contents at 70.7 and 72.0 wt%, respectively. The first GD12 and GD14 melts = 67.4 and 68.4 wt% SiO 2 , respectively, while GF36 and GF45 are less evolved (61.7 and 61.9 wt% SiO 2 , respectively; Fig. 4, left column). Melt compositional evolution with increasing melt fraction and temperature is consistent for all starting materials and follows the expected trend (i.e. towards the bulk rock composition; Fig. 4 and Tab. 4).

In order to better track the effect of the different protoliths on the extent of enrichment or depletion with regard to the different elements at different melting degrees (or temperatures), all melt compositions were normalized to the starting material composition (Fig. 4, right column). These plots show that the degree of enrichment or depletion of a specific element in the melt is distinctly different depending on the protolith. A strong discrepancy between the different protoliths, even within the same lithological subtype (D10 compared to D11 and GD12 compared to GD14), is distinctive for Al 2 O 3 and the alkalis, which are mainly controlled by plagioclase crystallization (present in all experiments at 100 MPa except for GD14 at T=1030°C). With respect to Al 2 O 3 , starting material D11 is characterized by an increase with increasing temperature, while GD12, GF36, and GF45 decrease. D10 and GD14 increase initially up to T=970°C followed by a slight decrease at higher temperature. Al 2 O 3 in D11 is ~2.0 wt% lower at T≤970°C than are all other compositions, resulting in a weak enrichment when normalized to the starting material, while D10 is strongly enriched (Fig. 4). K 2 O is generally low in all experiments due to low concentrations in the protolith. However, the behavior for varying degrees of melting is significantly different. Although the K 2 O content is relatively constant for the anatectic melts of the altered basalt, anatectic melts of one "hydrous" (GD12) and one "dry" (GF45) granoblastic hornfels are strongly enriched with initial melting; this is clearly shown when normalized to the starting material (Fig. 4). In contrast, Na 2 O is only subtly lowered with increasing temperature within individual samples even though the variation between the different protoliths is significant (Fig. 4). Moreover, the normalized starting material diagram for Na 2 O reveals the challenge of achieving global equilibrium in partial melting experiments. The normalized value of sample D10 at T=940°C is far from the trend, suggesting that in this experiment the number of relict phases is particularly high. However, as this is the only obvious outlier, it can be assumed that, although present, relict phases in other experiments have less influence over the chemical composition of the melt. TiO 2 and FeO in the melt are strongly affected by a decrease in magnetite and ilmenite crystallization with increasing temperature. TiO 2 exhibits relatively constant increases up to T=1000°C where the highest content can be observed, followed by a slight decrease at higher temperature (Fig. 4). This peak is due to the absence of ilmenite at T ≥1000°C. P 2 O 5 contents in the melt (see Fig. 7c in the Discussion) are generally below 0.4 wt% except for GD12 and GD14 at T=970°C where an increase of up to 0.62 wt% is observed. Cl content in the melt (see Fig. 7d), which can be used as a tracer for sea water contamination due to hydrothermal circulation, is strongly dependent upon the Cl contents of the starting material (1000 ppm for GD12, 300-600 ppm for all others, Table 1) and is accordingly highest in the first melt using the GD12 starting material, reaching 0.91 wt% in the run performed at T=940°C (for a detailed description and illustration of Cl and P 2 O 5 see section "Comparison of the experimental melts with felsic rocks from Site 1256" in the Discussion).

Experiments at T=1000°C with water-undersaturated samples (i.e. ~0.8 wt% H 2 O, aH 2 O ~0.1 for D10; ~2.0 wt% H 2 O, aH 2 O ~0.45 for D11, GD12, and GD14) at 100 MPa correspond in terms of melt fraction and mineral proportions to those performed at T=940°C at water saturation (Fig. 3). Thus, GF36 and GF45 are below the solidus. D10 contains 65.7 wt% SiO 2 at aH 2 O ~0.1, while it shows minor enrichment at aH 2 O ~0.45 with variations between 54.9 and 58.9 wt%. Al 2 O 3 is distinctively lower at reduced aH 2 O resulting in contents from 11.3 to 14.4 wt%, in each case close to the starting material (Fig. 4). The FeO content at waterundersaturated conditions is notably higher, resulting in a potentially higher degree of Feoxide crystallization (e.g., magnetite) at lower temperature, while TiO 2 is only slightly affected by reduced aH 2 O and, thus, more reducing conditions. P 2 O 5 (0.4 to 1.0 wt%) and Cl (0.1 to 0.8 wt%) are higher in all water-undersaturated experiments than in water-saturated experiments (see Fig. 7).

200 MPa experiments (at T=1000 and 1030°C) display a high melt fraction (from 70 to 94 vol%) triggered by the higher water content (4.5 to 5.1 wt%) in the melt at higher pressure [START_REF] Berndt | A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts[END_REF]. This results in a melt composition similar to that of the starting material. SiO 2 contents do not exceed 54.4 wt% and the Al 2 O 3 content is too low to crystallize plagioclase, in contrast to experiments at shallower pressure (i.e., 100 MPa).

Compositions of newly-formed mineral phases

The average mineral compositions of the starting materials and the corresponding experiments are presented in Table 4. In most experiments, at low temperature and correspondingly low melt fraction, the assemblages and the compositions of the newly-formed minerals correspond to those of the natural granoblastic hornfels from the IODP Site 1256 or the Oman ophiolite dike/gabbro transition (e.g., [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF][START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF][START_REF] Zhang | Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge[END_REF]). At higher temperature and higher melt fraction, orthopyroxene is often replaced by minor amounts of olivine, which is not present in natural granoblastic hornfels. Because olivine is absent in all starting materials it is always newly formed. It is stable at T≥970°C, except for starting material GF45 (≥1000°C) and D11 (only at 1030°C) and mostly forms euhedral crystals (see BSE images in Fig. 2). Forsterite contents increase with temperature from 61.2 mol% (GD14 at 970°C) to 76.8 mol% (GD12 at 1030°C). All other olivine-bearing experiments fall within this range.

Newly formed clinopyroxenes are present in all experiments at 100 MPa both at the rim of residual protolith clinopyroxenes and as isolated euhedral crystals (see BSE images in Fig. 2)

Compositions are always homogeneous within individual experimental runs. The application of the Al-in-clinopyroxene thermometer of [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF], which was calibrated in a comparable system, reveals good agreement with the experimental temperature. While TiO 2 is in a narrow range within all experiments using one starting material (Fig. 5 Plagioclase is present in all 100 MPa experiments (except for GD14 at T=1030°C). It occurs either as euhedral crystals or as rim overgrowths around relic cores. The Anorthite (An) content (in mol%) varies systematically with temperature with an average increase of <2 %/10°C for D11 and up to ~5 %/10°C for GF36 and GF45. An contents vary up to 20 mol% between the different protolith compositions at the same temperature (Fig. 6). The FeO t content of plagioclase increases with temperature coinciding with the increase of FeO t in the melt, up to 1.39 wt% (D10 at 1000°C). In D10 and D11 metastable An was observed. As previously proposed by [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] this phase may form during the breakdown of prehnite, which is present as an alteration phase in the starting material, following the reaction:

1 prehnite ↔ 2 anorthite + 1 wollastonite + H 2 O.
Titanomagnetite is present in all experimental runs up to T=1000°C and, additionally, in GD12, GD14, and GF36 at T=1030°C. TiO 2 content varies between 9.09 wt% (GF36) and 16.52 wt% (D10). Ilmenite is only present in experiments up to T=970°C; its Al 2 O 3 content is generally relatively low (≤0.5 wt%, except for D11 at T=910°C), while MgO uniformly increases with temperature (from 2.79 to 4.99 wt%).

Elevated pressure (i.e. 200 MPa) has only a minor effect on the composition of the crystallized phases when compared to experiments at the same temperature at 100 MPa.

Lower aH 2 O (<0.5) influences the chemistry of crystallized phases; this is mainly due to the change to more reducing conditions. Olivine equilibrates at higher FeO contents (from +6 to +16 wt%), and, thus, to lower Mg# (down to 24 units lower). FeO in clinopyroxene and, in particular, orthopyroxene is also enriched (up to 11 wt% higher) compared to all other experiments at aH 2 O = 1, while plagioclase compositions transform to more evolved compositions that correspond to experiments performed at lower temperatures under water-saturated conditions. The Fe 2+ /ΣFe-ratio calculated for oxides is lower at water-saturated conditions (e.g., for D10: Fe 2+ /ΣFe = 0.8-0.9 at aH 2 O ~0.1, and Fe 2+ /ΣFe = 0.5 at aH 2 O = 1).

DISCUSSION

Influence of the starting material composition on anatectic products

One of the main objectives of the present study was to evaluate the influence of the starting material composition (modal and chemical) on the modal and chemical composition of experimental results. Our results will be compared hereafter to [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF], who examined partial melting effects of a fully altered dike from the Oman ophiolite, and to selected experiments of [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF], who studied the anatexis of greenschist and hornblende hornfels at 100 MPa. As a whole, anatectic melts produced in the present study display lower SiO 2 concentrations than melts produced at similar temperatures by Beard and Lofgren (1991) (Fig. 7). This is probably related to the more primitive and less altered character of our starting material. However, this discrepancy with our experimental melts is limited to higher temperature (or high partial melting degree). It is less pronounced for [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] first dry anatectic melts at lower temperature (i.e. ≤970°C), whereas their low temperature wet partial melts remain at relatively low SiO 2 . This is due to a distinctively higher melt fraction with added water (+10 to 15 vol% at T=950°C). However, in experiments with low melt fraction at temperatures close to the solidus (~900°C) all anatectic melts (wet and dry) of [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF], [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF], and altered basalts from this study (D10, D11) exhibit SiO 2 contents in a similar range (~70 to 75 wt%).

The alteration of protoliths mainly influences the melt composition for minor elements that are particularly sensitive to hydrothermal alteration (e.g., K, Cl). As an example, the observed enrichment in K 2 O is highly sensitive to the K 2 O content of the starting material. Anatectic melts are generally K 2 O-poor in our results (starting material K 2 O ~ 0.1 wt%), and strongly enriched in [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] and [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF]; i.e., starting material K 2 O=0.20 and 0.16 wt%, respectively. K 2 O can be enriched in the starting material due to alteration by seawater circulation, which is in agreement with the fully altered character of the [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] starting material and the amphibole rich/pyroxene poor starting materials of [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF].

Although K 2 O in sample GD12 (0.09 wt%) is similar to that in the other starting materials used it is significantly enriched in the first melts. That is not the case for the five other samples used in this study (Fig. 7b) which also include potentially different melt fractions.

Despite low K 2 O in the protolith, GD12 generally shows the strongest alteration imprint. This is expressed by the substitution of clinopyroxene for secondary amphibole. GD12 also shows the highest Cl concentration in the protolith (0.10 wt%), potentially caused by rock/seawater interaction during hydrothermal alteration with the consequence that Cl contents of the anatectic melts leads to the most pronounced enrichment trend (Fig. 7d). The same effect is obvious for phosphorous, where the protolith with the highest P 2 O 5 (GD12) shows the strongest enrichment in the experimental melts (Fig. 7c). However, the concentration of Cl and P 2 O 5 in protolith GD12 is relatively small compared to the average of Cl and P 2 O 5 in the D10, D11, and GD14 protoliths (Cl: 0.10 to 0.05; P 2 O 5 : 0.17 to 0.12; see Table 1). Moreover, despite the distinctive altered character of [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] starting material, that material is low in P 2 O 5 (0.12 wt%; Cl not given) and their experimental melts show only moderate enrichment trends for both Cl and P 2 O 5 . Thus, the degree to which the protoliths have been altered seems to have a minor influence on the concentration of phosphorus in anatectic melts, but rather the effective concentration of phosphorus in the starting material phase assemblages. Consequently, P 2 O 5 concentration in the melt is dominated by the amount of apatite and the degree of apatite melting. Although Cl is slightly enriched in GD12, an additional influence of the alteration degree on strong Cl enrichment is likely to be the major source for elevated Cl contents in seafloor alteration.

The MORB normalized diagram [START_REF] Gale | The mean composition of ocean ridge basalts[END_REF]) of all oxides for experiments with a melt fraction ≤20 vol% highlights the different influence of our starting materials and those of [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] and [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF] on the generated melts (Fig. 8). Anatectic The large span in crystallized plagioclase composition is also reflected in the difference in the starting materials (Fig. 6). In experiments at a given temperature, the spread in An values is ~ 20 mol% (T≤940°C), caused by varying Ca/Na ratios of the different starting materials (D11 > D10; GD12 > GD14). This spread decreases with increased temperature, revealing a deviation of only 6 mol% at 1030°C at generally high An values (>80 mol%).

Temperature evolution through the sheeted dike complex root zone

Equilibrium temperatures of the peak metamorphic overprint recorded by two-pyroxene thermometry [START_REF] Brey | Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers[END_REF][START_REF] Andersen | QUILF: A pascal program to assess equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, olivine, and quartz[END_REF]) of pyroxene hornfels, sampled at the AML roof either in modern oceanic crust or in ophiolites, give an estimation of temperatures of the anatectic processes in the dike/gabbro transition (e.g. [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF][START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF]Koepke and Goetze, in press).

Temperature calculations of granoblastic rocks from Hess Deep, for example, result in an average temperature of 900 to 950°C [START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF], perfectly matching the temperatures of our experiments for relatively small melt fractions, which produce highly-evolved partial melts under water-saturated conditions. Similar values were estimated for hornfels from the Troodos and Oman ophiolite dike/gabbro transitions (e.g., T=850-1050°C; [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF]. Amphibole-plagioclase thermometry generally yields lower temperatures for the studied sample suites (400-700°C), reflecting the record of hydrothermal alteration at relatively high temperatures (amphibolite facies) mainly after the contact metamorphic overprint [START_REF] Gillis | Anatectic Migmatites from the Roof of an Ocean Ridge Magma Chamber[END_REF][START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF][START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF].

For two-pyroxene hornfels of the dike/gabbro transition penetrated at Site 1256, several studies deal with the temperature evolution of this particular zone [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] Koepke | Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges[END_REF][START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF][START_REF] Zhang | Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge[END_REF]. While [START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF] applied the twopyroxene thermometer of [START_REF] Brey | Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers[END_REF] and report equilibrium temperatures of 800°C-924°C, the QUILF two-pyroxene thermometer of [START_REF] Andersen | QUILF: A pascal program to assess equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, olivine, and quartz[END_REF] applied by [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] Koepke | Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges[END_REF] and [START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF] determined distinctly higher temperatures for the peak metamorphic overprint (935°C-1045°C), where the metamorphic grade is highest (presence of two-pyroxene hornfels). In those granoblastic hornfels from the Site 1256 dike/gabbro transition, which underwent hydrothermal alteration after the metamorphic imprint, amphiboles are ubiquitous, enabling the application of amphibole thermometers (i.e., plagioclase-amphibole by [START_REF] Holland | Non-ideal interactions in calcic amphiboles and their bearing on amphiboleplagioclase thermometry[END_REF][START_REF] Holland | Non-ideal interactions in calcic amphiboles and their bearing on amphiboleplagioclase thermometry[END_REF]Ti-in-amphibole by Ernst and[START_REF] Ernst | Experimental phase-equilibrium study of Al and Ti-contents of calcic amphibole in MORB -A semiquantitative thermobarometer[END_REF]. As noted above, results reveal distinctly lower equilibrium temperatures for this event. [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF] obtained temperatures of 708°C-854°C and 650°C-747°C by using the plagioclase-amphibole thermometer and the Ti-in-amphibole thermometer, respectively. Even though [START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF] calculated temperatures with the Ti-in-amphibole thermometry of up to 913°C for individual samples, the majority of estimated temperatures are significantly lower, consistent with the hypothesis that hornfels cooling proceeded under hydrous conditions due to seawater-derived hydrothermal circulation.

Comparison of the experimental melts with felsic rocks from IODP Site 1256

The great advantage of using IODP Site 1256 samples as starting material is that experimental melts and residues can be directly compared to their natural equivalents. The compositions of mm-to-cm-thick felsic veins, consisting of tonalites cutting granoblastic hornfels in the drill core from Site 1256, are used for comparison. These are regarded as products of in-situ partial melting of AMC roof basalts [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF][START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF][START_REF] Zhang | Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge[END_REF]. In Figure 7 we also included the compositions of small felsic veins cutting hornfels sampled from the dike/gabbro transitions of the southern Oman Semail ophiolite blocks [START_REF] Freund | Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fast-spreading Pacific-Antarctic Rise by assimilation-fractional crystallisation[END_REF]; these are also regarded as anatectic melts generated in the roof rocks of AMLs [START_REF] France | Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges[END_REF][START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF]. It is important to note that plagiogranites in the Oman ophiolite, in general, are very heterogeneous in composition and size and are believed to have been formed by different processes (i.e., fractional differentiation of wet, subduction-zone related basalt or melting of sediments from the subducted slab [START_REF] Rollinson | New models for the genesis of plagiogranites in the Oman ophiolite[END_REF][START_REF] Haase | Melts of sediments in the mantle wedge of the Oman ophiolite[END_REF].

The relatively wide compositional range of the Oman felsic veins (i.e. SiO 2 ~ 58 -77 wt%) covers the complete range of SiO 2 contents of the experimental results at T≤ 970°C (Fig. 7).

While most major and minor elements of the experimental melts with relatively low melt fraction (≤20 vol%) fit quite well with 1256D tonalites, there is a large discrepancy with respect to Al 2 O 3 (Fig. 7). While the Oman plagiogranites show Al 2 O 3 contents similar to those of the experimental melts (Fig. 7), the tonalites from 1256D sampled to date are significantly lower in Al 2 O 3 . Al 2 O 3 in the melt is mainly influenced by plagioclase crystallization which is, in turn, controlled mainly by the prevailing aH 2 O (e.g., [START_REF] Gaetani | The influence of water on the petrogenesis of subductionrelated igneous rocks[END_REF][START_REF] Feig | Effect of water on tholeiitic basalt phase equilibria: an experimental study under oxidizing conditions[END_REF][START_REF] Koepke | Petrogenesis of crustal wehrlites in the Oman ophiolite: Experiments and natural rocks[END_REF]). Most of the experiments of this study were performed under watersaturated conditions with aH 2 O close to 1 (i.e. total H 2 O in the melt ~3.5 wt%) leading to high Al 2 O 3 contents in the melt (Fig. 7a), indicating that 1256D tonalitic melt generation proceeded under vast plagioclase crystallization as a function of reduced aH 2 O (see discussion below). K 2 O is very low in 1256D tonalites and Oman felsic veins (≤0.12 wt%, and ≤0.36 wt%, respectively) and, thus, accords with the experimental melt trends except for GD12, which is the sample showing strong hydrothermal alteration (Fig. 7b). While P 2 O 5 is generally low (0.11 to 0.38 wt%) for most experimental melts, it is significantly higher in less-evolved 1256D tonalites (0.58 to 0.64 wt%). Those high values are only observed in samples GD12 and GD14, which show a peak in P 2 O 5 at intermediate temperature/composition. At higher SiO 2 contents 1256D tonalites have the same P 2 O 5 range as experimental melts and even mirror the step towards lower P 2 O 5 between T=940°C and 910°C shown by altered basaltic dikes (Fig. 7c). This is most likely caused by stable apatite in the residue, which in turn would dramatically decrease the content of rare earth elements, Y, and Sr of the first melts because they are compatible in apatite and apatite is, thus, a trap for Y and Sr [START_REF] Prowatke | Trace element partitioning between apatite and silicate melts[END_REF]. This would be one explanation for the lower-than-expected trace element enrichments of anatectic melts in comparison to those predicted for melts generated by large extents of fractional crystallization [START_REF] Brophy | La-SiO[END_REF][START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF]. Oman plagiogranites are generally low in P 2 O 5 (≤0.36 wt%) and are in the range of experimental anatectic melts.

Chlorine concentrations are generally low, even in the very first experimental melts (≤0.35 wt%). This is in agreement with 1256D tonalites which are also low in Cl (max. 0.12 wt%) and which are well reproduced by the compositional trend of melts from D11 starting material. Only the melts generated with starting material GD12, which is the sample with the most pronounced secondary alteration, is again out of the range compared to the melts produced by the other protoliths and contains 0.91 wt% Cl. In Figure 7d we also plotted evolved lavas from the 9° N overlapping spreading center (OSC) at the EPR [START_REF] Wanless | Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers[END_REF]. Wanless et al.'s (2011) andesitic and dacitic lavas are enriched in Cl (up to 0.71 wt%) and reflect the trend to Cl-enriched experimental melts found in GD12. [START_REF] Wanless | Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers[END_REF] concluded that this enrichment is, in part, due to a combination of partial melting and assimilation of a previously hydrothermalized protolith at the roof of the AML. The results of our study show that such Cl enrichment in anatectic melts is possible, provided that the protoliths are strongly altered, as is the case for sample GD12.

Comparison of the experimental residue with granoblastic hornfels from IODP Site 1256

As discussed above, in most of our experiments performed at lower temperatures, the residual minerals coexisting with felsic melts are plagioclase, clino-and orthopyroxene, and Fe-Ti oxides; exactly the same minerals compose the granoblastic hornfels at the roof of AMLs. In the following, the experimental residual phases are compared with mineral data from natural granoblastic hornfels recovered from Hole 1256D [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF]Koepke and Goetze, in press).

With respect to the TiO 2 versus Al 2 O 3 trend (Fig. 5), the high-temperature experiments (i.e. 1000°C and 1030°C) show clinopyroxene enrichment in both TiO 2 and Al 2 O 3 in comparison to the starting material. With decreasing temperature, TiO 2 and Al 2 O 3 decrease. For the hydrated basalts as protolith, clinopyroxene compositions at T=910°C (see example for starting material D10, Fig. 5a) match those of natural "dry" granoblastic hornfels from 1256D, but those from higher temperature deviate significantly. When using the more refractory "hydrous" and "dry" granoblastic hornfels as starting material (GD and GF), clinopyroxenes at all temperatures fit quite well with the clinopyroxene compositions of the natural rocks, except for the highest temperature condition (see examples for starting materials GD14 and GF36, Fig. 5b andc). Thus, TiO 2 and Al 2 O 3 in the experimental clinopyroxenes are controlled by both starting material and temperature, if the starting material for the natural processes at the dike/gabbro transitions was already refractory due to previous dehydration processes (corresponding to our GD and GF samples). Our results imply that a partial melting event proceeded at T˂1030°C, which is also consistent with the composition of the coexisting melt which is too low in SiO 2 at T>1000°C compared to 1256D tonalites.

The plagioclase compositions in granoblastic hornfels from Site 1256 vary only slightly, between 49 and 56 mol% An. The corresponding equilibrium temperatures for the granoblastic hornfels were estimated by the two-pyroxene geothermometer [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF]Koepke and Goetze, in press) and are shown in Figure 6. The experimental plagioclases produced with D11 and GD12 starting materials are generally too high in An when compared to the natural hornfelses from Hole 1256D (Fig. 6), which is a consequence of the high Ca/Na ratios in these rocks (see above). Plagioclases from the other starting materials at T≤970°C are within the compositional range of the plagioclases from the natural hornfels. However, all these evaluations are based on experiments performed under water saturation. In the following, we argue that this was probably not the case during the anatectic processes at the roof of the Site 1256 AML.

The role of water activity

There are several lines of evidence that partial melting processes at the roof of the AML, producing felsic melts and granoblastic hornfels, proceed under reduced aH 2 O rather than at water-saturated conditions.

(I)

When comparing the Al 2 O 3 contents of the experimental melts generated under water-saturated conditions with 1256D tonalites, it is obvious that the experimental melts contain significantly higher Al 2 O 3 , provided that sampled 1256D tonalites are representative of the partial melt composition (Fig. 7a).

Most of our experiments were performed under water saturation; it is wellknown that water surpresses plagioclase stability (e.g., [START_REF] Gaetani | The influence of water on the petrogenesis of subductionrelated igneous rocks[END_REF][START_REF] Feig | Effect of water on tholeiitic basalt phase equilibria: an experimental study under oxidizing conditions[END_REF][START_REF] Koepke | Petrogenesis of crustal wehrlites in the Oman ophiolite: Experiments and natural rocks[END_REF] 3b) resulting in SiO 2 contents of less than 62 wt%, which are lower than the Site 1256 felsic veins (Fig. 9b). Only by reducing the aH 2 O can higher SiO 2 contents at temperatures corresponding to those calculated for the hornfels be reached.

(III) At T=1000°C, which is in the range of natural granoblastic hornfels formation temperatures, experimental An contents match the plagioclase composition from the natural hornfels at water-undersaturated conditions (Fig. 6). The best fit at that temperature results from a run with aH 2 O ~ 0.1.

(IV) At the equilibrium temperatures estimated for natural granoblastic hornfels (average ~ 970°C; [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF]Koepke and Goetze, in press) in most of our water-saturated experiments, olivine becomes a stable residual phase, crystallizing at the expense of orthopyroxene (Fig. 3a), which is not in accord with the natural phase assemblage where olivine is rare. Despite minor amounts of olivine coexisting with orthopyroxene, our experiments show that reduced aH 2 O leading to lower melt fractions has the same effect as lower temperatures, thus shifting the 'olivine-in' curve to higher temperatures, which is in accord with the natural record.

These observations raise the question of how anatectic processes can occur under strongly water-undersaturated conditions if distinct hydrothermal influence at the dike/gabbro transition is assumed. This is especially true at low degrees of melting, where a small amount Hence, for partial-melting processes, only minor amounts of water are available due to previously-altered protoliths. In our model most of the water released due to the breakdown of alteration phases migrates up at the prevailing supercritical conditions because it is an 'open cooking pot' rather than a closed system with a lid. Thus, an aH 2 O below 0.5 can be reached.

The absence of amphibole-rich rocks as possible protoliths for felsic melt generation Amphibole-rich protoliths are often used to explain the generation of felsic melts at midocean ridges (e.g., [START_REF] Haase | Nb-depleted andesites from the Pacific-Antarctic Rise as analogs for early continental crust[END_REF][START_REF] Brophy | A study of rare earth element (REE)SiO2 variations in felsic liquids generated by basalt fractionation and amphibolite melting: a potential test for discriminating between the two different processes[END_REF][START_REF] Rollinson | New models for the genesis of plagiogranites in the Oman ophiolite[END_REF][START_REF] Wanless | Dacite Petrogenesis on Mid-Ocean Ridges: Evidence for Oceanic Crustal Melting and Assimilation[END_REF]).

However, one surprising aspect of this study is that amphibole was never produced as a residual phase in our experiments, although most experiments were performed under watersaturated conditions. The reason for the absence of amphibole is probably the very low pressure at which the experiments were performed, since it is well-known that too-low pressure destabilizes amphibole (e.g., [START_REF] Rutherford | Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in the 1995-2002 Soufrière Hills Magma[END_REF]. The absence of amphibole is in agreement with the natural drilling record from IODP Site 1256, where amphibole exists in those two-pyroxene hornfelses from the lowermost horizon of the sheeted dikes only as a secondary phase generated by a stage of hydrothermal overprint after the peak metamorphism (e.g. [START_REF] Zhang | Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge[END_REF]Koepke and Goetze, in press). Amphibole as part of the prograde metamorphic paragenesis occurs only at a higher crustal level where the heat supply of the underlying AML is minor, away from the zone where the main anatectic processes are ongoing. [START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF] and [START_REF] Alt | Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256[END_REF] present equilibration temperatures varying from 650°C to 854°C and from 464°C to 913°C, respectively, for amphiboles in hornfelses, and similar amphibole-bearing hornfelses with equilibrium temperatures between 850 and 900°C were reported from Pito Deep (EPR) and the Troodos ophiolite [START_REF] Gillis | The roof of an axial magma chamber: A hornfelsic heat exchanger[END_REF]). These temperatures are in the range of the solidus of altered basalts [START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF]this study). Thus, such rocks may be regarded as residues remaining after a partial melting process, provided the aH 2 O was very high; this possibility can be excluded for the anatectic conditions at Site 1256. It seems that the critical point is whether water liberated during the partial melting reaction can escape the zone of reaction or not; such escape is, in turn, is depending on the velocity of the uprising heat source (heat emanating from the AML after replenishment), and the efficiency of establishing a fluid pathway system in the caprocks overlying the reaction zone.

CONCLUSION

We experimentally simulated anatectic processes occurring at the roof of an oceanic magma chamber at fast-spreading ridges with special focus on IODP drilling Site 1256, representing 15 Ma old crust generated at the EPR. To accomplish this, we partially melted different lithologies drilled at Hole 1256D and compared the experimental results (melts and residues)

with the natural counterparts from the same location (felsic rocks and 2-pyroxene hornfels).

Our results lead to the following conclusions:

(I)

The experimental melt composition is strongly influenced by the degree of alteration/recrystallization of the starting material. The first melts in the comparative experimental studies of [START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] Moreover, the use of granoblastic two-pyroxene hornfels as starting material leads to low F (e.g., F~10 vol% at T=970°C). However, the corresponding melts are too low in SiO 2 when compared with the natural tonalites, implying that these rocks are unlikely to be protoliths (Fig. 9).

( lithostratigraphic column (modified after [START_REF] Wilson | Drilling to Gabbro in Intact Ocean Crust[END_REF]. The sampled section of the column is shown by the small black rectangle in the schematic cross-axis view of the magmatic system at fast-spreading ridges (modified after [START_REF] France | Contamination of MORB by anatexis of magma chamber roof rocks: Constraints from a geochemical study of experimental melts and associated residues[END_REF]). This model (not to scale, vertical extent ~6 km) highlights the position of the AML (mainly composed of pure melt, yellow color) and the underlying crystal mush (less than 20% melt). At the base of this crystal mush another melt lens is illustrated (yellow) which corresponds to a model from [START_REF] Natland | Paired melt lenses at the East Pacific Rise and the pattern of melt flow through the gabbroic layer at a fast-spreading ridge[END_REF]. The conductive boundary layer (CBL) is highlighted in purple; it consists mainly of granoblastic hornfels This curve highlights the result that that D samples can only be affected by anatexis when the AML and, thus, the temperature curve migrates upwards.

  ), Al 2 O 3 and Mg# (Mg# = Mg/(Mg+Fe total )) continuously increase with temperature from Al 2 O 3 = 1.35 wt% and Mg# = 65 (D10 at 910°C) to Al 2 O 3 = 3.19 wt% and Mg# = 87 (D10 at 1030°C). Clinopyroxene compositions of all other experiments fall within this range. Orthopyroxene typically grows at the rim of clinopyroxene relics and only rarely forms euhedral crystals. Mg# (62.3 to 70.0), Al 2 O 3 (0.90 to 1.45 wt%), TiO 2 (0.25 to 0.34 wt%), and CaO (1.71 to 2.21 wt%) increase systematically with temperature, best demonstrated by sample D11, where orthopyroxene is stable over the widest temperature range (i.e. from 910°C to 1000°C).

  melts from other experimental studies generally exhibit lower MORB normalized values and are distinctly enriched in K 2 O. Moreover, a different evolution in MORB normalized values is apparent for France et al.'s (2010) anatectic melts which exhibit a step from low TiO 2 to enriched Al 2 O 3 , followed by low values for FeO, MgO, and CaO and strong enrichment in Na 2 O and K 2 O finalized by pronounced low P 2 O 5 values. This deviation from the anatectic melts of this study highlights again the difference between the Oman ophiolite protolith used by France et al. (2010) and the IODP Site 1256 protoliths used in this study.

  of water in the protolith can lead to high aH 2 O in the melt. Hence, for low aH 2 O virtually no water can be present close to the melt lens and water-bearing alteration phases of potential protoliths must lose their water at the initial stage of partial melting. Additionally, pressure fluctuations should be minor because decreasing pressure, caused by an ongoing upward movement of the AML, in turn increases the aH 2 O. A possible model (Fig.10b) allows the hydrothermal flux to penetrate most of the oceanic crust until deep in the sheeted dike complex. However, at a certain distance from the AML (~50 to 100m according to[START_REF] Zhang | Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge[END_REF] prevailing conditions hamper water migration deeper into the conducting boundary where anatexis starts. In addition, due to the metamorphic imprint triggered from below by the heat of an advancing melt lens, alteration phases containing H 2 O or OH -groups produced by an earlier hydrothermal alteration lose their water, causing the protolith to become "dry".

FIGURE CAPTIONS Fig. 1

 CAPTIONS1 FIGURE CAPTIONS

Fig. 2

 2 Fig. 2 Backscattered electron images of selected experimental results under water-saturated conditions. On the left, the complete temperature range (i.e. 910°C to 1030°C) of starting material D11 is shown. On the right, all different starting materials are compared for T=970°C. The scale (200 µm) is the same for each image. Mineral abbreviations are the same as in Table 3

Fig. 3 a

 3 Fig. 3 a Phase proportions over the experimental temperature range for the different starting materials estimated by ImageJ (Schneider et al. 2012). Mineral abbreviations are the same as in Table 3; magnetite and ilmenite are summarized as FeTi-ox. In the box at the upper right of each graph the composition of experiments performed at 200 MPa and low aH 2 O, respectively, at the corresponding temperature are inserted as pie charts. Colors refer to the proportions of the corresponding phases indicated in the diagrams b Comparison of the melt fraction (F) evolution with increasing temperature for the different starting materials. Experiments at 200MPa and low aH 2 O, respectively, performed at T=1000°C are highlighted. Black data points refer to France et al.'s (2010) experiments.

Fig. 4

 4 Fig. 4 Compositional evolution of experimental melts of different starting materials in wt% as a function of temperature (left column). The compositional evolution of the starting material was normalized in order to remove the influence of varying starting material composition (right column). Experiments performed at low aH 2 O are highlighted with blue data points. Black data points refer to France et al.'s (2010) experiments.

Fig. 5

 5 Fig. 5 Clinopyroxene compositions as a function of TiO 2 and Al 2 O 3 . Shown are the average residual clinopyroxene compositions from experiments with starting materials D10 (a), GD14 (b), and GF36 (c) with a comparison to the clinopyroxene composition of natural "dry" (i.e. amphibole-free) granoblastic hornfels from IODP Site 1256. For this comparison, clinopyroxene compositions of "dry" granoblastic hornfels from this study (GF36 and GF45; black dots) are shown, as well as those measured by Koepke et al. (2008) and Koepke and Goetze (in press). The starting composition in c is shown by the black data points. Error bars represent the standard deviation of the single measurements.

Fig. 6

 6 Fig.6An content of plagioclase as a function of temperature. Shown is the average plagioclase composition of the different starting materials and the average composition of newly-formed plagioclases from the experiments. Blue data points highlight experimental plagioclases formed under reduced aH 2 O (aH 2 O=0.1 for D10, aH 2 O~0.5 for the others). Note that the plagioclases of the starting material from D10, D11, and GD12 correspond to the primary magmatic plagioclase while those from GD14, GF36, and GF45 are of the metamorphic granoblastic assemblage. The grey field corresponds to natural plagioclases of granoblastic hornfels from IODP Site 1256. The temperatures were obtained by[START_REF] Koepke | Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes[END_REF] andKoepke and Goetze (in press) by applying the two-pyroxene geothermometer in the corresponding natural rocks.

Fig. 7

 7 Fig.7Harker diagrams (oxide versus SiO 2 ). Comparison of experimental melts from this study with experimental studies from[START_REF] Beard | Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb[END_REF] and[START_REF] France | Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations[END_REF]. Also shown are natural tonalite rock data from IODP Site 1256 (this study) and data from Oman ophiolite felsic veins[START_REF] Freund | Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fast-spreading Pacific-Antarctic Rise by assimilation-fractional crystallisation[END_REF]. Instead of Oman felsic veins for Cl (d), a comparison to dredged lavas from the 9°N overlapping spreading center (OSC) at the EPR is plotted[START_REF] Wanless | Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers[END_REF]. The bulk rock composition of the starting materials is marked with stars, experiments performed at reduced aH 2 O are highlighted with blue data points.

Fig. 8

 8 Fig. 8 Major element-MORB normalized diagram (using all MORB from Gale et al. 2013) for experimental melts and natural tonalites from IODP Site 1256 (grey field). Shown are all experiments from this study and from Beard and Lofgren (1991) and France et al. (2010) with a melt fraction ≤20 vol% in comparison to 1256D tonalites. Experiments performed at reduced aH 2 O are highlighted with blue data points.

Fig. 9 a

 9 Fig. 9 a Al 2 O 3 versus SiO 2 as shown in Figure 7a focusing on experiments performed at reduced aH 2 O. Trends for the different aH 2 Os are highlighted with arrows. b SiO 2 (wt%) of the experimental melts as a function of temperature as shown in Figure 4a including the range of natural 1256D tonalite rock data obtained by this study and the temperature calculations of Koepke et al. (2008). The shift due to reduced aH 2 O is highlighted.

Fig. 10

 10 Fig. 10 Schematic model of the proposed stratigraphy at the oceanic magma chamber roof under a) water-saturated conditions and varying temperature or b) constant temperature (1000°C) and different aH 2 O. Shown are the generated felsic melts with melt volume (in vol%) and SiO 2 content for the different starting materials (with the average for each lithology) at the relative depth where they were sampled as a function of temperature (a) and aH 2 O (b). The granoblastic overprint is highlighted with purple dots. Blue dashed lines represent the hydrothermal cells (a and b) and potential dehydration at reduced aH 2 O (b only). The red curve in (a) is based on calculated 2-pyroxene maximum temperatures from Koepke et al. (2008) which are adapted for the depth from which the GD samples were taken. This curve highlights the result that that D samples can only be affected by anatexis when the AML and, thus, the temperature curve migrates upwards.

  2 , Al 2 O 3 , and K 2 O and depleted in TiO 2 for a given MgO content as consequence of formation in a fore arc of a subduction zone (e.g. MacLeod et al. 2013). The starting material used in France et al. (2010) was thus relatively enriched in SiO 2 , Al 2 O 3 , and K 2 O (54.1, 16.2, and 0.2 wt%, respectively, on an anhydrous basis); this rarely corresponds to the more primitive MORBs at Site 1256. Moreover, their starting material was fully altered (greenschist facies condition)

  , resulting in melts which are strongly enriched in Al 2 O 3 compared to corresponding experiments with reduced aH 2 O.

		However, this enrichment in Al 2 O 3 relative to starting compositions is not
		present in experiments performed at 1000°C and aH 2 O ≈ 0.5 (D11, GD12,
		GD14) and at aH 2 O ≈ 0.1 (D10) (Fig. 9a). Moreover, the trend of 1256D
		tonalites is perfectly matched by the strongly water-undersaturated D10 sample
		(aH 2 O ~ 0.1). Figure 9a shows that either a very low aH 2 O is required during
		the anatectic processes (i.e. aH 2 O ≤0.1) at T~1000°C or a moderately-reduced
		aH 2 O (e.g., aH 2 O ~0.5) is required at T˂1000°C. It is important to add a note
		of caution here, since our database on tonalites from the drilled core at Site
		1256 comprises only three samples, a number which is too low to guarantee
		full representativeness. However, these rocks form a coherent group in terms
		of petrographic features and geochemical compositions. Thus, we believe that
		the conclusion drawn here is justified.
	(II)	The two-pyroxene geothermometer of Andersen et al. (1993) applied to the
		granoblastic hornfels (data from Koepke et al. 2008; Koepke and Goetze, in
		press) indicated equilibrium temperatures from 930 to 1045°C and is, thus, in
		accord with scenarios discussed above for very low or moderately-reduced
		aH 2 O. Irrespective of the high Al 2 O 3 contents, water-saturated melts over this
		temperature range exhibit a relatively high melt fraction (up to 50 vol%, see
		Fig.

  IV) At low F and high SiO 2 (>70 wt%), the low P 2 O 5 values of 1256D tonalites are experimentally reproduced. This implies that apatite is stable at these conditions. The relatively large amounts of rare earth elements, Y, and Sr found in apatite are held back from the melt, due to the high distribution Low aH 2 O is also indicated by the An content of plagioclase in the natural hornfels, which is best mirrored in the experimental residues at reduced aH 2 O.

	(VI) The lack of amphibole in our experimental residue supports the observations in
		two-pyroxene hornfels from IODP Site 1256 where amphibole only occurs as a
		secondary phase, generated by a late stage hydrothermal overprint (e.g. Zhang
		et al. 2014; Koepke and Goetze, in press). Thus, amphibole-rich protoliths for
		anatexis at the dike/gabbro transition from present day fast-spreading ridges,
		which are often used for chemical modeling of felsic rock suites in the oceanic
		crust, are probably less important.
		coefficients apatite has for these elements. This can explain the expected
		relatively low enrichment of anatectic melt in these trace elements (e.g.,
		Brophy 2009) compared to evolved melts generated by fractional
		crystallization.
	(V)	Because experimental melts are generally higher in Al 2 O 3 than are 1256D
		tonalites, the assumption of partial melting under water saturation has to be
		scrutinized. Experiments performed at water-undersaturated conditions exhibit
		a good correlation to natural counterparts with the best fit at aH 2 O=0.1.
		Moreover, only at reduced aH 2 O can a low melting degree and thus high SiO 2
		content in anatectic melts be realized in the temperature range obtained for the
		peak metamorphic event (930°C to 1045°C), when using the most likely

protolith (i.e. moderately-altered basaltic dike).

Table 1

 1 Comparison of the different starting compositions

Table 2

 2 Experimental conditions

Table 3

 3 Phase relations and reactions for the different starting materials

Table 4

 4 Composition of the experimental melt obtained from the different starting materials Table A1 electronic supplementary material Petrography and mineral modes of the different starting materials Table A2 electronic supplementary material Composition of the experimental residue and the corresponding phases of the starting rock

ACKNOWLEDGMENTS

We thank Otto Dietrich and Julian Feige for their careful sample preparation. The manuscript has been substantially improved after thorough reviews by M. Perfit and two anonymous reviewers. This research used samples and/or data provided by the International Ocean Drilling Program (IODP). IODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of the Consortium for Ocean Leadership (COL). Funding for this research was provided by grants from the Deutsche Forschungsgemeinschaft (KO 1723/13).

 (2014)