Mathieu Rospabé
email: mathieu.rospabe@get.omp.eu

Mathieu Benoit

Georges Ceuleneer

Florent Hodel

Mary-Alix Kaczmarek

Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers

Keywords: Oman ophiolite, dunitic mantle-crust transition zone, trace elements, melt-rock reactions, melt percolation, refertilization

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Introduction

The boundary between the Earth mantle and the oceanic crust is underlined by a dunitic horizon in most ophiolites worldwide, regardless the tectonic setting in which they evolved (mid-ocean ridges vs. supra-subduction zone) (e.g. [START_REF] Moores | The Troodos Massif, Cyprus and other Ophiolites as Oceanic Crust: Evaluation and Implications[END_REF][START_REF] Prinzhofer | Structures in the new caledonia peridotites-gabbros: Implications for oceanic mantle and crust[END_REF][START_REF] Boudier | Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains[END_REF][START_REF] Quick | Petrology and petrogenesis of the Trinity Peridotite, an upper mantle diapir in the Eastern Klamath Mountains, Northern California[END_REF][START_REF] Jan | The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan[END_REF]Kelemen et al., 1997a;[START_REF] Bédard | Formation of chromitites by assimilation of crustal pyroxenites and gabbros into peridotitic intrusions: North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada[END_REF][START_REF] Ceuleneer | The Trinity ophiolite (California): The strange association of fertile[END_REF][START_REF] Bouilhol | Magma and fluid percolation in arc to forearc mantle: Evidence from Sapat (Kohistan, Northern Pakistan)[END_REF]. Since dunites are observed in present-day oceans in different tectonic settings, associated to mantle harzburgites and to troctolitic-gabbroic veins and sills like in the dunitic mantle-crust transition zone (DTZ) (e.g. [START_REF] Arai | Petrology of gabbro-troctolite-peridotite complex from Hess Deep, equatorial Pacific: implications for mantle-melt interaction within the oceanic lithosphere[END_REF][START_REF] Dick | Late-stage melt evolution and transport in the shallow mantle beneath the East Pacific Rise[END_REF][START_REF] Parkinson | Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for mantle melting and melt-mantle interaction in a supra-subduction zone setting[END_REF]Savov et al., 2005;[START_REF] Godard | Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneath a slow spreading ridge[END_REF], it is reasonable to suppose that ophiolitic DTZ formed prior to the intra-oceanic thrusting leading to obduction.

A residual origin for dunites after very high (>40%) degrees of partial melting of the mantle [START_REF] Green | The genesis of basaltic magmas[END_REF] requires unrealistic thermal regimes, at least in the modern Earth and in oceanic spreading contexts [START_REF] Herzberg | Density constraints on the formation of the continental Moho and crust[END_REF]. Alternatively, dunite may result from the accumulation of olivine grains issued from fractional crystallization from a melt sufficiently rich in Mg to remain during a significant time in the fractionation field of olivine alone (+/-spinel) [START_REF] Bowen | The later stages of the evolution of the igneous rocks[END_REF]O'Hara, 1965;[START_REF] Elthon | High magnesia liquids as the parental magma for ocean floor basalts[END_REF]. Finally, dunite may be replacive after interaction ("reactional melting") between melts and peridotite in dry [START_REF] Bowen | The Origin of ultra-basic and related rocks[END_REF][START_REF] Berger | Les dunites en enclaves dans les basaltes alcalins des iles oceaniques: approche petrologique[END_REF] or hydrated systems [START_REF] Bowen | The System MgO-SiO 2 -H 2 O[END_REF]. The interaction between harzburgite and a melt undersatured in silica accounts for the formation of dunite through the dissolution of pyroxenes, leading to the enrichment in silica of the melt with concomitant crystallization of olivine [START_REF] Dick | Evidence of partial melting in the Josephine peridotite[END_REF][START_REF] Quick | Petrology and petrogenesis of the Trinity Peridotite, an upper mantle diapir in the Eastern Klamath Mountains, Northern California[END_REF][START_REF] Kelemen | Reaction between ultramafic rock and fractionating basaltic magma I. phase relations, the origin of calc-alkaline magma series, and the formation of discordant dunite[END_REF][START_REF] Kelemen | Formation of harzburgite by pervasive melt/rock reaction in the upper mantle[END_REF]. Petrological evolutions across a densely sampled cross section along a exclusively derive from melt-peridotite interaction, its shallowest part (topmost 50 m) showing fractional crystallization trends [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF].

In the melt-rock reaction model, it was proposed that reactional melting and orthopyroxene consumption leads to high intergranular permeability enhancing the efficiency of melt extraction from the mantle [START_REF] Ortoleva | Geochemical self-organization II: the reactive-infiltration instability[END_REF][START_REF] Daines | The transition from porous to channelized flow due to melt/rock reaction during melt migration[END_REF]Kelemen et al., 1995a;Kelemen et al., 1997b). In this way, the DTZ appears to be a reactive interface where melts are focused, transformed and potentially accumulated, then distributed beneath the midocean ridge. Crystallization associated to melt migration through the mantle-crust transition zone accounts for the wide diversity of lithological facies observed in the DTZ. Dunites evolve from "pure dunites", i.e. compacted at very high temperature when complete melt extraction occurred, to "impregnated dunites", highly refertilized before incipient cooling and that potentially leads in extreme cases to the formation of hybrid olivine-rich troctolites or plagioclase-bearing wehrlites (e.g. [START_REF] Arai | Petrology of gabbro-troctolite-peridotite complex from Hess Deep, equatorial Pacific: implications for mantle-melt interaction within the oceanic lithosphere[END_REF][START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF]Sanfilippo and Tribuzio, 2013a). Although interstitial minerals in ophiolites or abyssal DTZ are regularly found in equilibrium with melts that fed the overlying crust (e.g. [START_REF] Drouin | Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge[END_REF][START_REF] Ghosh | Moho transition zone in the Cretaceous Andaman ophiolite, India: A passage from the mantle to the crust[END_REF][START_REF] Nicolle | Major and trace element and Sr and Nd isotopic results from mantle diapirs in the Oman ophiolite: Implications for off-axis magmatic processes[END_REF], the Oman ophiolite DTZ was also percolated by melts with more exotic compositions [START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF] that, for a part of them, may be attributable to hybridization with a hydrothermal fluid component [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF].

Especially the presence of orthopyroxene, amphibole, garnet and diopsides both interstitially between olivine grains and in inclusion in chromite calls for early, high temperature, hydration of the DTZ. In this frame, to define the origin of the DTZ has direct implications on melt generation, transfer and evolution from the mantle to the crust, as well as on the deep hydrothermal circulations that may influence both the chemical exchanges and the thermal structure of the oceanic lithosphere.

Most geochemists use the trace element contents of interstitial clinopyroxene and plagioclase (if any) scattered in dunites to build petrological scenario (e.g. Kelemen et al., 1995a;[START_REF] Nicolle | Major and trace element and Sr and Nd isotopic results from mantle diapirs in the Oman ophiolite: Implications for off-axis magmatic processes[END_REF]Akizawa et al., 2016a). However, it concerns actually the genesis of crystallization products from melt fractions that travelled through the dunites, which might be risky to extrapolate to the genesis of the dunitic matrix itself. Very few whole rock analysis or olivine trace element contents are available due to their very low content in olivine. The sporadic studies providing trace elements data on dunite show that re-equilibration processes occurred between the olivine matrix, selectively enriched in some trace elements, and the melts percolating interstitially between olivine grains (e.g. [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002;[START_REF] Sanfilippo | Mantle-crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine[END_REF][START_REF] Sanfilippo | Water, lithium and trace element compositions of olivine from Lanzo South replacive mantle dunites (Western Alps): New constraints into melt migration processes at cold thermal regimes[END_REF].

To better constrain the processes leading to the formation of both pure and impregnated dunitic products constituting the DTZ as well as to their chemical signature, we combined major and trace elements in whole rock and in mineral phases for 79 samples of dunite from the Oman ophiolite mantle-crust transition zone. We show that the important chemical variability recorded by the dunites at Moho level is issued from a combination of harzburgite dunitization, re-equilibration associated to melt percolation, and refertilization from unmixed and contrasted MORB-derived melt batches variably hybridized with hydrous fluids.

Geological background

Geology of the Oman ophiolite

The Oman ophiolite exposes a 30.000 km 2 remnant of the Tethyan Ocean. Its creation occurred ~95-97 million years ago [START_REF] Tilton | Uranium-lead isotopic ages of the Samail Ophiolite, Oman, with applications to Tethyan ocean ridge tectonics[END_REF][START_REF] Tippit | The biostratigraphy of sediments in the volcanic unit of the Samail ophiolite[END_REF][START_REF] Rioux | Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust[END_REF] and was directly followed by an oceanic detachment close to the ridge axis that eventually led to its obduction on the Arabian margin [START_REF] Lanphere | K-Ar ages of metamorphic rocks at the base of the Samail Ophiolite, Oman[END_REF][START_REF] Boudier | Kinematics of oceanic thrusting in the Oman ophiolite: model of plate convergence[END_REF][START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF][START_REF] Montigny | K-Ar and Ar study of metamorphic rocks associated with the Oman ophiolite: Tectonic implications[END_REF]. The synoptic mapping of mafic-ultramafic dykes cropping out in the mantle section [START_REF] Python | Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite[END_REF][START_REF] Python | Chromian spinels in mafic-ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite[END_REF] revealed that two main magmatic suites contributed to the igneous evolution of the ophiolite: (1) a MORB suite mostly present in the southeastern massifs of the ophiolite (Nakhl, Sumail and Wadi Tayin) and in some restricted areas in the north, and (2) a more widespread depleted-andesitic type attributed either to subduction or to hydrated re-melting of the shallow lithosphere during spreading [START_REF] Pearce | The Oman ophiolite as a Cretaceous arc-basin complex: evidence and implications[END_REF][START_REF] Benoit | The remelting of hydrothermally altered peridotite at mid-ocean ridges by intruding mantle diapirs[END_REF][START_REF] Python | Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite[END_REF]Yamasaki et al., 2006;MacLeod et al., 2013) (Fig. 1a).

Along the Oman ophiolite, spatially constrained vertical flow structures within the mantle section were interpreted as former asthenospheric diapirs distributed along the oceanic ridge [START_REF] Ceuleneer | Mantle flow pattern at an oceanic spreading centre: the Oman peridotite record[END_REF][START_REF] Ceuleneer | Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge[END_REF][START_REF] Jousselin | Detailed mapping of a mantle diapir below a paleo-spreading center in the Oman ophiolite[END_REF][START_REF] Bouldier | Accretion of Oman and United Arab Emirates ophiolite -Discussion of a new structural map[END_REF]. The Sumail massif, focus of the present study, exposes in the Maqsad area a well preserved paleo mantle diapir and its associated N130 paleo-spreading center (Ceuleneer, 1986;Ceuleneer et al., 1988;[START_REF] Ceuleneer | Evidence for a Paleo-Spreading Center in the Oman Ophiolite: Mantle Structures in the Maqsad Area[END_REF][START_REF] Jousselin | Detailed mapping of a mantle diapir below a paleo-spreading center in the Oman ophiolite[END_REF]. This structure is located in the central part of the largest (80 × 25 km) MORB district of Oman [START_REF] Python | Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite[END_REF] (Fig. 1).

The MORB affinity has been evidenced by the crystallization sequence and confirmed by trace elements and isotopic data on both mantle and crustal rocks [START_REF] Benoit | Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)[END_REF][START_REF] Ceuleneer | Thermal structure of a fossil mantle diapir inferred from the distribution of mafic cumulates[END_REF][START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF][START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF][START_REF] Godard | Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions[END_REF][START_REF] Clénet | Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism[END_REF].

The dunitic transition zone around the Maqsad diapir

The thickness of the DTZ reaches 300 to 350 m above the central part of the Maqsad mantle diapir [START_REF] Rabinowicz | Melt segregation and flow in mantle diapirs below spreading centres: evidence from the Oman ophiolite[END_REF][START_REF] Boudier | Nature of the moho transition zone in the Oman ophiolite[END_REF][START_REF] Jousselin | The Moho transition zone in the Oman ophiolite-relation with wehrlites in the crust and dunites in the mantle[END_REF][START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF] and thins progressively off-axis due to tectonic transposition by mantle plastic flow as it diverges from the diapir [START_REF] Ceuleneer | Evidence for a Paleo-Spreading Center in the Oman Ophiolite: Mantle Structures in the Maqsad Area[END_REF]. In this area the DTZ was mainly described as the uppermost part of the mantle, modified after complete orthopyroxene consumption resulting from MORB-peridotite interaction [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002), while its upper part (top most ~50 m) displays all the characteristic of cumulates [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF]. The interstitial plagioclase and clinopyroxene located between olivine grains exhibit chemical signature in good agreement with a fractionation from a percolating MORB [START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF][START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF][START_REF] Nicolle | Major and trace element and Sr and Nd isotopic results from mantle diapirs in the Oman ophiolite: Implications for off-axis magmatic processes[END_REF]. It was also recently evidenced that some horizons of the DTZ contain interstitial orthopyroxene and pargasite that support the percolation of a hybrid liquid, i.e. a blend of variably evolved MORB and of hydrous silica-rich liquids [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. The ubiquitous occurrence of hydrothermal diopside and grossular, both in interstitial position between olivine grains and enclosed in disseminated chromite together with orthopyroxene, pargasite, biotite and aspidolite, suggests them as primary high temperature features and further challenges the pure igneous origin of the DTZ. According to our observations, they are not systematically associated to cracks affecting chromite grain, as would be expected if they were low temperature alteration products resulting from serpentinization and Ca-rich metasomatism having affected mafic and ultramafic rocks (e.g. [START_REF] Palandri | Geochemical models of metasomatism in ultramafic systems: Serpentinization, rodingitization, and sea floor carbonate chimney precipitation[END_REF][START_REF] Python | Diopsidites and Rodingites: Serpentinisation and Ca-Metasomatism in the Oman Ophiolite Mantle[END_REF]Akizawa et al., 2016b) (see also [START_REF] Tamura | Geochemistry of spinel-hosted amphibole inclusions in abyssal peridotite: Insight into secondary melt formation in melt-peridotite reaction[END_REF]. The possible involvement of high temperature hydrothermal fluids during the magmatic stage that edified the DTZ allows reconsidering the former hypothesis of a hydrothermal origin for dunites, originally proposed by [START_REF] Bowen | The System MgO-SiO 2 -H 2 O[END_REF].

Methods

Sampling strategy

The Sumail massif was slightly tilted during the Miocene uplift of the Djebel Akhdar. Its regional dip does not exceed 10° to the SE. For this reason, to perform sampling along distributed cross sections is a straightforward way to reconstruct the three-dimensional structuration of the DTZ around the Maqsad diapir and its associated paleo-ridge axis. In this study we report on whole rock and mineral major elements and whole rock trace elements for 79 samples, and on interstitial clinopyroxenes trace elements for 11 impregnated dunites.

Samples were collected systematically every 10 to 20 m vertically along four cross sections (Fig. 1b). Their location and mineral contents are presented in Table 1.

Analytical methods

In situ mineral major element concentrations were acquired by electron microprobe using a Cameca SX 100 (Microsonde Ouest, Brest, France) and a Cameca SXFive (Centre de MicroCaractérisation Raimond Castaing, Toulouse, France) with a 20 kV accelerating voltage, a beam current of 20 nA, an electron beam diameter of 1 μm for all analyses and a counting time of 10 s on peak for each element and 5 s on backgrounds on both sides of the peak. In situ clinopyroxene trace element concentrations were obtained by laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Institute of Earth Sciences, University of Lausanne (Switzerland). We used a NewWave UP-193 ArF Excimer based laser ablation system coupled with a Thermo Scientific™ ELEMENT XR™ high resolution ICP-MS. Spot size ranged between 75 and 100 μm and operating conditions were a 20 Hz pulse rate and an energy density of 6.0 J/cm 2 . The SRM612 NIST was used as external standard following the values proposed by [START_REF] Jochum | Determination of reference values for NIST SRM 610-617 glasses following ISO guidelines[END_REF] and 29 Si was used as internal standard.

Whole rock major element concentrations were obtained by X-ray Fluorescence Spectrometry at the Central Analytical Facilities, Stellenbosch University (South Africa). Whole rock trace element concentrations were acquired on a Thermo Scientific™ ELEMENT XR™ HR-ICP-MS at the Géosciences Environnement Toulouse laboratory (Observatoire Midi-Pyrénées), Université of Toulouse III. The sample preparation consisted in a HF-HClO 4 digestion procedure (Yokoyama et al., 1999) combined with a thulium addition [START_REF] Barrat | Determination of Rare Earth Elements in Sixteen Silicate Reference Samples by Icp-Ms After Tm Addition and Ion Exchange Separation[END_REF].

The concentrations in Li and Large Ion Lithophile Elements (LILE; Rb, Sr, Cs and Ba) and of most of transition elements (Cr, Mn, Co, Ni, Cu, Zn) and Ga were acquired after a direct digestion/dilution method. Sc, V, Rare Earth Elements (REE) and Y, High Field Strength Elements (HFSE; Zr, Hf, Nb, Ta, Ti) and U, Th and Pb contents were determined after the double coprecipitation procedure detailed in [START_REF] Rospabé | Determination of Trace Element Mass Fractions in Ultramafic Rocks by HR-ICP-MS: A Combined Approach Using a Direct Digestion/Dilution Method and Pre-concentration by Co-precipitation[END_REF]. BHVO-2 was used as calibrator following the values proposed by [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn Isotopes[END_REF]. The yield was monitored and concentrations calculated using the Tm anomaly appearing in the samples REE pattern [START_REF] Barrat | Determination of Rare Earth Elements in Sixteen Silicate Reference Samples by Icp-Ms After Tm Addition and Ion Exchange Separation[END_REF]. The international rock standards UB-N (serpentinite) and DTS-2B (dunite) were analysed as unknown during the course of this study. They are given in Appendix A together with the samples major and trace element compositions.

Lithological diversity in the Maqsad DTZ

Olivines exhibit a recrystallized equigranular mosaic texture with frequent triple junctions at 120° (Fig. 2a). Its average grain size is about 1 mm. All samples are affected to some degree by serpentinization, which ranges from about 30 to 60%. Submillimeter round-shaped chromite grains are interstitial between olivine grains. They are generally disseminated but chromite schlierens occur in some samples. Small grains of clinopyroxene are also frequently observed at olivine triple junctions or along the olivine grain rims, with a grain size no larger than few tens of microns. Samples with less than 0.5% of clinopyroxene are considered as "pure dunites" (38 samples).

The remaining 41 samples contain well-developed oikocrystic of clinopyroxene and/or other interstitial minerals. These interstitial phases are classically interpreted as crystallization products fractionated from a melt that percolated through the dunitic matrix and are referred to as "impregnated dunites" (Benn et al., 1988;[START_REF] Boudier | Nature of the moho transition zone in the Oman ophiolite[END_REF][START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF][START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF]. Two contrasted clinopyroxene compositions are observed; one consistent with an igneous origin and the other one calling for crystallization from hydrothermal fluids or water-rich melts [START_REF] Python | Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres[END_REF][START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF].

Accordingly, we use "clinopyroxene" and "diopside" to distinguish clinopyroxene with igneous and hydrothermal origin respectively. Clinopyroxene is the most common impregnating phase (14 samples are cpx-bearing dunites). Its texture changes with its modal abundance, from small interstitial at olivine grains boundaries (up to hundreds microns width in size) to oikocrystic crystals (millimeter to up to centimetre in size) (Fig. 2b). Smaller interstitial diopsides are also sometimes observed both in pure and impregnated dunites, showing different texture and chemical content than the clinopyroxene oikocrysts (Fig. 2c).

Plagioclase appears as a well-developed interstitial intercumulus phase, generally as oikocrysts (Fig. 2d), and is regularly associated to clinopyroxene, characterizing the pl/cpxbearing dunites (14 samples). Plagioclase-bearing cpx-free dunites are rarely observed (one sample only). Plagioclase/olivine contacts are also regularly underlined by a clinopyroxene corona. In addition to clinopyroxene and plagioclase, the dunites collected in the DTZ from the Maqsad area show orthopyroxene oikocrysts in pl/cpx-bearing dunites (7 samples) (Fig. 2e), and amphibole, observed as subhedral crystals in few samples with no preference to the other minerals present (7 samples) (Fig. 2f). Interstitial subhedral grossular garnet was observed in 12 impregnated dunites [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. All these interstitial minerals are also present as round-shaped primary inclusions in chromite grains together with micas (phlogopite and aspidolite), the latter being never observed interstitially within the dunitic matrix.

Results

Whole rock major and minor elements

The dunites from the DTZ plot above the mantle fractionation array in the MgO/SiO 2 vs.

Al 2 O 3 /SiO 2 diagram (Fig. 3a). Pure dunites and slightly impregnated ones (<5% of cpx or cpx + pl) exhibit a more refractory characteristic than the most depleted harzburgites from the mantle section with higher MgO/SiO 2 (>1.1) and lower Al 2 O 3 /SiO 2 (<0.02) ratios.

Impregnated samples are generally characterized by a progressive decrease of MgO/SiO 2 and an increase of Al 2 O 3 /SiO 2 that reflects an increasing amount of clinopyroxene and plagioclase. The loss on ignition (LOI) is comprised between 6.9 and 10.9 wt.% for pure dunites and 4.7 and 10.8 wt.% for impregnated dunites, and reflects primarily the degree of serpentinization that is similar in both dunite types (Appendix A). The variation of FeO and MgO in pure and slightly impregnated dunites perfectly mimics the stoichiometric variation of the olivine Fe-Mg composition (FeO + MgO = 66.67 in mol%) while other higher impregnated samples contain a lower MgO content similar to mantle harzburgites (mainly ~43-47 wt.%) (Fig. 3b).

Low CaO and Al 2 O 3 also characterize the pure and slightly impregnated dunites with concentrations lower than 1 wt.% (Fig. 3c). The higher CaO and Al 2 O 3 contents are strongly correlated to the abundance of impregnating minerals. The modal proportion of clinopyroxene controls the CaO content that increases fast with increasing amount of cpx (Al 2 O 3 /CaO<1 in cpx-bearing dunites) while plagioclase and amphibole exert the main control on the Al 2 O 3 content in other impregnated dunites (Al 2 O 3 /CaO>1).

In pure dunites the Mg# (100 × molar Mg/(Mg + Fe total )) is more variable (from 86 to 92.3, except a pure dunite containing schlierens of chromites with a whole rock Mg# = 83.4) than in impregnated dunites (from 87.2 to 90.9) and peridotites from the mantle section (from 89.8 to 91.6). The dunites from the DTZ also display a much more variable Ni (1207-2870 μg/g) and a higher Co (110-155 μg/g) contents than mantle peridotites (Ni = 1747-2671 μg/g; Co = 95-122 μg/g) (Fig. 3d and3e). These compositions are not clearly related to the degree of impregnation minerals with high Ni and Co concentrations also observed in the most impregnated samples.

Mineral major elements

The Fo in olivine (100 × molar Mg/(Mg + Fe total )) in dunites from the DTZ generally decreases from 92.5 to 87.8 concomitantly with the NiO content (0.45 to 0.19 wt.%) (Fig. 4a). This compositional range is broader than in the peridotites from the mantle section (89.5<Fo<92.1; 0.25<NiO<0.53 wt.%). The CaO content in olivine, mainly <0.05 wt.% in mantle harzburgites, reaches 0.32 wt.% in the DTZ. Higher CaO values are generally observed in olivine grains from pure dunites, also displaying the lower Fo and NiO contents (Fig. 4b), and may consequently not be attributed to re-equilibration with other CaO-rich minerals. Fo in olivine and Mg# in whole rocks are highly correlated (Fig. 4c).

The Cr# (100 × molar Cr/(Cr + Al)) variation range in chromite is more restricted in dunites from the ) than in harzburgites from the mantle section (20.6-70). Cr# gently decreases with the Fo and progressively shifts from the olivine-spinel mantle array (OSMA) [START_REF] Arai | An estimation of the least depleted spinel peridotite on the basis of olivinespinel mantle array[END_REF][START_REF] Arai | Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation[END_REF]) (Fig. 4d). Mg# (100 × molar Mg/(Mg + Fe 2+ )) in chromite is quite similar between the different types of dunite (43.3-63.4) while the TiO 2 content is on the contrary much more variable depending on the host lithological facies (Fig. 4e), being lower (0.20-0.60 wt.%) in pure dunites than in impregnated ones (mainly comprised between 0.15 and 0.9 wt.% and occasionally reaching 1.3 wt.%). On the contrary YFe 3+ (100 × molar Fe 3+ /(Cr + Al + Fe 3+ )) is slightly higher in chromite from the pure dunites (Appendix B).

Minute clinopyroxenes were observed at the triple junction of olivine grains in what we defined as pure dunites. They exhibit a variable Mg# (86.9-94.2) and a CaO content exceeding 24.5 wt.% that contrast with the clinopyroxene oikocrysts from the impregnated dunites (Mg# = 89.6-92.9; CaO = 21.1-24.8 wt.%) (Appendix B). All the clinopyroxenes display a wide variation range in TiO 2 (0.16-0.71 wt.%) (Fig. 4f), Cr 2 O 3 (0.79-1.32 wt.%) and Al 2 O 3 (2.6-4.9 wt.%) (Appendix B). Zoning was observed in few clinopyroxenes only, with the concomitant increase of Mg#, SiO 2 and TiO 2 together with the decrease of Al 2 O 3 and Cr 2 O 3 from the core to the rim. Small interstitial diopside composition ranges between igneous and hydrothermal end-members, with higher Mg# and lower TiO 2 (Fig. 4f), Cr 2 O 3 and Al 2 O 3 contents (see also [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF].

Oikocrystic orthopyroxene grains observed in interstitial position in dunites present a high TiO 2 content (0.15-0.23 wt.%), specific to the dunites from the Maqsad area [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF] (Appendix B). They display Mg# and Al 2 O 3 contents varying from 88.8 to 90.8 and 1.15 to 2.22 wt.% respectively. As clinopyroxene, orthopyroxene rarely exhibits zoned compositions with the increase of SiO 2 concomitantly to the decrease of Al 2 O 3 , and Cr 2 O 3 from the core to the rim. Mg# is quite variable as MgO and FeO both increase with SiO 2 .

Plagioclase was not systematically analysed in each pl-bearing dunites due to its higher degree of alteration than other phases. It presents rather primitive compositions in (opx/)pl/cpx-bearing dunites (An (100 × molar Ca/(Ca + Na + K)) = 86.3-90.1) while being more evolved in one amphibole/pl/cpx-bearing dunite (An = 74.6) (Appendix B).

Whole rock trace elements

The dunites from the DTZ exhibit variable trace element compositions (Appendix A). Pure dunites are highly depleted in lithophile trace elements with concentrations below both the chondritic and the Primitive Mantle (PM) values (mostly comprised between 0.001 and 0.1 times the PM concentrations). Pure dunites are characterized by U-or V-shape REE chondrite-normalized patterns (Fig. 5a), reflecting the major MREE (Sm, Eu and Gd) depletion relative to LREE (La CN /Sm CN <3.76; 1.9 in average) and HREE (0.04< Gd CN /Yb CN <0.28). We define the U-or V-shape appellation depending on the presence or absence of negative Eu anomaly. The (Eu/Eu*) CN ratio ranges between 0.22 and 1 in 29 samples, while 9 samples exhibit a positive Eu anomaly with (Eu/Eu*) CN <2.4 ((Eu/Eu*) N = Eu CN /√ S CN × Gd CN )). Pure dunites display relative homogeneous HREE concentrations (Yb CN = 0.10-0.24) in comparison to more variable MREE (0.005< Gd CN <0.059; 0.004< Sm CN <0.065) and especially LREE (0.004< La CN <0.17). The depletion from HREE to MREE is not linear with several samples exhibiting instead concave-upward patterns with a progressive Gd CN /Dy CN increase for a constant Er CN /Yb CN . Pure dunites U-shaped PM-normalized multi-elements patterns exhibit significant enrichments in LILE, Th, U, Nb and Ta relative to LREE (Rb PMN /La PMN = 1.5-85; Th PMN /La PMN = 0.7-5.3) and strong Pb and Sr positive anomalies similar to mantle peridotites (2.19< Pb PMN /Ce PMN <1294) (Fig. 5b). Zr and Hf are generally enriched relative to MREE (Zr PMN /Sm PMN = 0.6-27; 2.9 in average) as well as Ti that displays a strong positive anomaly (1.3< Ti PMN /Gd PMN <76).

Cpx-bearing dunites exhibit variable REE and multi-element patterns partly attributable to variations in the interstitial clinopyroxene content. Slightly impregnated ones (<5% cpx) exhibit linear LREE-depleted or slightly concave-upward REE patterns (La CN /Yb CN = 0.06-0.97) with similar HREE concentrations (Yb CN = 0.12-0.29) and more restricted LREE and MREE variation ranges (La CN = 0.01-0.23; Gd CN = 0.02-0.12) than pure dunites (Fig. 5c).

Dunites containing a higher amount of clinopyroxene have higher HREE contents (Yb CN = 0.56-0.83). They exhibit clear convex-upward patterns with a nearly flat slope of the HREE segment (Gd CN /Yb CN = 0.74-0.85) followed by a progressive depletion from MREE to LREE (La CN /Sm CN = 0.06-0.09). It is worth noting that the cpx-bearing dunites display La and Ce contents within the variation range of pure dunite (La CN = 0.01-0.23). This observation may be extended to other incompatible trace elements like Th, U, HFSE that are not more concentrated in cpx-bearing dunites than in pure dunites, excepting one sample that displays spectacular HFSE positive anomalies (Fig. 5d). Cpx-bearing dunites exhibit similar enrichment in LILE relative to LREE and the same strong positive Pb and Sr anomalies than in pure dunites. The positive Ti anomaly observed for the slightly impregnated samples disappears for the more impregnated ones in which the Ti content is buffered around 0.25 times the PM value.

The pl-bearing dunite displays a W-shaped REE pattern with strong depletion in MREE (Sm CN = 0.005; Gd CN = 0.003) relative to LREE (La CN /Sm CN = 4.3) and HREE (Gd CN /Yb CN = 0.002) bordering a strong positive Eu anomaly ((Eu/Eu*) CN = 54) (Fig. 5e). Other pl/cpxbearing chondrite-normalized REE patterns evolve from concave-upward to convex upward with the increasing amount of clinopyroxene. This evolution is accounted by a progressive decrease of the HREE segment slope (Gd CN /Yb CN from 0.17 to 0.63). They are also characterized, in regard to pure and cpx-bearing dunites, by higher Li and Rb contents, a higher range of Cs, a well depletion in their Th and U contents, and a slighter positive Pb anomaly (Fig. 5f).

Opx/pl/cpx-bearing dunites and amphibole-bearing dunites (± opx, pl and cpx) have REE contents similar to pl/cpx-bearing dunites with patterns intermediate between highly impregnated pl-bearing and cpx-bearing dunites (Fig. 5g). They also display similar PMnormalized multi-elements patterns than pl/cpx-bearing dunites, with Th, U, and with few exceptions Nb and Ta contents similar to LREE (averaged Th PMN /La PMN = 0.9) (Fig. 5h).

Clinopyroxene minor and trace elements

Trace elements concentrations were obtained for unzoned crystals of clinopyroxene in 11 impregnated dunites (2 cpx-bearing, 6 pl/cpx-bearing, 3 opx/pl/cpx-bearing, and 1 amph/opx/pl/cpx-bearing). Their composition is homogeneous within a given sample. In clinopyroxenes, lower trace elements contents are observed in the amph/opx/pl/cpx-bearing dunite while higher contents much characterize cpx-bearing dunites; clinopyroxenes in (opx/)pl/cpx-bearing dunites display both low and high trace elements concentrations.

Regarding the chondrite-normalized REE patterns, all the clinopyroxenes are depleted in LREE relative to MREE (La CN /Sm CN = 0.03-0.32) and the slope in HREE is highly variable, from strongly positive to moderately negative (Gd CN /Yb CN = 0.61-2.28) (Fig. 6a). The most LREE-depleted patterns are rather observed in (amph/)opx/pl/cpx-bearing dunites whatever the REE concentrations, while the less depleted ones are observed in cpx-bearing dunites.

Negative anomalies in Nb and Ta (Nb PMN /La PMN = 0.08-1) relative to LREE and in Zr and Hf relative to MREE (Zr PMN /Sm PMN = 0.14-0.43) are systematically observed whatever the host lithology (Fig. 6b). With few exceptions, negative anomalies in Pb and Sr relative to LREE-MREE are also observed as well as in Ti for the most enriched clinopyroxenes.

Discussion

The dunitic transition zone from the Maqsad area exposes various types of dunites, characterized from their mineralogical content [START_REF] Boudier | Nature of the moho transition zone in the Oman ophiolite[END_REF][START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF][START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF][START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF] and from their geochemical diversity within each lithological facies. Previous studies proposed different processes to explain the formation of pure and impregnated dunites and/or their geochemical signature: (1) serpentinization or weathering, (2) fractional crystallization and accumulation of olivine and other minerals, (3) melt-rock reactions comprising harzburgite reactional melting and reequilibration related to melt percolation and, (4) refertilization associated to melt transport. In the next sections we will discuss the relative influence of each process in regard to the geochemical variations revealed by our study, then we will propose a more general model for the Maqsad DTZ genesis.

Serpentinization or secondary weathering effects

The dunites are partly serpentinized with a serpentine abundance of about 40 to 50% in average, according to thin sections observation and loss of ignition (LOI) values.

Serpentinization is known to be nearly isochemical regarding the bulk rock major element concentrations (e.g. [START_REF] Coleman | A chemical study of serpentinization-burro mountain[END_REF][START_REF] Deschamps | Geochemistry of subduction zone serpentinites: A review[END_REF]. In our samples, the LOI is not or poorly correlated with any major element contents attesting that the serpentinization did not extensively affect bulk rock major element concentrations (see also [START_REF] Rospabé | Etude pétrologique, géochimique et structurale de la zone de transition[END_REF]. More specifically, the samples plot above the terrestrial array in the MgO/SiO 2 vs. Al 2 O 3 /SiO 2 diagram (Fig. 3a) and do not show any evidence for MgO loss or SiO 2 enrichment as reported in pervasively serpentinized abyssal peridotites or talc-bearing serpentinites [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Bach | Seawater-peridotite interactions: First insights from ODP Leg 209[END_REF][START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15°20'N D L g 209 : Implications for fluid/rock interaction in slow spreading environments[END_REF][START_REF] Boschi | Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30°N): Insights from B and Sr isotope data[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]. The perfect consistency between the Fo in olivine and the Mg# in whole rock (Fig. 4c) further demonstrates the absence of influence of serpentinization on the bulk dunite major element compositions.

Regarding trace elements, with few exceptions, the La content is strongly correlated to the Th content, in both pure and impregnated dunites (Fig. 7a). The Th, as well as HFSE, are immobile during post-magmatic aqueous alteration (You et al., 1996;[START_REF] Kogiso | Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts[END_REF][START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15°20'N D L g 209 : Implications for fluid/rock interaction in slow spreading environments[END_REF]. Thus, correlated enrichments of La with Th testify that LREE enrichments derived from magmatic processes, as seen in mantle harzburgites from the Sumail massif [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002). A poorer but significant correlation exists between HFSE (Zr, Hf, Nb, Ta) and U contents on one hand and Th and LREE contents on the other (Fig. 7b, c andd). Rb and Ba seem to be vaguely correlated to HFSE in pure dunites (especially to Zr and Hf) as well as to Th (Fig. 7e andf) and thus were probably enriched during an initial igneous stage before being potentially affected again during secondary weathering processes. Other LILE, such as Sr, and Pb contents are not correlated to the LOI but the absence of correlation with Th suggests that their enrichment could be related to a secondary alteration/serpentinization processes post-dating the igneous history. This is confirmed by the fact that all the dunitic bulk rock analyses from the DTZ exhibit strong positive Sr and Pb anomalies, while interstitial igneous clinopyroxenes exhibit the opposite negative Sr and Pb anomalies. These trace element evolutions in regard to immobile elements have been already described in Oman ophiolite mantle harzburgites (Gerbert-Gaillard, 2002). The co-variations of Th, U, LREE and HFSE at least may thus be interpreted in the framework of igneous processes.

Cumulative origin

A cumulative origin for dunites [START_REF] Bowen | The later stages of the evolution of the igneous rocks[END_REF][START_REF] Bowen | The System MgO-SiO 2 -H 2 O[END_REF]O'Hara, 1965;[START_REF] Elthon | High magnesia liquids as the parental magma for ocean floor basalts[END_REF] implies to preserve a high-Mg parental melt in the crystallization field of the single olivine (+/-spinel) during differentiation. This is quite problematic to account for thick layers (several hundred meters or large bodies of dunites), and the production of such high temperature melts is inconsistent with the inferred thermal conditions in the present day Earth interior (Verhoogen, 1954;[START_REF] Mckenzie | The volume and composition of melt generated by extension of the lithosphere[END_REF][START_REF] Tilley | Melting relations of some oceanic basalts[END_REF]. However, it was proposed that the shallowest part of the Maqsad DTZ, where both Fo and NiO in olivine decrease together upsection, was generated by olivine crystallization and accumulation while the main part of the DTZ was a residue of reaction melting [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF].

Dunites from the Maqsad DTZ exhibit Ni and Co contents that are much closer to the mantle peridotites compositions (Fig. 3d and3e) than to the most primitive troctolite cumulates cropping out within the mantle harzburgitic section in the Maqsad area (Ni < 650 μg/g; Co < 40 μg/g; [START_REF] Benoit | Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)[END_REF]. This observation, together with the absence of clear correlation between Mg# and Ni in whole rock (Fig. 3d), the negative correlation between Mg# and Co in whole rock (Fig. 3e), and with the consistent compositional variations between the different types of dunites, suggests that both pure and impregnated dunites cannot be related by fractional crystallization from a common parent melt. Regarding the pure dunites, a pure cumulative origin does not account for the normalised U-/V-shaped REE patterns (Fig. 5a and 5b). The enrichment in LREE relative to MREE is not predicted by REE partition coefficients between olivine and melt [START_REF] Frey | Integrated models of basalt petrogenesis: A study of quartz tholeiites to olivine melilitites from South Eastern Australia utilizing geochemical and experimental petrological data[END_REF]McKenzie and O'Nions, 1991;[START_REF] Kelemen | Relative depletion of niobium in some arc magmas and the continental crust: partitioning of K, Nb, La and Ce during melt/rock reaction in the upper mantle[END_REF][START_REF] Hauri | C g f plumes: A trace element study of peridot x h f S ' W S[END_REF][START_REF] Lee | Extension of lattice strain theory to mineral/mineral rare-earth element partitioning: An approach for assessing disequilibrium and developing internally consistent partition coefficients between olivine, orthopyroxene, clinopyroxene and basaltic melt[END_REF][START_REF] Sun | An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites[END_REF] and may better be explain by melt-rock reactions leading to peridotite metasomatism (e.g. [START_REF] Navon | Geochemical consequences of melt percolation: the upper mantle as a chromatographic column[END_REF][START_REF] Godard | Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes : A chromatographic approach[END_REF]Vernières et al., 1997). Models presented in Appendix C evidence that the olivine matrix hosting impregnations exhibit similar REE patterns than for whole rock pure dunites, with the same range of variability. In this way, impregnated dunites may be definitely interpreted as hybrid lithologies, made of a mantle-derived olivine matrix associated to other minerals crystallized during melt percolation (e.g. [START_REF] Rabinowicz | Melt segregation and flow in mantle diapirs below spreading centres: evidence from the Oman ophiolite[END_REF][START_REF] Nicolas | Mantle-crust transition zone and origin of wehrlitic magmas: Evidence from the Oman ophiolite[END_REF][START_REF] Boudier | Nature of the moho transition zone in the Oman ophiolite[END_REF][START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF][START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF][START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. The modelling allows us to conclude that the whole rock trace element signatures of impregnated dunites result from a blend of three main variables: the amount of impregnating mineral phases and their chemical signatures as expected, but also the composition of the dunitic matrix itself (Appendix C).

Melt-rock reaction product: dunitization and melt transport

State of melt-rock reactions leading to incompatible trace element enrichments in mantle peridotites

Incompatible trace element enrichments in peridotites, as U-shaped REE patterns, are documented in numerous peridotites from various tectonic settings, from ophiolitic and orogenic massifs (e.g. [START_REF] Prinzhofer | Residual peridotites and the mechanims of partial melting[END_REF][START_REF] Bodinier | Mechanisms of Mantle Metasomatism: Geochemical Evidence from the Lherz Orogenic Peridotite[END_REF][START_REF] Gruau | The origin of U-shaped rare earth patterns in ophiolite peridotites: assessing the role of secondary alteration and melt/rock reaction[END_REF][START_REF] Bouilhol | Magma and fluid percolation in arc to forearc mantle: Evidence from Sapat (Kohistan, Northern Pakistan)[END_REF] to abyssal peridotites at mid-ocean ridges [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Godard | Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneath a slow spreading ridge[END_REF] or in forearc context [START_REF] Parkinson | Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for mantle melting and melt-mantle interaction in a supra-subduction zone setting[END_REF]. It has been proposed that melt-rock reactions may lead to some chromatographic effects such as melt-peridotite re-equilibration or to trapped melt between olivine and pyroxene grains. This can be accounted for by (1) a twostages process involving mantle partial melting later percolated by a LREE-enriched melt (i.e. cryptic metasomatism in the peridotitic matrix without mineralogical reaction) [START_REF] Navon | Geochemical consequences of melt percolation: the upper mantle as a chromatographic column[END_REF] or, (2) a single-stage process by combining partial melting (or melt-rock reaction at increasing melt mass; Vernières et al., 1997) and melt transport concomitantly, leading to a MREE depletion relative to HREE whereas LREE remain constant [START_REF] Godard | Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes : A chromatographic approach[END_REF]. In the Oman ophiolite, U-shaped REE patterns were described in mantle harzburgites [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002;[START_REF] Girardeau | The Wuqbah peridotite, central Oman Ophiolite: Petrological characteristics of the mantle in a fossil overlapping ridge setting[END_REF][START_REF] Mée | Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF][START_REF] Hanghøj | Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; Major and trace element geochemistry, and Os isotope and PGE systematics[END_REF], especially in the Maqsad diapir area where it is attributed to an association between partial melting, deformation related to mantle flow and MORB percolation [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002).

Extreme trace elements enrichments in DTZ dunites: an early dunitization-related signature

The melt-rock reaction leading to dunite formation from a harzburgite through orthopyroxene dissolution is expected to result in a high intergranular permeability of the dunite, that becomes a porous media for efficient melt transfer and extraction [START_REF] Toramaru | Connectivity of melt phase in a partially molten peridotite[END_REF]Kelemen et al., 1995b). Dunite channels and horizons, filled or not with troctolitic cumulates, have been mapped within the Maqsad mantle section and interpreted in terms of frozen compaction waves [START_REF] Ceuleneer | Thermal structure of a fossil mantle diapir inferred from the distribution of mafic cumulates[END_REF][START_REF] Rabinowicz | The effect of sloped isotherms on melt migration in the shallow mantle: a physical and numerical model based on observations in the Oman ophiolite[END_REF]. They are common features in the Maqsad diapir but not significant in terms of volume. Hence, it is unlikely that the DTZ formed by the transposition in the solid state of such dunitic bodies generated at greater depth, especially since the DTZ dunites do not show any sign of deformation. They better represent the final in situ reacting residue of complete dunitization of the uppermost few hundred meters of the mantle induced by melt migration and extraction.

Regarding REE, pure dunites from the DTZ display lower HREE, similar and more variable MREE, and higher and more variable LREE contents than harzburgites from the Maqsad mantle section (Fig. 5a) [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002). The lower HREE content is in good agreement with a more refractory signature, i.e. depleted, following the complete resorption of orthopyroxene. However, their LREE, Zr, Hf and Th contents are systematically higher than in mantle harzburgites, as Nb, Ta and U in many cases (Fig. 5a and 5b). Considering a reacting melt with MORB-like affinities (e.g. [START_REF] Ceuleneer | Thermal structure of a fossil mantle diapir inferred from the distribution of mafic cumulates[END_REF][START_REF] Benoit | Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)[END_REF]Kelemen et al., 1997a;[START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF], these signatures are inconsistent with a genesis at the expense of the surrounding harzburgites alone following the single-stage process described above [START_REF] Godard | Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes : A chromatographic approach[END_REF]Vernières et al., 1997). Moreover, the occurrence of interstitial minerals implies that the percolating melt suffered crystallization after the harzburgite dunitization, associated to the last stage of percolation during the cooling of the system. Then, one can envision the overprinting of two-or several contemporaneous or successive stages processes in order to explain the dunite trace element signatures. It is worth mentioning that the single-and two-stages processes were proposed in order to mimic harzburgite signatures [START_REF] Navon | Geochemical consequences of melt percolation: the upper mantle as a chromatographic column[END_REF][START_REF] Godard | Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes : A chromatographic approach[END_REF]Vernières et al., 1997), but it may be different for dunites.

As the trace element enrichment in pure dunites relative to harzburgites cannot be accounted for by V1-type (e.g. [START_REF] Godard | Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions[END_REF] MORB percolation alone, it therefore calls for the additional involvement of a more enriched melt. In the same spirit [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF] proposed the pervasive infiltration of volatile-and LILE-enriched small melt fractions to account for the enrichment in Th, Nb and Ta relative to LREE in mantle harzburgites. The interaction between an incompatible trace elements-enriched melt or fluid and the surrounding dunites does not contradict the percolation of MORB through the DTZ, and the migration of different kinds of melt is further supported by the variable composition of clinopyroxenes located in interstitial position between olivine grains (Fig. 6) (see also [START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF] and by the presence of amphibole and orthopyroxene among other which are not expected in the fractionation path of primitive MORB [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. Furthermore, two strong arguments in favour to MORB percolation through the DTZ are the particularly high CaO content in olivine especially observed in pure dunites (Fig. 4b), consistent with MORB-dunite equilibration before cooling [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF], and the high TiO 2 content in disseminated chromites and impregnant pyroxenes (Fig. 4e andf) (see also [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. In a profile view along the base (pure dunites) of the Buri cross section (Fig. 1b), chemical evolutions from about 640 m to 700 m high show that high CaO values in olivine are strongly correlated to other chemical species (Fig. 8), independently from the very small amount of interstitial clinopyroxene (Table 1). This includes high Gd CN /Yb CN ratio in whole rock (Gd increases from 2.5 to 9.5 ng/g respectively) and a higher concavity of the REE patterns (i.e. well-defined U-/V-shaped REE patterns), but also higher Co content in whole rock and TiO 2 content both in chromite and scarce minute clinopyroxenes. On the contrary, the 700 m level is characterized by lower Fo in olivine (and Ni content both in olivine and whole rock) and La CN /Sm CN ratio in whole rock (faster increase of Sm (from 4.8 to 7.8 ng/g) relative to La (from 1.1 to 5.0 ng/g)). Assuming that high CaO in olivine recorded the last re-equilibration with a percolating MORB before cooling [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF], these co-evolutions highlight that dunites had overprinted a prior enrichment in LREE and a strong concave-upward REE pattern signature before the accumulation and the reequilibration with MORB at 700 m followed by the compaction of the dunitic matrix.

These observations have several major implications: (1) considering that such enrichments in incompatible elements may be attributable to a low melt-rock ratio (i.e. low porosity) and/or to chromatographic effects induced by the migration of a highly enriched melt [START_REF] Navon | Geochemical consequences of melt percolation: the upper mantle as a chromatographic column[END_REF][START_REF] Godard | Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes : A chromatographic approach[END_REF]Vernières et al., 1997), the low CaO in olivine for the most LREE-enriched samples better supports the re-equilibration with an incompatible elementsrich melt or fluid early in the magmatic history of the DTZ; (2) dunites displaying the highest La CN /Sm CN ratios do not represent an end-member that was percolated by a higher amount of melt [START_REF] Navon | Geochemical consequences of melt percolation: the upper mantle as a chromatographic column[END_REF] but result from the more or less significant interaction with this early enriched melt or fluid; (3) the percolation of MORB-type melts did not occur all over the DTZ and seems to have been focused at some particular horizons at an advanced stage of the DTZ structuration (i.e. here the 700 m high level); (4) the increase of the La, Sm and Gd contents until 700 m implies that the re-equilibration with MORB indeed enhanced the enrichment in the most incompatible LREE, but mostly affected and re-enriched the dunitic matrix in the previously most depleted MREE. Especially, from the base to the elevation of 700 m along the Buri cross section, the faster increase of Gd CN /Dy CN (0.33 to 0.53) relative to Er CN /Yb CN (0.47 to 0.53) follows the smoothing of MREE-HREE segment pattern. In this way the linear REE patterns in dunites and harzburgites have been interpreted as resulting from re-equilibration induced by MORB-type melt transport [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF], while concave-upward patterns observed through the DTZ and in mantle harzburgites from the Maqsad area should be better related to the dunitization process. Therefore, MORB percolation led to a decrease of the LREE/MREE and an increase and MREE/HREE ratios, and to smoother REE patterns (i.e. exhibiting a lower concavity). Accordingly, dunites from the DTZ appear to be generated after a multi-stages process: (1) a complete harzburgite reactional melting during which enriched melt batches are generated probably in low volume (i.e. melt generated by the incongruent melting of orthopyroxene associated with the contribution of fluids and minor amount of MORB) followed by (2) a chemical reequilibration with larger amount of percolating MORB. In opposition with the model of [START_REF] Navon | Geochemical consequences of melt percolation: the upper mantle as a chromatographic column[END_REF], we propose that the incompatible trace elements-enriched signature is a feature acquired early during the dunitization rather than later by the percolation of enriched melts.

About the habitus of incompatible trace elements in the dunitic matrix

As previously discussed, the trace element enrichment in dunite, especially in LREE, may better reflect magmatic processes rather than secondary alteration events (paragraph 6.1). It supports the view that trace elements are hosted by the olivine itself rather than by interstitial serpentine surrounding preserved grains and is in accordance with recent trace element data in olivine obtained by laser ablation methods (e.g. [START_REF] Drouin | Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge[END_REF][START_REF] Rampone | Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine[END_REF][START_REF] Stead | Rare earth element partitioning and subsolidus exchange behaviour in olivine[END_REF]. Sub-solidus re-equilibration between olivine and other mineral phases may eventually occur [START_REF] Sun | An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites[END_REF] and lead to trace element enrichments at olivine grains boundary [START_REF] Stead | Rare earth element partitioning and subsolidus exchange behaviour in olivine[END_REF], while other studies propose that these particular geochemical signatures reflect analytical artefacts or the entrapment of small LREE-rich melt inclusions (e.g. [START_REF] Lee | Extension of lattice strain theory to mineral/mineral rare-earth element partitioning: An approach for assessing disequilibrium and developing internally consistent partition coefficients between olivine, orthopyroxene, clinopyroxene and basaltic melt[END_REF]. The analytical method we developed [START_REF] Rospabé | Determination of Trace Element Mass Fractions in Ultramafic Rocks by HR-ICP-MS: A Combined Approach Using a Direct Digestion/Dilution Method and Pre-concentration by Co-precipitation[END_REF] allowed us to establish with confidence that the extensive enrichment in LREE relative to MREE in pure dunites (Fig. 5) is a reliable result. These enrichments thus cannot be satisfactorily explained both by analytical effects or re-equilibration with other minerals. Furthermore, we did not detect melt inclusions through microscopic observations and during the electron microprobe sessions, although the existence of inclusions smaller than about 10 μ cannot be excluded. Finally, the absence of zoning in olivine grains, at least in terms of major elements, does not militate for a possible concentration of incompatible elements at grain boundaries. All these observations support the hosting of the incompatible trace elements by crystal lattices in olivine consecutively to re-equilibration of the olivine matrix with interstitial melts during their migration through the DTZ. Sub-solidus re-equilibration between different olivine grains may lead to the trace element content smoothing between rims and cores, but stays a hypothetical track at this stage and should be explored in future works.

Refertilization induced by melt transport

Interstitial minerals in impregnated dunites may be considered as crystallization products from a melt percolating through the residual and porous dunitic media (e.g. Benn et al., 1988;[START_REF] Rabinowicz | Melt segregation and flow in mantle diapirs below spreading centres: evidence from the Oman ophiolite[END_REF]Kelemen et al., 1995a;[START_REF] Boudier | Nature of the moho transition zone in the Oman ophiolite[END_REF][START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF], rather than as trapped melts not perfectly extracted (e.g. [START_REF] Faul | Permeability of partially molten upper mantle rocks from experiments and percolation theory[END_REF][START_REF] Sundberg | Trapped melt in the Josephine peridotite: Implications for permeability and melt extraction in the upper mantle[END_REF]. This reasonable assumption is supported by the systematic high Mg# in pyroxenes and generally high An in plagioclases, as well as by the low REE contents both in clinopyroxenes and whole rocks, that are all together not expected in the case of MORB-like melt entrapment between olivine grains (see also [START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF]. Basically, the whole impregnated dunite composition is strongly impacted by the modal abundance of these interstitial minerals, especially by the amount of clinopyroxene (Fig. 5 and Appendix C).

Diversity of the percolating melts within the Maqsad DTZ

Calculated melts in equilibrium with clinopyroxenes impregnating the DTZ appear much more variable than the melts that migrated through the mantle section [START_REF] Benoit | Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)[END_REF][START_REF] Benoit | C g h p ' g g f ' ph[END_REF] and the V1 lavas constituting the upper crust in the Sumail massif [START_REF] Godard | Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions[END_REF] (Fig. 9). Three main types of patterns are observed: clinopyroxenes in equilibrium with (1) a N-MORB type melt (1 sample of cpx-bearing dunite) (Fig. 9a), (2) LREE-enriched melts (mainly in (pl/)cpx-bearing dunites) (Fig. 9b), (3) strongly LREE-depleted melts (mainly in (amph/)opx/pl/cpx-bearing dunites) (Fig. 9c). HREE melt concentrations are basically in the variation range of the MORB-type (or V1, [START_REF] Godard | Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions[END_REF] lavas but are much more variable in LREE. The presence of minerals supposed to be in equilibrium with both LREE-enriched and LREE-depleted melts through the DTZ may reflect either the mixing between various melt batches, including a high proportion of deep melts issued from low mantle partial melting degrees, or the reaction at shallow depth between a primitive MORB and residual depleted peridotites [START_REF] Sobolev | Melt Incusions in Minerals as a source of princple petrological information[END_REF]Kelemen et al., 1997b;[START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF].

The first hypothesis would seem unlikely here since the absence of such derived enriched-like cumulates in the mantle section. Indeed, MORB-parent melts that crystallized primitive troctolite sills cropping out in the Maqsad harzburgites show quite geochemical variations attributable to different degrees of partial melting within the diapir [START_REF] Benoit | C g h p ' g g f ' ph[END_REF], but that are much more restricted than the melt diversity described here.

Early generation of enriched melt batches during dunitization

The Maqsad area is characterized by the absence of cumulates crystallized from an enriched parent melt within both the mantle section and the DTZ [START_REF] Benoit | Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)[END_REF]Kelemen et al., 1997a;Python and Ceuleneer, 2013). According to this and to the trace element enrichments in pure dunites (Fig. 5), we propose the early production of LREE-enriched melts at Moho level (Fig. 9b) further related to melt-rock reactions rather than originated from depth. These melts can be related to the SiO 2 -enriched melts produced during incongruent melting of the orthopyroxene and expected following the study of other ophiolitic massifs [START_REF] Dick | Evidence of partial melting in the Josephine peridotite[END_REF][START_REF] Quick | Petrology and petrogenesis of the Trinity Peridotite, an upper mantle diapir in the Eastern Klamath Mountains, Northern California[END_REF][START_REF] Kelemen | Reaction between ultramafic rock and fractionating basaltic magma I. phase relations, the origin of calc-alkaline magma series, and the formation of discordant dunite[END_REF][START_REF] Kelemen | Formation of harzburgite by pervasive melt/rock reaction in the upper mantle[END_REF]. The early involvement of an enriched melt responsible of the incompatible trace element enrichment in olivine, deduced from the whole pure dunite composition, is in accordance with this interpretation.

The genesis of highly enriched melts during dunitization, and the incipient crystallization, may be related to reactive crystallization [START_REF] Collier | The case for reactive crystallization at mid-ocean ridges[END_REF]. This process was identified as the main cause of clinopyroxene chemical zoning in olivine-rich troctolites from slow spreading environments: cores were generated by fractional crystallization from a percolating melt while enriched rims result from the fractionation after the contamination of the melt by assimilation of the surrounding peridotite (e.g. [START_REF] Suhr | Microstructures in Hole 1274A peridotites, ODP Leg 209, Mid-Atlantic Ridge: Tracking the fate of melts percolating in peridotite as the lithosphere is intercepted[END_REF][START_REF] Drouin | Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge[END_REF]Sanfilippo et al., 2013[START_REF] Sanfilippo | Hybrid troctolites from mid-ocean ridges: Inherited mantle in the lower crust[END_REF][START_REF] Rampone | Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine[END_REF]. Especially, the resorption of orthopyroxene seems to be a credible source for this particular signature, allowing to important Zr CN /Nd CN fractionation (until 1.5) with the increase of REE contents [START_REF] Sanfilippo | Hybrid troctolites from mid-ocean ridges: Inherited mantle in the lower crust[END_REF]. In this way, the dunitization, when directly followed by interstitial mineral fractionation, can be presented as a kind of reactive crystallization process considering the enrichment in silica and incompatible trace elements of the residual melt. However, in this study, clinopyroxene grains analyzed for their trace elements are not zoned and present low Zr CN /Nd CN ratios (0.34-0.74; 0.47 in average) with no observed correlation with Yb, similar to clinopyroxene cores in olivine-rich troctolites. This highlights that the chemical evolution of enriched melt batches is probably, in the frame of the Maqsad DTZ, firstly dominated by the residual orthopyroxene assimilation process by MORB (i.e. reactive crystallization in the case of clinopyroxene fractionation at this stage). Following the complete orthopyroxene resorption, fractional crystallization occurs while MORB issued from partial melting within the Maqsad diapir are introduced and transferred through the porous olivine matrix without any further reaction.

Differentiation and hybridization of the percolating melts within the Maqsad DTZ

Oppositely, further hypotheses may be proposed concerning the LREE-depleted melts (Fig. 9c), which may reflect (1) the assimilation of the surrounding dunite at an advanced refertilization stage (i.e. reactive crystallization products), (2) the later sub-solidus reequilibration between clinopyroxene crystals and neighbouring mineral phases (e.g. [START_REF] Sun | An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites[END_REF], (3) the result of hybridization between a latter percolating MORB and hydrous fluids [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]). The two first hypotheses difficultly account for the large chemical variations in clinopyroxene, that is the most common and abundant impregnant phase in DTZ dunites, especially concerning the variable slope of the HREE segment (Figs. 6 and9). However, the third hypothesis is supported by the fact that the LREE-depleted signature is observed mainly in samples containing interstitial orthopyroxene and/or amphibole, minerals previously interpreted as fractionated from a hybrid melt between MORB and hydrous fluids [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. In this scenario this hybrid component may contribute to the exotic melt involved in the formation of disseminated chromites and chromitite ore bodies as suggested by the numerous inclusions of hydrous minerals in chromite [START_REF] Lorand | Silicate and base-metal sulfide inclusions in chromites from the Maqsad area (Oman ophiolite, Gulf of Oman): A model for entrapment[END_REF][START_REF] Leblanc | Chromite crystallization in a multicellular magma flow: Evidence from a chromitite dike in the Oman ophiolite[END_REF][START_REF] Borisova | A new view on the petrogenesis of the Oman ophiolite chromitites from microanalyses of chromite-hosted inclusions[END_REF][START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF][START_REF] Martin | Fluids are bound to be involved in the formation of ophiolitic chromite deposits[END_REF].

To conclude, our study reveals that the variability in the composition of the melts responsible of the dunite formation and of their petrological and geochemical evolution is extremely pronounced (see also [START_REF] Sanfilippo | Mantle-crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine[END_REF][START_REF] Sanfilippo | Water, lithium and trace element compositions of olivine from Lanzo South replacive mantle dunites (Western Alps): New constraints into melt migration processes at cold thermal regimes[END_REF]. A working hypothesis is to consider that such wide melt variability is a specificity of the DTZ and reflects local melt/fluid-rock reactions rather than a complex mixing of deep-seated melt batches. This assumption is supported by the fact that some clinopyroxene grains exhibiting different patterns were analysed in samples collected in nearly same locations (Table 1 and Appendix B). According to the observed clinopyroxene chemical heterogeneity, it appears that the DTZ evolved as an open reactive system in which melts fractions issued from strong melt-rock interactions evolved by fractional crystallization and hybridization and were far from being fully homogenised before being supplied to the lower crust.

Synthetic model for the formation of the Maqsad DTZ

As pointed out here above, none of the four main processes that are discussed -(1) serpentinization or secondary weathering, (2) fractionation and accumulation of olivine, (3) harzburgite reactional melting and re-equilibration related to melt percolation into dunite porosity, and (4) refertilization through minerals crystallization from percolating melts interstitially between olivine grains -may account alone for the wide spectrum of geochemical signatures observed in whole rock and mineral compositions of the DTZ, although each can contribute to account for part of this variability, depending on the elements considered. This indicates that the DTZ records an overprinting of several processes.

In attempt to propose a general scenario describing the development of the Maqsad DTZ, Figure 10 summarizes the different steps deduced from the whole rock and mineral major and trace element signatures. The starting lithology, i.e. the harzburgite from the specific Maqsad area, present a slight but significant LREE enrichment relative to the harzburgites from the main mantle section located far away from the diapir (Fig. 10a) [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]. Acting as a filter for ascending melts, the top of the diapir is progressively transformed into dunites by interaction with interstitial melts, probably of MORB affinity (Fig. 10b). This process leads to the depletion in HREE in the residual dunite due to the resorption of the last pyroxenes and to the enrichment in LREE and other incompatible trace elements (HFSE, Th, U and possibly some LILE as Rb and Ba). This signature is better explained by the early generation of small batches of enriched melts in response to orthopyroxene incongruent melting (i.e. enriched residual melt). At start, the reaction is driven by MORB/harzburgite interaction but the olivine matrix records the solely incompatible trace elements enrichment, calling for a very low porosity needed to preserve this signature. It has been proposed that high temperature hydrothermal fluids were involved in dunite formation or as early as the mantle dunitization process [START_REF] Bowen | The System MgO-SiO 2 -H 2 O[END_REF][START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF][START_REF] Martin | Fluids are bound to be involved in the formation of ophiolitic chromite deposits[END_REF]; the enriched signature described in this paper may be another evidence for the early hydration of the DTZ. Following this, the dunitization will increase the permeability, as MORB-like melts delivered by the mantle continue to invade the matrix. Those melts no longer contribute to mineralogical reaction following the complete orthopyroxene resorption within the host rock, and then may accumulate at Moho level or feed the crust (Fig. 10c and10d). Chemical re-equilibration occurs during MORB percolation, leading to a slight LREE and mainly MREE enrichment in the host dunite, i.e. the re-equilibration process seems to mainly affect the most depleted trace elements in the dunitic matrix independently from the partition coefficient between those elements and olivine. The chemical overprinting of this stage occurs until the compaction of the matrix and the resulting melt extraction. It is worth mentioning that all the intermediate cases may exist between a pure dunite having preserved its prior enriched signature (Fig. 10e) and another one having been fully re-equilibrated with MORB (Fig. 10f). Therefore, pure dunites may be considered as a witness of a magmatic system closed at very high temperature (i.e. above the olivine-plagioclase-clinopyroxene ternary cotectic). Depending on the thermal state of the DTZ, and certainly on the primitive or evolved nature of the percolating melt, crystallization will or not occur interstitially. As supposed following the various REE patterns calculated for the olivine matrix hosting some impregnating clinopyroxene (Appendix C), mineral fractionation occurs concomitantly with the melt percolation and re-equilibration processes (Fig. 10g). Among other, the circulation of hydrous fluids at Moho level during the structuration of the DTZ explains: (1) the higher amount of impregnations approaching the base of the crust witnessing the former thermal gradient across DTZ [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]; see also [START_REF] Dygert | Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges[END_REF], (2) the harzburgite dunitization in the specific context of the DTZ, and not to deeper dunite channels within the mantle section, and (3) the interstitial mineralogy indicative of the percolation of exotic hybrid melts certainly involved in the transport of high amount of Cr -potentially released from orthopyroxene during the dunitization (Johan et al., 2017) -and its concentration in disseminated chromites and chromitite pods [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. In the case of a much colder environment as in slower-spreading environment [START_REF] Collier | The case for reactive crystallization at mid-ocean ridges[END_REF], extreme refertilization potentially occur through a more efficient assimilation of the olivine matrix coupled to fractional crystallization (reactive crystallization), leading to the formation of olivine-rich troctolites as observed in samples collected along present-day mid-ocean ridges or within ophiolites DTZ (e.g. [START_REF] Suhr | Microstructures in Hole 1274A peridotites, ODP Leg 209, Mid-Atlantic Ridge: Tracking the fate of melts percolating in peridotite as the lithosphere is intercepted[END_REF][START_REF] Drouin | Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge[END_REF][START_REF] Dick | Mantle melting, melt transport, and delivery beneath a slow-spreading ridge: The paleo-AR f 23°15'N 23°45[END_REF]Sanfilippo and Tribuzio, 2013b;Sanfilippo et al., 2013[START_REF] Sanfilippo | Mantle-crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine[END_REF][START_REF] Sanfilippo | Hybrid troctolites from mid-ocean ridges: Inherited mantle in the lower crust[END_REF][START_REF] Rampone | Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine[END_REF].

At this stage, it cannot be excluded that the fractional crystallization dominated the refertilization process within the Maqsad DTZ, relative to reactive crystallization, due to the fast spreading environment in which the Oman ophiolite likely evolved. Another possibility consists in the fast cooling of this oceanic lithosphere fragment, related to the oceanic thrusting that directly followed the lithosphere accretion and that fossilized the system in a high temperature configuration [START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF]. Finally, in regard to the whole dunites and mineral major and trace elements, it appears that both pure and variably impregnated dunites represent end-members that can be generated independently.

As shown, the re-equilibration with percolating MORB did not occur equally all along the DTZ but was more efficient at some specific levels (Fig. 8) (see also [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF], suggesting that the dunitic matrix was either unequally compacted when MORB migrated through it or that the MORB percolation was locally focused. This feature may be partially explained by a model of progressive downward dunitization of the mantle harzburgites at the top of the Maqsad diapir [START_REF] Abily | The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?[END_REF]. During this process, the compaction may progress downward, possibly in response from cooling from above, through successive steps and then may spatially constrain the porous fronts in which MORB may have accumulated. Clearly, some horizons of the DTZ remained isolated early after dunitization and other horizons were intensely re-equilibrated with MORB more or less hybridized with hydrous, likely hydrothermal fluids.

Conclusion

Major and trace elements of 79 pure to highly impregnated dunites from the Oman ophiolite and the composition of the impregnation minerals revealed a large petrological and chemical variability. The dunitic matrix -measured pure dunites and modelled olivine matrix in impregnated dunites -is characterized by a more or less pronounced concave-upward REE pattern and a variable enrichment in other incompatible HFSE, Th, U, Rb and Ba. Their REE and multi-elements patterns are highly scattered implying that no dominant single process controlled the dunite formation. Chemical evolutions suggest that the enriched signature was recorded early during the dunitization by interaction between the surrounding olivine matrix and incompatible trace elements-rich small melt batches at low melt/rock ratios. The latest MORB percolation leads to a cryptic metasomatism expressed as smoothest REE patterns, among others, inducing flattest slopes in HREE and LREE patterns. This scenario contradicts the later percolation of a deep-seated enriched melt and better supports that hybrid enriched melts are generated by melt/rock reactions at an early magmatic stage within the DTZ beneath mid-ocean spreading centers.

Our survey shows that these different signatures are variably distributed along the DTZ. This implies that some horizons have been compacted early immediately following the mantle harzburgites dunitization, while others maintained a high enough permeability to drain MORB and make possible re-equilibration. The physical processes that condition the heterogeneous chemical distribution along the DTZ remains quite enigmatic at this stage.

Pure and impregnated dunites may be considered as two lithological end-members starting from a common dunitized peridotitic matrix: the first have been compacted at very high temperature after various chemical re-equilibration degrees and the complete extraction of interstitial percolating melts, while the others have been modally refertilized before the extraction of the residual melt. In this frame, the combination of whole dunites and in situ minerals major and trace elements compositions reveals undoubtedly the multi-stages processes involved in the DTZ generation.

Finally, based on the chemical variability within the different minerals observed in dunites from the DTZ, we conclude that if MORB magmas generated by mantle partial melting may have been partly homogenized within the uppermost mantle before being supplied to the mantle-crust transition zone, melt-rock reactions and hybridization processes occurring within the DTZ may generate new, exotic and/or hybrid, melt batches that will participate to the oceanic crust genesis and potentially to the MORB heterogeneity. The precise relationship between the evolution of the Maqsad diapir, the N130 paleo-ridge axis, and the vertical petrological and geochemical signatures across the DTZ is a complex three-dimensional problem and could be deciphered by a more comprehensive structural, petrological and geochemical survey of the area. [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002;[START_REF] Takazawa | Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF][START_REF] Hanghøj | Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; Major and trace element geochemistry, and Os isotope and PGE systematics[END_REF][START_REF] Khedr | Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity[END_REF][START_REF] Nicolle | Major and trace element and Sr and Nd isotopic results from mantle diapirs in the Oman ophiolite: Implications for off-axis magmatic processes[END_REF] They are plotted whatever their hosting lithological facies, displaying a composition range spanning between igneous clinopyroxenes and pure hydrothermal diopsides [START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. -Gaillard, 2002;[START_REF] Takazawa | Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)[END_REF][START_REF] Monnier | Along-ridge petrological segmentation of the mantle in the Oman ophiolite[END_REF][START_REF] Hanghøj | Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; Major and trace element geochemistry, and Os isotope and PGE systematics[END_REF][START_REF] Khedr | Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity[END_REF][START_REF] Nicolle | Major and trace element and Sr and Nd isotopic results from mantle diapirs in the Oman ophiolite: Implications for off-axis magmatic processes[END_REF] and to the main mantle harzburgites from the Sumail massif (light grey field), the harzburgites from the Maqsad diapir area (medium grey field) and few other dunites from the Maqsad DTZ (dark grey field) [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF]Gerbert-Gaillard, 2002). In other panels each lithological facies is compared to the field of pure dunites from this study (blue fields).

Normalizing chondrite and Primitive Mantle values are from [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn Isotopes[END_REF] and [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF] respectively. Calculated melts are compared to the Oman ophiolite V1 (red field) and V2 (green fields) lavas [START_REF] Godard | Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions[END_REF][START_REF] Godard | A MORB source for low-Ti magmatism in the Semail ophiolite[END_REF] and boninites (dark green field) [START_REF] Ishikawa | Boninitic volcanism in the Oman ophiolite: Implications for thermal condition during transition from spreading ridge to arc[END_REF][START_REF] Ishikawa | Trace element characteristics of the fluid liberated from amphibolite-facies slab: Inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis[END_REF]. N-MORB values are from [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF].

Partitioning coefficients used in the calculation are from [START_REF] Bédard | A procedure for calculating the equilibrium distribution of trace elements among the minerals of cumulate rocks, and the concentration of trace elements in the coexisting liquids[END_REF] and normalizing chondrite values are from [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn Isotopes[END_REF]. It is worth mentioning that clinopyroxene is the most abundant impregnating phase, and is supposed to display the higher trace element contents in dunites, together with amphibole. In regard of the large HREE content variations in clinopyroxene, it is difficult to account for this variability following sub-solidus reequilibration with other minerals present (e.g. [START_REF] Sun | An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites[END_REF], especially in the restricted temperature range expected with the cotectique ol-pl-cpx. Thus it is assumed that re-equilibration between clinopyroxene and neighbouring olivine, orthopyroxene, plagioclase and/or amphibole is negligible as far as the REE are concerned (i.e. the assumption of a low REE diffusivity at subsolidus temperature following [START_REF] Van Orman | Rare earth element diffusion in diopside: influence of temperature, pressure, and ionic radius, and an elastic model for diffusion in silicates[END_REF]. Then, the calculation of the inferred melt in equilibrium from which the clinopyroxene crystallized allows investigating the nature of the melt that percolated and fractionated interstitial minerals through dunitic rocks. Altitude (m)

  Interstitial amphiboles in the studied samples are pargasites, pargasitic-or edenitichornblende. They display Mg# ranging from 85.7 to 91.4 and extremely variable Al 2 O 3 (11.0-16.3 wt.%), Na 2 O (1.98-3.75 wt.%), TiO 2 (0.02-1.24 wt.%) and Cr 2 O 3 (0.01 to 1.81 wt.%) contents (Appendix B).

Figure 1 .

 1 Figure 1. Geological map of the Oman ophiolite showing (1) the spatial distribution of the

Figure 2 .

 2 Figure 2. Photomicrographs illustrating the mineralogical diversity that allowed to define the

Figure 3 .

 3 Figure 3. Whole rock major and minor element compositions of the studied dunites in the

  . (a) MgO/SiO 2 versus Al 2 O 3 /SiO 2 , (b) total iron as FeO versus MgO, (c) Al 2 O 3 versus CaO, and (d) Ni and (e) Co as a function of the Mg# ratio (100 × molar Mg/(Mg + Fe total )). Compositions are recalculated on a volatile-free basis. Grey bar in panel (a) represents the silicate Earth differentiation trend (or " " (Jagoutz et al., 1979). Dashed black line in panel (b) represents the variation of olivine composition constrained by FeO + MgO = 66.67 mol% following Godard et al. (2000). Dashed grey lines in panels (b) and (c) represent constant Mg# and Al 2 O 3 /CaO ratios respectively.

Figure 4 .

 4 Figure 4. Mineral major element compositions of the studied dunites from the Maqsad area

Figure 5 .

 5 Figure 5. Whole rock chondrite-normalized REE (a, c, e and g) and Primitive Mantle-

Figure 6 .

 6 Figure 6. Clinopyroxene chondrite-normalized REE (a) and Primitive Mantle-normalized

Figure 7 .

 7 Figure 7. Trace element co-evolutions in pure dunites represented as (a) La, (b) U, (c) Zr, (d)

Figure 8 .

 8 Figure 8. Chemical evolutions along the base of the Buri cross section, made of pure dunites,

Figure 9 .

 9 Figure 9. Calculated equilibrium melts with the interstitial clinopyroxenes in impregnated

Figure 10 .

 10 Figure 10. Synthetic model for the genesis of the dunites from the Maqsad DTZ,

  Figure1

  Figure 5

  Figure 9

Table 1 .

 1 Location and modal composition of the samples analysed in this study. The modal abundance of each mineral was calculated using both whole rock and mineral major elements. Whole rock major and trace element compositions.Verhoogen J. (1954) Petrological evidence on temperature distribution in the mantle of the earth. Eos, Trans. Am. Geophys.Union. Vernières J., Godard M. and Bodinier J.-L. (1997) A plate model for the simulation of trace

	Mode calculations were systematically compared to and are in good agreement with thin
	section observations.

Appendix A.

Yokoyama T., Makishima

A. and Nakamura E. (1999)

Evaluation of the coprecipitation of incompatible trace elements with fluoride during silicate rock dissolution by acid digestion. Chem.

Geol. 157,[175][176][177][178][179][180][181][182][183][184][185][186][187]

You C. F., Castillo P. R., Gieskes J. M., Chan L.

H. and Spivack, A. J. (1996)

Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones. Earth Planet.

Sci. Lett. 140,[41][42][43][44][45][46][47][48][49][50][51][52]

  Fo in olivineCaO in ol (wt.%) La CN /Sm CN in WR Gd CN /Yb CN in WR

	Figure 8
									100
	Concentration / chondrite	a MORB-type melts		b LREE-enriched melts	10	c LREE-depleted melts
		(La CN /Yb CN ~0.48)		(La CN /Yb CN ~1.50)		(La CN /Yb CN ~0.16)
										1
	La Ce Pr Nd	Sm Eu Gd Tb Dy Ho Er Tm Yb Lu	La Ce Pr Nd	Sm Eu Gd Tb Dy Ho Er Tm Yb Lu	La Ce Pr Nd	Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
		.	.	.		.	.	.	.	.	.
		This study:					Previous studies:
		calculated melts in equilibrium with clinopyroxene	V1 lavas	boninite
		(colours refer to the facies of the host rock)		V2t1 lavas	N-MORB
		averaged melt in equilibrium with clinopyroxene		V2t2 lavas
		used in models (Appendix C)

Acknowledgments

We are grateful to F. de Parseval, J.-F. Ména and L. Menjot for thin sections preparation, S. Gouy, J. Langlade and P. de Parseval for technical assistance during microprobe analysis, and to F. Candaudap and D. Roggero for technical assistance during whole rock trace elements analysis by HR-ICP-MS and clinopyroxene trace elements acquisition by laser ablation (LA-ICP-MS) respectively. Y. Liang, T. Morishita and A. Sanfilippo are thanked for their highly constructive comments that helped to improve the paper. Informal discussions with and questions/comments from N. Dygert were also greatly appreciated. This work has benefited from a financial support provided by the Centre National de la Recherche Scientifique-Institut N S ' CNRS-INSU). M.-A. Kaczmarek thanks the Swiss National Science Foundation (SNSF) for support from Ambizione grants PZ00P2_142454 and PZ00P2_166933. For their hospitality and assistance H. Al Azri as well as M. Al Araimi, M. Al Batashi, A. Al-Rajhi and other people from the Ministry of Commerce and Industry, Sultanate of Oman, are greatly acknowledged.