Delphine Klaessens
email: delphine.klaessens@univ-lorraine.fr

Laurie Reisberg

David Jousselin

Marguerite Godard

Claire Aupart

Osmium isotope evidence for rapid melt migration towards the Moho in the Oman ophiolite

Keywords: Oman ophiolite, Mantle, Osmium isotopes, Melt migration, Dunite Highlights

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

Introduction

Melt migration processes beneath mid-oceanic ridges remain controversial, engendering a diversity of models based on structural [START_REF] Nicolas | A Melt Extraction Model Based on Structural Studies in Mantle Peridotites[END_REF], geochemical [START_REF] Kelemen | Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels[END_REF], petrological [START_REF] Morgan | An experimental and numerical study of the kinetics of harzburgite reactive dissolution with applications to dunite dike formation[END_REF] and numerical studies [START_REF] Ahern | Magma migration beneath an ocean ridge[END_REF][START_REF] Stevenson | Spontaneous small-scale melt segregation in partial melts undergoing deformation[END_REF]. One strong constraint is the existence of disequilibrium between shallow mantle peridotites and mid-ocean ridge basalts (MORBs) [START_REF] O'hara | Primary magmas and the origin of basalts[END_REF][START_REF] Stolper | A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis[END_REF], manifested notably by undersaturation of orthopyroxene (opx) in MORBs. Melt-peridotite interactions can result in pyroxene dissolution in the host peridotite [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] Lambart | An experimental study of focused magma transport and basalt-peridotite interactions beneath mid-ocean ridges: Implications for the generation of primitive MORB compositions[END_REF][START_REF] Morgan | An experimental and numerical study of the kinetics of harzburgite reactive dissolution with applications to dunite dike formation[END_REF], forming dunite if continued to completion. The origin of ophiolitic dunite bodies is commonly attributed to this process (Boudier andNicolas, 1977, 1995;[START_REF] Büchl | Melt percolation monitored by Os isotopes and HSE abundances: A case study from the mantle section of the Troodos Ophiolite[END_REF][START_REF] Büchl | Os mobilization during melt percolation: The evolution of Os isotope heterogeneities in the mantle sequence of the Troodos ophiolite, Cyprus[END_REF][START_REF] O'driscoll | Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland[END_REF], which may generate dunite channels providing transport pathways for ascending melts [START_REF] Kelemen | Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels[END_REF]. In such channels, melts are isolated from equilibration with the surrounding peridotite, but may equilibrate with the dunite.

The Oman ophiolite is well-suited for the study of melt migration beneath mid-oceanic ridges. This large (>20 000 km 2 ), well-exposed segment of oceanic lithosphere, formed ~95 Ma ago along a rapid spreading center [START_REF] Boudier | EPR microplates, a model for the Oman Ophiolite[END_REF]Nicolas et al., 2000a;[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], provides a window into the uppermost mantle, which outcrops over >50% of its surface. The Re-Os radiometric system, in which 187 Re decays to 187 Os (t 1/2 = 41.6 Ga, λ= 1.666 × 10 -11 ) [START_REF] Smoliar | Re-Os Ages of Group IIA, IIIA, IVA, and IVB Iron Meteorites[END_REF], is a particularly useful tool for studying mantle melt transport. The contrasting behavior of these two elements during partial melting (rhenium is moderately incompatible while osmium is compatible) creates Earth reservoirs with strikingly different Re/Os ratios, producing over time contrasting Os isotope ratios. Recycled oceanic crust and sediments develop highly radiogenic Os compositions while mantle peridotites remain unradiogenic. When melt migrates through dunite, the isotopic and elemental compositions of incompatible elements (e.g. light rare earth elements) are rapidly overprinted in the dunite by equilibration with the magma, which has much higher concentrations of these elements. For Os, the mass balance is reversed, allowing gradual changes in Os isotope composition to be recorded in the dunite as melt progressively passes through.

Here, we present structural observations and geochemical data on melt migration and dunite formation from peridotites from the southeastern Oman ophiolite. We focus on Os isotopes, tracers exceptionally well-adapted to studying melt-peridotite interactions. Implications of our results for melt transport mechanisms are modeled in light of the structural context of each sample.

The Oman ophiolite

Internal structure

As documented by early studies (e.g. [START_REF] Boudier | Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains[END_REF]; [START_REF] Lippard | The Ophiolite of Northern Oman[END_REF]; Nicolas et al. (2000a)), the Oman ophiolite consists of a >15 kilometer thick dominantly harzburgitic mantle section, overlain by a complete mafic crust. Our study focuses on the mantle section. This domain was first detailed by [START_REF] Boudier | Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains[END_REF], based on a cross-section in the Wadi Tayin massif. The uppermost few hundred meters beneath the crust correspond to the Moho Transition Zone (MTZ), a primarily dunitic layer of variable thickness. The MTZ has been extensively studied (e. g. [START_REF] Boudier | Nature of the Moho Transition zone in the Oman ophiolite[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] Rospabé | Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite : Implications for melt/fluidrock reactions at Moho level beneath oceanic spreading centers[END_REF]) and is interpreted as a melt storage and reaction zone, where ascending melts stagnate beneath the crust-mantle interface (Moho). These melts dissolve orthopyroxene of the host harzburgite, producing both pure dunite and "melt-impregnated" dunite with interstitial plagioclase, pyroxene and occasional amphibole, as well as rare cumulate gabbro lenses (<5%). In the lowermost MTZ, interfingering relationships between harzburgite and dunite with orthopyroxene relics suggest a melt-peridotite reactional origin for the dunite [START_REF] Nicolas | Cumulative or Residual Origin for the Transition Zone in Ophiolites: Structural Evidence[END_REF]. Furthermore, the olivine Mg# [= Mg/(Mg+Fe total ) at ] is similar in the MTZ and underlying harzburgite, whereas spinel TiO 2 content in the MTZ is higher, indicating dunite arose from equilibration with TiO 2 -rich primary melts rather than crystal fractionation [START_REF] Koga | Petrogenesis of the crust-mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite[END_REF].

Beneath the MTZ, the primarily harzburgitic main mantle section (MMS) comprises most of the ophiolite volume. The exact distribution of dunite within the MMS is unclear and its estimated proportion varies between studies (e. g. <5% for [START_REF] Boudier | Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains[END_REF] and 5-15% for [START_REF] Lippard | The Ophiolite of Northern Oman[END_REF]). Dunite occurs either as planar and irregular bodies with dimensions up to ~20 meters [START_REF] Bartholomew | The primary structures and fabrics of the upper mantle and lower crust from ophiolite complexes[END_REF], crosscutting the mantle fabric, or more rarely, as thin concordant bands.

Crosscutting gabbro and pyroxenite dikes are occasionally observed.

At the bottom of the mantle section near the basal thrust, a highly deformed, basal banded unit (BBU) is often observed. In this section, [START_REF] Boudier | Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains[END_REF] report subequal proportions of harzburgite (or occasionally lherzolite) and concordant dunite bands, varying in thickness from 0.1 to several meters. [START_REF] Lippard | The Ophiolite of Northern Oman[END_REF] also note significantly increased dunite abundance near the base. These basal dunites have never been thoroughly nor systematically studied. [START_REF] Braun | Dunite distribution in the Oman Ophiolite: Implications for melt flux through porous dunite conduits[END_REF] determined a power law relationship between dunite width and cumulative abundance, which they used to support a model of melt transport to the ridge in coalescing dunite channels. The mid to large-sized dunites in their study were measured in the Muscat massif (Fig. 1). The proximity with the basal thrust and the exceptionally high dunite abundance (18-46%) of this small massif suggest that it is derived from the BBU.

Geodynamic context

The geodynamic context of the Oman ophiolite has long been controversial. The ophiolite presents several characteristics akin to those of fast spreading ridges (e. g. magma chamber shape, mantle structure, crustal continuity, structure and thickness variation…) [START_REF] Boudier | EPR microplates, a model for the Oman Ophiolite[END_REF]Nicolas et al., 2000a). Moreover, basalts above the sheeted dike complex (V1, Geotimes unit) display MORB-like compositions [START_REF] Ernewein | The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)[END_REF][START_REF] Godard | A MORB source for low-Ti magmatism in the Semail ophiolite[END_REF]. However, upper lavas (V2, Lasail unit, found in northern Oman) overlying hydrothermal deposits display island arc affinities [START_REF] Alabaster | The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex[END_REF][START_REF] Ernewein | The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)[END_REF], suggesting ophiolite formation in a supra-subduction setting. As these upper units clearly postdate most of the crustal accretion, and probably the recorded mantle flow, this origin has been disputed. However, reappraisal of the Geotimes unit (MacLeod et al., 2013) and dating of the metamorphic sole [START_REF] Guilmette | Forced subduction initiation recorded in the sole and crust of the Semail ophiolite of Oman[END_REF] strongly argue for accretion of the entire ophiolite from a hydrated mantle above a subducting slab. Nevertheless, the critical parameters controlling magmatic accretion at the ridge are spreading rate and presence or absence of a magma chamber, regardless of the broader tectonic context. Thus, the Oman ophiolite remains a well-suited laboratory for studying melt migration in the mantle beneath accretion centers, as long as we focus on features contemporaneous with the asthenospheric mantle flow. Systematic petrostructural mapping (Nicolas et al. (2000a) and references therein) revealed the deformation temperatures of the peridotite and their spatial variation. Coarse-grained granular and porphyroclastic textures with well-recovered sub-structures are ascribed to asthenospheric deformation at the spreading center. Finer-grained porphyroclastic textures with varying degrees of recrystallization into neoblasts, mostly restricted to the borders of the massifs, reflect overprinting low-temperature deformation, associated with ophiolite emplacement. Later faulting is limited. Thus, the ophiolite can be seen as a fossilized piece of oceanic lithosphere, dissected by erosion and slightly tilted following the dip of the Moho.

In the southeastern part of the ophiolite, the focus of our study, a NW-SE paleo-spreading center was identified from dike orientations, which trend NW-SE in the center of the Sumail massif and perpendicularly (NE-SW) to the north and south of this central zone (Fig. 1) [START_REF] Boudier | EPR microplates, a model for the Oman Ophiolite[END_REF]. The position of this spreading center is inferred from the alignment of several harzburgitic mantle diapirs, defined by steeply plunging lineations with radial horizontal lineations at their outskirts [START_REF] Jousselin | Detailed mapping of a mantle diapir below a paleospreading center in the Oman ophiolite[END_REF]. Solid-state flow lineations in the harzburgite and MTZ, and magmatic lineations in the overlying crust are remarkably parallel, indicating that the mantle lineations formed before the gabbro was frozen. These observations show that harzburgite diapirs, MTZ formation and ridge accretion were contemporaneous, with diapirs being likely feeding centers for the overlying ridge segments. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)

Definition of three types of dunite in the mantle section

To clarify dunite distribution, we made observations and collected samples along six cross-sections traversing the mantle section, in diapiric and non-diapiric areas in the Nakhl-Rustaq, Sumail (Maqsad diapir) and Wadi Tayin massifs (Fig. 1 Our observations distinguish three types of dunite, each associated with a specific part of the mantle section (Fig. 12; dunite thicknesses in Table A.1):

(1) Moho Transition Zone (MTZ) (Fig. 2a): As noted by previous studies [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] Rospabé | Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite : Implications for melt/fluidrock reactions at Moho level beneath oceanic spreading centers[END_REF], the MTZ is composed of pure and plagioclase/clinopyroxeneimpregnated dunites, and reaches thicknesses of 50-500 meters above harzburgite diapirs, and usually <30 meters far from diapirs. Dunitic lenses several meters wide, interlayered with harzburgite, are sometimes found in the hundred meters underlying the MTZ. The MTZ volume is so large that it can be represented on maps including several massifs (scales of 1/1 000 000 to 1/500 000) (Fig. 1).

(2) Harzburgitic main mantle section (MMS): Discordant gabbroic and pyroxenitic dikes rimmed by dunitic reaction zones crosscut sporadically the MMS harzburgite (Fig. 2b left), as described by [START_REF] Boudier | Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains[END_REF]. In some cases, all of the melt was extracted from the dike, leaving only the dunitic reaction zones, several centimeters to 5 meters wide, crosscutting the mantle fabric (henceforth such features referred to as "dunite veins") (Nicolas et al. (2000b) and references therein). Angular contacts, harzburgite blocks trapped between dunite veins, and stockwork-like geometry suggest a hydrofracturing origin for the magmatic dikes from which the dunitic reaction zones formed (Fig. 2b right). The crosscutting nature of these veins indicates they post-date asthenospheric flow, but several examples of partially tectonically transposed dunite veins suggest they also could form in the flowing mantle. Centimetric dunites parallel to the foliation are also observed in the MMS (henceforth "dunite bands") (insert in Fig. 2b), possibly representing extreme examples of transposed veins. All the MMS dunites, even those in harzburgite diapirs, represent a negligible, unmappable fraction of the mantle volume.

(3) Basal banded unit (BBU): This zone often hosts abundant, mainly concordant dunitic bands (Fig. 2c). These are usually centimeters to meters thick, rarely up to 20 meters, with the largest being mappable locally at the 1/10 000 scale. They are sometimes composed of opx-poor harzburgite instead of dunite. Coarse-grained samples sometimes occur between fine-grained ones (Table A .2) and at the top of the unit, suggesting that the transition from the low-temperature to the hightemperature domain is irregular, and that the BBU is thicker than the low-temperature domain. In the Wadi Tayin and Nakhl-Rustaq massifs, the top of this unit, where coarse-grained asthenospheric deformation is preserved, includes 3-20 meter "blobs" of dunite, often with several dunite veins branching into them, exactly as drawn by [START_REF] Bartholomew | The primary structures and fabrics of the upper mantle and lower crust from ophiolite complexes[END_REF]. Such blobs are rare in the MMS (none in our cross-sections). The concentration of dunitic bands in the BBU is poorly understood. [START_REF] Lippard | The Ophiolite of Northern Oman[END_REF] suggested that ophiolite thrust-related extreme deformation of scattered dunite bodies in the mantle section could explain the basal dunite banding; however, this would not explain the higher dunite content in this section relative to the MMS, and the fact that parts of the banding, as well as the dunitic blobs, display asthenospheric deformation textures.

An exceptional fourth type of dunite is exposed in the Batin area, consisting of a dunite body >9 kilometers long, 2 kilometers wide and several hundred meters thick (Fig. 1), evident on the USGS Wadi Tayin geological map and clearly visible in the field and on aerial photographs. This body was sampled by the ICDP Oman Drilling Project and several dedicated studies, including our own, are ongoing. Apart from the Batin area, no dunite larger than 30 meters, or similar to those at the base of the ophiolite, was found in the MMS. This is true even in the diapiric areas, thought to represent feeding zones for the overlying crust.

Samples and analytical techniques

Samples were selected from the three types of dunite (19 from 81 collected dunites, chosen for freshness and variety) and host harzburgite (20 from 140 collected harzburgites) in the mantle sections of the Nakhl-Rustaq, Sumail, Wadi Tayin and Haylayn massifs (Fig. 1; UTM coordinates in Tables A.2-3). In six cases, dunite and harzburgite were collected in close proximity (<1 meter) (Table A.1). Sampling localities include both off-axis (e.g. Nakhl-Rustaq and Wadi Tayin massifs, Mansah diapir in Sumail massif) and on-axis (e.g. Maqsad diapir in Sumail massif) settings.

Dunite structural data and petrographic descriptions of all analyzed samples are given in Tables A.1-2 (complementary descriptions of non-analyzed samples in Table A.9). Harzburgites are moderately to highly serpentinized and have coarse-grained porphyroclastic textures, except for four fine-grained samples from the BBU. Dunites are moderately to highly serpentinized, with higher degrees of serpentinization in the MTZ and the BBU. Sulfides are very rare and small in both lithologies. Medium to high-temperature porphyroclastic and granular textures are observed among all three types of dunite. Grain size, spinel abundance, crystal shape and serpentinization degree do not vary systematically, making it difficult to distinguish different dunite types simply from examination of hand specimens or thin sections. Nevertheless, basal dunites (1) never contain plagioclase, and (2) often contain orthopyroxene relics. Our dunitic samples include seven pure dunites (98.0-99.5% Ol and <0.5-2.0% Spl ), eight containing pyroxene relics (<1.5% Px ), three impregnated with plagioclase and/or pyroxene, and one olivine troctolite.

Whole-rock major and trace element compositions (Tables A.3-5) were determined respectively by the SARM at CRPG using an ICP-OES iCap6500 and at Géosciences Montpellier (AETE-ISO University of Montpellier, France) using an Agilent 7700X quadrupole ICP-MS. After chemical separation, Os isotopic analyses (Tables A.6-8) were done at CRPG by NTIMS using a Finnigan MAT262 mass spectrometer.

Details of analytical methods are provided in the Supplementary Material.

Variations in major element and Os isotope compositions with structural context

Harzburgites from both the main and basal sections have Mg# mostly from 0.900 to 0.916 and Os concentrations from 2.27 to 6.29 ppb (Fig. 3b,4a; Table 1 andTables A.3, 6-7). Basal harzburgites generally have lower Al 2 O 3 (0.39-0.63 wt.%) than MMS harzburgites (0.48-0.96 wt.%), except for two samples (16OL27B and 16OL29), whose compositions (0.92 and 1.21 wt.%) approach those of clinopyroxene-bearing harzburgites from [START_REF] Godard | Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section in the Oman ophiolite[END_REF] (Fig. 3a). Plagioclase-impregnated, orthopyroxene-poor harzburgite 16OD5C from the uppermost MMS displays the highest FeO (8.76 wt.%) and lowest Os content of all of the harzburgites (Fig. 3b,4a). Values of 187 Os/ 188 Os 95Ma vary from 0.1131 to 0.1345, with most between 0.12 and 0.13, and no systematic differences are observed between MMS and basal harzburgites (Fig. 4a and Table 1). These values span nearly the entire global abyssal peridotite range ( Dunites have highly variable compositions correlating with their structural contexts (Fig. 3, 4b-c):

(1) MTZ dunites have generally low Mg# (0.897-0.920; mean: 0.904) with higher Al 2 O 3 (0.09-1.71 wt.%) and lower Os concentration (0.39 to 1.17 ppb) than most other dunites. Their 187 Os/ 188 Os 95Ma ratios and coherent with those of Oman MTZ chromitites [START_REF] Ahmed | Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary Osrich PGM[END_REF]. Plagioclase-impregnated dunite 10OM25 and especially troctolite 16OL34B are enriched in magmatophile elements (Fe, Al, Ca) (Fig. 3). The Os concentration and 187 Os/ 188 Os 95Ma of dunite 10OM25 do not differ from those of other MTZ dunites, but troctolite 16OL34B is less radiogenic (0.1319) and has a higher Os concentration (2.30 ppb) (Fig. 4b).

(2) Relative to MTZ dunites, most MMS dunites display a restricted range of Mg# (0.893-0.908; mean: 0.902), lower Al 2 O 3 (mostly 0.11-0.51 wt.%), higher Os concentration (1.12 to 5.95 ppb) and lower 187 Os/ 188 Os 95Ma (0.1249 to 0.1330). One unusual sample (16OD9A) located ~1 km below the MTZ has atypical Os characteristics (11.80 ppb Os; 187 Os/ 188 Os 95Ma = 0.1483) but an unexceptional major element composition (Mg#= 0.905; Al 2 O 3 = 0.33 wt.%). In MMS dunite/harzburgite pairs, the dunite is always much more radiogenic than the harzburgite (Fig. 4c).

(3) Basal dunites display higher overall Mg# (0.908-0.923; mean: 0.914) and lower Al 2 O 3 (≤0.8 wt.%) than the other dunites. Their Os concentrations vary substantially (0.77 to 7.17 ppb), while their [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]. In all panels, olivine troctolite (16OL34B) plots outside of the graph area, with higher Al 2 O 3 and lower FeO and MgO concentrations. Published Oman peridotite data are shown for comparison: MTZ dunites [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]; MMS dunites [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] 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]; MMS 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][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] and basal 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]. Attribution of samples reported in the literature to the three domains defined in our study was based on their geographic positions in the mantle section. 187 Os/ 188 Os 95Ma vs.

Fig. 4. Os isotope compositions of harzburgites and the three types of dunite. (a)

[Os] in harzburgites; (b) 187 Os/ 188 Os 95Ma vs. [Os] in dunites; (c) Variation in dunite 187 Os/ 188 Os 95Ma with approximate height in the stratigraphic section, normalized to the total height (Moho to base) of each section to allow comparison of samples from different cross-sections. Harzburgites taken adjacent to six of the dunites are also shown, with blue and red contours delimiting harzburgite-dunite pairs in the MMS and the BBU, respectively. Dunite 16OD4C (dashed blue contour) is associated with the 16OD4D Dun/Hzb pair but was collected at a greater distance from the harzburgite (30 cm). In all panels, light purple fields represent the estimated 187 Os/ 188 Os ratio for the Primitive Upper Mantle (PUM) (~0.1289 at 95 Ma) [START_REF] Meisel | Osmium isotopic compositions of mantle xenoliths : A global perspective[END_REF]. Light grey, dark grey and green curves on both sides of the diagrams show the probability density distributions of 187 [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] (symbols as in Fig. 3), with attribution to each mantle domain based on geographic positions in the mantle section.

Discussion

Our coupled geochemical and structural observations underscore three findings:

(1) Throughout the mantle section, harzburgite Mg#, Al 2 O 3 contents, Os concentrations and isotopic compositions span a range comparable to that of abyssal peridotites (Becker and Dale, 2016) (Fig. 3,4a). Dunites display more variable compositions linked to their structural contexts, with 187 Os/ 188 Os 95Ma and Al 2 O 3 content increasing and Mg# generally decreasing from the BBU through the MMS to the MTZ (Fig. 3, 4b-c).

(2) In the BBU, dunite Os isotope compositions closely resemble those of adjacent harzburgites (Fig. 4c). In contrast, dunites in the overlying MMS have more radiogenic compositions than associated harzburgites.

(3) In the MTZ, dunite 187 Os/ 188 Os 95Ma ratios are much more radiogenic than those of abyssal peridotites or even MORB [START_REF] Gannoun | Highly siderophile element and Os isotope systematics of volcanic rocks at divergent and convergent plate boundaries and in intraplate settings[END_REF], and dunite Os concentrations are low (Fig. 4b-c).

Evidence against seawater contamination

We first consider whether the radiogenic compositions of the MTZ dunites result from hydrothermal circulation of seawater, which has high 187

Os isotope evolution during melt-rock interaction

First order considerations

Dunite bodies are generally viewed as reaction products formed as melts migrate through host harzburgite, dissolving pyroxene and precipitating olivine. Focused melting experiments show that melt fractions of 0.25-0.30 are required to dissolve all pyroxenes at 0.5 GPa [START_REF] Lambart | An experimental study of focused magma transport and basalt-peridotite interactions beneath mid-ocean ridges: Implications for the generation of primitive MORB compositions[END_REF].

However, once all pyroxenes have been removed, the amount of additional melt migrating through a dunite body is difficult to quantify. Signatures of incompatible element tracers, which are dramatically less abundant in dunite than in melt, will be rapidly overprinted by equilibration with the liquid during melt percolation. In contrast, peridotites, including dunites, have higher Os concentrations than mantle melts. Thus, as melts equilibrate with peridotite during melt percolation, dunite Os concentration and 187 Os/ 188 Os should change slowly and progressively, allowing this element to serve as a tracer of the extent of melt-peridotite interaction.

To roughly estimate the proportion of melt needed to obtain the initial isotopic compositions of the three types of dunite, we first consider a two-component mixture between peridotite ( 187 Os/ 188 Os= 0.124, typical of Oman harzburgites) and melt (0.2 ppb Os; 187 Os/ 188 Os= 0.150). The melt 187 Os/ 188 Os was chosen to be slightly more radiogenic than the MTZ dunites. This value is higher than those of all MORB and most arc lavas (except for extrusives with extremely low Os contents, whose isotope compositions are easily modified by assimilation of radiogenic crustal material; see data compilation in

Melt-rock interaction during melt percolation

Single-cell percolation model

While two-component mixing models allow qualitative assessment of the melt fractions needed to produce the radiogenic compositions of MMS and MTZ dunites, melt percolation models taking the Os partition coefficients (k d ) of the pertinent phases into account are more realistic. One critical point is the compatible behavior of Os in common mantle rocks partly due to its extremely chalcophile nature (k d in sulfide ~10 6 ) [START_REF] Mungall | Partitioning of platinum-group elements and Au between sulfide liquid and basalt and the origins of mantle-crust fractionation of the chalcophile elements[END_REF]. During percolation of sulfide-undersaturated melts in the uppermost mantle, the sulfide fraction in the peridotite decreases progressively. As long as the peridotite contains sulfide, Os is trapped in the residue. When the peridotite loses its final trace of sulfide, the Os partition coefficient of the peridotite is likely to be lowered significantly causing Os concentration to decrease with continuing melt percolation. Concomitantly, if the percolating melts have a more radiogenic composition, the 187 Os/ 188 Os of the host peridotite increases gradually.

In a simple model, a representative elementary volume (cell) of harzburgite is flushed repeatedly by increments of 2% melt of a constant radiogenic composition (details in Supplementary Material). As in most more complex models (e.g. [START_REF] Vernières | A plate model for the simulation of trace element fractionation during partial melting and magma transport in the Earth's upper mantle[END_REF]), chemical and isotopic equilibrium with the peridotite is assumed to be attained after each melt increment is added. After equilibration, the melt moves out of the cell and is replaced by fresh melt of the original composition. We suppose that the initial Os compositions of melt and harzburgite are the same as those used in the two-component mixture, and that transformation of harzburgite to dunite occurs after interaction with 25-30% of melt, corresponding to 10-15 melt increments. For simplicity, we also consider that complete sulfide removal is coincident with full transformation of harzburgite into dunite, i.e. when opx depletion is complete. If sulfide exhaustion occurs slightly before or after transformation to dunite, the results will not change substantively. In our model, the harzburgite contains 100 ppm sulfide, equivalent to ~40 ppm S, assumed to decrease to zero over 10 increments of 2% melt.

After each increment of new melt equilibrates with the peridotite, the Os isotope composition is controlled by mass balance, while the new Os concentrations of the melt and the residual peridotite are controlled by the Os partition coefficients of the phases present. Osmium is strongly retained by the residue as long as sulfides remain. As sulfur is removed into the silicate melt, the IPGE (Iridiumgroup PGE: Os, Ir, Ru, Rh) are increasingly concentrated in the refractory sulfide residues, leading eventually to exsolution of platinum group minerals (PGM) such as Ir-Os alloys [START_REF] Fonseca | New constraints on the genesis and long-term stability of Os-rich alloys in the Earth's mantle[END_REF]. to a lesser extent in olivine, suggesting that Os could also be hosted by these phases. Thus, Os may behave as a moderately compatible element in dunite even after all sulfides have been removed.

Ir
Nevertheless, dunites that have been fluxed by large quantities of magma have lower Os concentrations than most mantle harzburgites [START_REF] Büchl | Melt percolation monitored by Os isotopes and HSE abundances: A case study from the mantle section of the Troodos Ophiolite[END_REF], suggesting unsurprisingly that the compatibility of Os in peridotite is lowered in the absence of sulfide. Taking these observations into consideration, we consider a bulk partition coefficient (D Os ) of 2 for dunites, which are assumed to have lost all of their original sulfide content. The effect of varying this value is explored below. Further constraint of D Os will require a better understanding of the behavior and stability of Os-bearing PGM during melt fluxing.

Results for the three types of dunite in the model framework

Results of the model are compared with the data in Fig. 6. The model is obviously simplistic. The harzburgitic protolith is treated as a single composition rather than the wide range of harzburgite compositions observed in the ophiolite, likely explaining much of the mismatch with the data.

Similarly, incoming melt composition and D Os , treated as constants, most probably vary with time.

Nevertheless, this model provides a conceptual framework for examining the extent of melt-rock reaction experienced by each type of dunite. While the geometries and contexts of the three types of dunite are quite different, the melt-rock reaction process can be modeled in each case by melt fluxing through a volume of rock, using the parameters given in the caption to Fig. 6. [Os] during this process. As repeated increments of melt (taken here as 2%) traverse the harzburgite, the pyroxenes will dissolve progressively, resulting in dunite formation after ~10-15 increments have passed (zone indicated as D-H on diagram). Concomitantly, any sulfides present in the harzburgite will also be dissolved in the percolating melt, with complete sulfide dissolution assumed to approximately coincide with the transition from harzburgite to dunite. As Os will be retained as long as sulfide is present, Os concentrations will not start to decrease before this transition is nearly complete. In the example shown, the starting harzburgite is assumed to contain 100 ppm sulfide (~40 ppm S), k d,Os = 10 6 for sulfide and D Os = 2 for dunite (see discussion of parameter choices in text and Supplementary Material).

After transition to dunite, continued fluxing with repeated increments of melt (tick marks placed at 100, 200, 300 and 400 increments on the curve) will cause the Os isotopic ratio and concentration to move from the starting position in the abyssal peridotite field along the black curve towards the higher shown for comparison with the results of the melt percolation model (symbols as in Fig. 3).

In the MTZ, about 300 increments of melt fluxing, equivalent to a melt/rock ratio ~6, would be needed to lower the Os contents and raise the 187 Os/ 188 Os ratios of the initial harzburgite to the values (~0.2-1.0 ppb and 187 Os/ 188 Os 95Ma ~0.138-0.149) observed in MTZ dunites. Such high melt/rock ratios are consistent with the intense melt-peridotite interactions thought to occur in this zone [START_REF] Boudier | Nature of the Moho Transition zone in the Oman ophiolite[END_REF][START_REF] Rospabé | Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite : Implications for melt/fluidrock reactions at Moho level beneath oceanic spreading centers[END_REF].

In the underlying MMS, nearly all dunites have 187 Os/ 188 Os 95Ma in the range of 0.125-0.133, and in each pair studied, the dunites are more radiogenic than the associated harzburgite. For the same melt composition, the degree of melt-rock interaction required to obtain MMS-like Os isotopic signatures is smaller than for MTZ dunites (less than 150 increments of melt, corresponding to melt/rock ratios <3; Fig. 6). This is consistent with dunite formation by wall-rock reaction in melt dikes traversing the MMS (Fig. 2b, 7a), a process reported in other peridotitic bodies [START_REF] Becker | Effects of melt percolation on the Re-Os systematics of periodites from a Paleozoic convergent plate margin[END_REF][START_REF] Büchl | Melt percolation monitored by Os isotopes and HSE abundances: A case study from the mantle section of the Troodos Ophiolite[END_REF].

Wall-rock reaction would not have significantly modified the composition of melt flowing rapidly through the center of the dike, because of the limited contact with the peridotite relative to the large volume of melt. In one case, two dunites were collected in the reaction zone of the same dike. Dunite 16OD4C, sampled ~30 cm from the dike edge and impregnated with plagioclase and clinopyroxene, has a moderately radiogenic 187 Os/ 188 Os 95 Ma ratio (0.1327). Clinopyroxene-impregnated dunite 16OD4D Dun , from the border with the host harzburgite (16OD4D Hzb ), is less radiogenic (0.1268), suggesting lesser degrees of melt-rock interaction than near the dike center (Fig. 7a). This dunite is nevertheless more radiogenic than the harzburgite (0.1169). The unusual MMS dunite band 16OD9A, with very high Os concentration (11.80 ppb) and very radiogenic 187 Os/ 188 Os 95Ma (0.1483) cannot be explained by this model and instead requires a specific mechanism of PGM or radiogenic sulfide precipitation. The Os isotopic composition of this sample is similar to the melt end-member used in the model, suggesting that such phases could have precipitated from magmas similar to those that interacted with harzburgite to form the MTZ and MMS dunites.

Individual basal dunites show Os isotope compositions in the abyssal peridotite range, indicating that each experienced much lower extents of melt-rock interaction than MTZ or MMS dunites (<<100 melt increments; Fig. 6), though, given the high proportion of dunite, the total melt flow through the BBU may have been quite high. Unlike the MMS dunites, BBU dunites have 187 Os/ 188 Os 95Ma ratios very similar to those of adjacent harzburgites (Fig. 4c). They also have higher Mg#s and lower Al 2 O 3 contents than MMS and MTZ dunites (Fig. 3), arguing for lesser degrees of interaction with fertile melt. Furthermore, some sections include opx-bearing dunite and/or opx-poor harzburgite bands, likely formed by small degrees of melt-rock interaction insufficient to fully dissolve orthopyroxene.

Two dunites have low Os concentrations coupled with very unradiogenic compositions, implying formation from harzburgites with very low 187 Os/ 188 Os (though within the abyssal peridotite range) and perhaps interaction with less radiogenic melts than those required for MTZ dunites.

Insensitivity to parameter choices

The pertinence of the model parameter choices was evaluated by changing each parameter individually while holding the others constant (Fig. 8). The instantaneous melt fraction modifies the number of melt increments needed to obtain a given 187 Os/ 188 Os ratio, but has little effect on the corresponding melt/rock ratio (Fig. A.3) or on the relationship between 187 Os/ 188 Os and [Os] (Fig. 8a).

The Os bulk-rock partition coefficient after sulfide exhaustion affects the rapidity with which Os in the dunite is lost to the passing melts, and accordingly the rate at which 187 Os/ 188 Os is modified by repeated introduction of new melt (Fig. 8b A mechanism is needed to transport the radiogenic melt to the upper MMS and the MTZ. In one commonly accepted model [START_REF] Kelemen | Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels[END_REF], the melt is transported by porous flow in dunite channels. Once the olivine-rich channels are established, the melt is protected from equilibration with the surrounding peridotite, but equilibrates with the dunite through which it travels (Fig. 7b). To consider whether this mechanism is consistent with our Os results, we model a dunite channel as a stack of reaction cells, each similar to that described above (Fig. 9a and details in Supplementary material). Each increment of fresh radiogenic melt enters the bottom cell and equilibrates with the peridotite, and then migrates upwards into the overlying cell and is replaced by a new melt increment with the original composition. In this idealized model, the melt leaving this bottom cell will have a much less radiogenic Os composition than the entering melt, because of the strong buffering effect of the Os-rich peridotite (Fig. 9b). Similarly, the 187 Os/ 188 Os ratios of the overlying cells will be successively less affected by melt interaction, with the ratio of the uppermost cell being hardly modified at all (Fig. 9c). Therefore, melt leaving the dunite channel will have a 187 Os/ 188 Os ratio close to that of the original harzburgite from which the dunite formed (Fig. 9c). This implies that melts that have traveled through such channels, equilibrating with the dunite along the way, cannot retain the highly radiogenic compositions needed to explain the high 187 Os/ 188 Os 95Ma ratios of the MTZ dunites and the overlying lower crust. Instead, a mechanism without (or with little) equilibration with dunite or harzburgite is needed to bring melts with a radiogenic Os signature from depth towards the Moho.

This lack of equilibration suggests rapid transfer of large melt volumes through the upper mantle section, also proposed by [START_REF] Kogiso | Length scales of mantle heterogeneities and their relationship to ocean island basalt geochemistry[END_REF] to explain the existence of linear correlations between Os and Sr, Nd and Pb isotopes in some ocean island basalts. The quantitative constraint that preservation of radiogenic Os compositions places on melt migration rates will depend on the geometry of melt pathways and on Os diffusion coefficients. While there are few experimental constraints on the latter, on theoretical grounds, Kogiso et al. ( 2004) estimated Os diffusion coefficients in olivine to be similar to those of Sr and Pb.

In addition to the melt brought to the MTZ through a rapid process, some melt may have percolated through the harzburgite in mantle diapirs [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] Lorand | Platinum-group element signature of the primitive mantle rejuvenated by melt-rock reactions: evidence from Sumail peridotites (Oman Ophiolite)[END_REF]. Coexistence of two transport processes could be consistent with modeling of melt migration beneath mid-ocean ridges based on U-series isotope systematics [START_REF] Elkins | Testing pyroxenite versus peridotite sources for marine basalts using U-series isotopes[END_REF]. However, any melt percolating slowly through harzburgite will acquire the non-radiogenic isotope composition of the peridotite, of which no trace is found in the MTZ dunites. Such a scenario would require the melts traveling rapidly to have had an even more radiogenic composition before mixing in the MTZ with melts that may have traveled slowly.

Concordance of model with geological observations

The radiogenic Os composition of the MTZ dunites and overlying lower crust implies that they cannot have been fed by melts percolating through and equilibrating with a network of coalescing dunite channels. This inference is consistent with the scarcity of several meter thick dunites throughout the MMS. These observations contrast with the common view that large tabular dunites are frequent If, as the geochemical and structural data indicate, melt did not migrate to the Moho through dunite channels, either vertically or along a putative permeability barrier, how was it transported? It appears to have traveled rapidly across the mantle section, possibly leaving very little chemical and isotopic imprint, except perhaps as small dunite reaction zones similar to those developed at the margins of the later cross-cutting veins (Fig. 2b,7a). If such dunites formed while the mantle was still flowing under asthenospheric conditions, they could have been transposed and stretched. These dunites may be difficult to identify now, or may correspond to the rare thin concordant bands observed in the MMS (insert in Fig. 2b and Table A.1). The modeling of [START_REF] Stevenson | Spontaneous small-scale melt segregation in partial melts undergoing deformation[END_REF] and the experimental results of [START_REF] Daines | The transition from porous to channelized flow due to melt/rock reaction during melt migration[END_REF] predict the formation of veins in the melting mantle. Rapid melt transport through hydrofractures in the asthenosphere is a physically complex process [START_REF] Richardson | Melt flow in a variable viscosity matrix[END_REF][START_REF] Rubin | Propagation of Magma-Filled Cracks[END_REF], whose viability remains to be better established, but this process would fit our structural and geochemical data.

Conclusions

Dunite is distributed in the mantle section of the southeastern Oman ophiolite in three distinct domains: (1) the Moho Transition Zone, a horizontal dunitic melt storage and reaction zone at the summit of the mantle section, representing by far the largest volume of dunite; (2) the harzburgitic main mantle section, just below the MTZ, crosscut by sporadic dikes rimmed by dunitic reaction zones and rarer concordant bands; and (3) the basal banded unit, with many concordant dunitic bands above the metamorphic sole.

Though these three types of dunite cannot be distinguished petrographically, their Os isotope compositions are strongly correlated with their structural context. Basal dunites, like harzburgites throughout the massifs, have 187 Os/ 188 Os 95Ma ratios in the range of abyssal peridotites, while the dunitic veins and bands of the MMS have variable but somewhat more radiogenic compositions.

Critically, MTZ dunites have highly radiogenic Os isotope compositions, similar to those of the overlying lower crust. Such compositions are inconsistent with models that call on melt transport to the ridge by porous flow through dunite channels, as the high 187 Os/ 188 Os ratios of the Os-poor melts would be erased by equilibration with the unradiogenic compositions of the Os-rich dunites through which they travel. Furthermore, the absence of thick vertical dunite bodies in mantle diapirs, thought to represent magmatic feeder zones for the overlying crust, also argues that dunite channels are not the dominant pathway of melt transport to the ridge. Thus, our results reveal the need for a process, possibly hydrofracturing, that can rapidly bring radiogenic melts from the source region to the Moho without significant equilibration with dunite or harzburgite of the surrounding mantle.

Fig. 1 .

 1 Fig. 1. Sample locations and structure of the southeastern part of the Oman ophiolite. (a) Simplified

  and Fig. A.1). Complementary observations were made at sites near Muscat, at the base of the Haylayn massif and in the Batin area in the eastern part of Wadi Tayin.

Fig. 2 .

 2 Fig. 2. Field photographs illustrating the three types of dunite observed in the mantle section of the SE

  Becker and Dale, 2016), with the least radiogenic samples having Redepletion model ages (T RD ) ~2.2 Ga (using Primitive Upper Mantle (PUM) evolution curve of Meisel et al. (2001)). T RD ages are obtained by comparing measured 187 Os/ 188 Os values directly with the mantle evolution curve, implicitly assuming Re concentration equals zero in residual peridotite after melt extraction. However, this assumption is often false, as moderate melt extraction leaves peridotites with non-zero Re contents. Therefore, T RD ages are always minimum values that could dramatically underestimate true melt extraction ages. Nevertheless, the T RD ages >0.5 Ga of most of the harzburgites imply that they experienced some melt extraction hundreds of million years before possible remelting beneath the Oman spreading center.

  (0.1415 to 0.1490) are higher than the estimated PUM value (~0.1289 at 95 Ma; Meisel et al. (2001))

  187 Os/ 188 Os 95Ma ratios (0.1165 to 0.1280) are similar to abyssal peridotite values. Unlike in the MMS, paired dunites and harzburgites collected in the BBU have very similar isotope ratios. When placed in the structural framework defined here, previous Mg#, Al 2 O 3 , Os concentration and Os isotope results from harzburgites and dunites of the Oman ophiolite (Godard et al., 2000; Hanghøj et al., 2010; Senda et al., 2020; Xiong et al., 2020a,b) are consistent with these observations.

Fig. 3 .

 3 Fig. 3. Whole-rock major element compositions of harzburgites and the three types of dunite

  [START_REF] Bezard | Os isotopic composition of western Aleutian adakites: Implications for the Re/Os of oceanic crust processed through hot subduction zones[END_REF]). Results are shown for two peridotite Os concentrations (Fig.5; details in Supplementary Material), representing typical harzburgite (4 ppb) and MTZ dunite (0.5 ppb). The peridotite187 Os/ 188 Os ratio rises slowly with increasing melt fraction and is nearly unchanged at the melt fraction (0.25-0.30) needed to fully transform harzburgite to dunite. Much higher melt fractions, 0.75-0.96 and ≥0.94 respectively using the assumed parameters, are needed to attain 187 Os/ 188 Os values of MMS (~0.130) or MTZ (~0.140) dunites.

Fig. 5 .

 5 Fig. 5. Two-component mixture between peridotite ( 187 Os/ 188 Os= 0.124) and melt ( 187 Os/ 188 Os= 0.150;

Fig. 6 .

 6 Fig. 6. Effect of percolation of radiogenic melt ( 187 Os/ 188 Os= 0.150; [Os]= 0.2 ppb) through a representative elementary volume (i.e., a single cell) of typical harzburgite ( 187 Os/ 188 Os= 0.124; [Os]= 4 ppb). Black curve shows evolution of peridotite 187 Os/ 188 Os vs. [Os] during this process. As repeated

187Os/ 188

 188 Os 95 Ma ratios and lower Os concentrations found in the MTZ dunites. Dotted grey curve represents percolation of a different melt ([Os]= 0.2 ppb; MORB-like 187 Os/ 188 Os= 0.133 -Gannoun et al. (2016)), interacting with harzburgite with 187 Os/ 188 Os towards the bottom of the abyssal peridotite range (Becker and Dale, 2016), possibly explaining low [Os] and unradiogenic compositions of two of the basal dunites. Os concentrations and initial Os isotope ratios for the three types of dunite are

Fig. 7 .

 7 Fig. 7. Schematic diagrams of two end-member melt transport mechanisms: hydrofracturing (a) vs.

  and Fig. A.4). As discussed above, this parameter depends on the behavior and stability of rare PGM, and though modeled as a constant, probably changes during melt fluxing. To match both the Os isotope compositions and concentrations of the MTZ dunites, D Os values between about 0.5 and 5 are required (Fig. 8b). For input melt of a fixed Os isotope composition, the Os concentration will control the number of iterations required to obtain a given 187 Os/ 188 Os ratio. The data field can be matched by melt Os concentrations ranging from about 0.02 to 0.5 ppb (Fig. 8c and Fig. A.5). To produce MTZ dunites, the starting melt must have a 187 Os/ 188 Os ratio of at least 0.15 (Fig. 8d and Fig. A.6), but melts with much higher ratios do not fit the data. The reasonableness of all of the successful parameters supports the plausibility of the model.

Fig. 8 . 3 .

 83 Fig. 8. Effect of varying (a) instantaneous melt fraction (𝑓) (0.5%, 2% and 4%), (b) Os bulk partition

Fig. 9 .

 9 Fig. 9. Multi-cell model (see text) showing Os evolution during melt percolation through dunite channel

  throughout the mantle section, as first expressed in the schematic diagram of[START_REF] Lippard | The Ophiolite of Northern Oman[END_REF]. In the southern massifs of the Oman ophiolite, dunites are abundant in only three areas: (1) the MTZ, which structural, petrographic and geochemical evidence[START_REF] Boudier | Nature of the Moho Transition zone in the Oman ophiolite[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] Rospabé | Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite : Implications for melt/fluidrock reactions at Moho level beneath oceanic spreading centers[END_REF] suggest formed in situ, while the overlying crust was still partially molten; (2) the BBU above the metamorphic sole; (3) the vicinity of the spectacular, yet atypical, giant Batin dunite body. In contrast, dunites are very rare even in the cores of mantle diapirs, thought to represent magmatic feeder zones for the overlying crust. The model of porous melt transport in vertical dunite channels[START_REF] Kelemen | Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels[END_REF] would predict the existence of large vertical dunite roots beneath the thick MTZ in these areas. Yet, such dunite conduits have not been found within any of the mantle diapirs. Alternatively, it has been proposed that upwardly migrating melts pool along a permeability barrier (e.g. the top of the melting region, the base of the lithosphere or the locus of plagioclase crystallization) and then migrate laterally, "uphill", along this boundary towards the ridge axis through porous flow channels or as "porosity waves"[START_REF] Hebert | Generation of permeability barriers during melt extraction at mid-ocean ridges[END_REF][START_REF] Sparks | Melt extraction from the mantle beneath spreading centers[END_REF]. At first glance, the basal dunite bands could correspond to the geometry suggested by this model. However, their Os isotope compositions are equivalent to those of the surrounding harzburgite, indicating they did not serve as pathways for the much more radiogenic melts that reached the MTZ and the lower crust.

  Os/188 Os ratios for abyssal peridotites (compilation in[START_REF] Becker | Re-Pt-Os isotopic and highly siderophile element behavior in oceanic and continental mantle tectonites[END_REF]), MORB (MORB sulfide data compiled in[START_REF] Gannoun | Highly siderophile element and Os isotope systematics of volcanic rocks at divergent and convergent plate boundaries and in intraplate settings[END_REF] and Oman lower crust[START_REF] Peucker-Ehrenbrink | Rhenium-osmium isotope systematics and platinum group element concentrations in oceanic crust[END_REF] respectively. Published Oman peridotite data are shown for comparison: MTZ dunites(Xiong et al., 2020a,b); MMS dunites[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] Xiong et al., 2020a,b) and MMS harzburgites

Table 1 .

 1 Mg#, Re and Os concentrations and Os isotope compositions of the studied Oman peridotitesOs isotopic compositions in whole rock were analyzed at the CRPG laboratory in Nancy (France) using N-TIMS (Finnigan MAT 262) and MC-ICP-MS (Neptune +). All uncertainties are 2σ and include all recognized sources of uncertainty, including in-run analytical precision, reproducibility of standard analyses, uncertainties related to the variability of the blank and its isotope composition, uncertainties related to the age correction (95 Ma). Samples belonging to harzburgite/dunite pairs collected in close proximity (<1 meter) are indicated. For each sample, an approximate estimation of its height in the mantle section was obtained by normalizing to the total section height to compare samples from different cross-sections. Duplicate analyses were performed for several samples (noted "-d") and UB-N standard. More details are presented in the Supplementary Material.

	Sample	Massif	Lithology	Hzb/Dun pairs	Normalized height	Mg#	[Os]	[Re]	187 Re/ 188 Os	( 187 Os/ 188 Os)m	( 187 Os/ 188 Os)95 Ma	2σ error
					in the section		(ppb)	(ppb)
	Moho Transition Zone (MTZ) peridotites
	10 OM 25	Sumail (Maqsad)	impregnated dunite		0.030	0.897	0.399	0.114	1.376	0.14214	0.13996	0.00055
	10 OM 45C	Sumail (Maqsad)	dunite		0.030	0.900	0.947	0.050	0.255	0.14456	0.14415	0.00064
	18 OK 26	Sumail (Maqsad)	dunite		0.040	0.920	0.628	0.409	3.149	0.14993	0.14494	0.00086
	10 OM 5C*	Sumail (Mansah)	dunite		0.025	0.908	1.063			0.14235	0.14115	0.00052
	10 OM 5C-d							0.168	0.760
	07 OD 1	Sumail (Mansah)	dunite		0.050	0.900	0.390	0.199	2.462	0.14151	0.13761	0.00057
	16 OL 11B	Wadi Tayin	pure dunite		0.000	0.903	1.173	0.033	0.137	0.14922	0.14900	0.00085
	16 OL 34B	Wadi Tayin	olivine troctolite		0.028	0.899	2.305	0.053	0.111	0.13209	0.13192	0.00200
	Main mantle section (MMS) peridotites
	16 OD 5C	Nakhl-Rustaq	depleted harzburgite		0.007	0.890	2.274	0.448	0.948	0.11976	0.11826	0.00199
	16 OD 5D	Nakhl-Rustaq	harzburgite		0.007	0.916	4.211	0.152	0.174	0.11362	0.11334	0.00116
	16 OD 7	Nakhl-Rustaq	harzburgite		0.053	0.909	2.921	0.025	0.042	0.12517	0.12510	0.00160
	16 OD 4DHzb	Nakhl-Rustaq	harzburgite	16 OD 4DDun	0.073	0.900	5.519	0.358	0.312	0.11741	0.11691	0.00054
	16 OD 9C	Nakhl-Rustaq	harzburgite	16 OD 9A	0.089	0.908	3.691	0.130	0.170	0.12871	0.12844	0.00134
	16 OD 2	Nakhl-Rustaq	harzburgite		0.241	0.907	4.314	0.500	0.559	0.12990	0.12902	0.00114
	16 OD 1A	Nakhl-Rustaq	harzburgite	16 OD 1B	0.337	0.906	4.164	0.599	0.693	0.12350	0.12241	0.00059
	16 OD 10	Nakhl-Rustaq	harzburgite		0.419	0.906	4.855	0.381	0.378	0.13112	0.13052	0.00106
	11 OD 15A	Sumail	harzburgite		0.280	0.912	3.894	0.152	0.187	0.11782	0.11753	0.00057
	07 OD 24	Sumail	harzburgite		0.254	0.907	4.826	0.460	0.459	0.13451	0.13378	0.00051
	07 OD 27B	Sumail	harzburgite		0.344	0.904	4.314	0.221	0.247	0.12817	0.12778	0.00052
	07 OD 8C	Sumail	harzburgite		0.150	0.910	5.246	0.379	0.348	0.12961	0.12906	0.00066
	07 OD 8C-d						5.228	0.321	0.295	0.12570	0.12523	0.00056
	16 OD 4C	Nakhl-Rustaq	impregnated dunite (vein)	(16 OD 4DHzb)	0.073	0.893	1.120	0.305	1.312	0.13474	0.13267	0.00106
	16 OD 4DDun	Nakhl-Rustaq	impregnated dunite (vein)	16 OD 4DHzb	0.073	0.899	5.954	0.414	0.335	0.12729	0.12676	0.00055
	16 OD 9A	Nakhl-Rustaq	pure dunite (band)	16 OD 9C	0.089	0.905	11.802	0.431	0.177	0.14862	0.14834	0.00063
	16 OD 1B	Nakhl-Rustaq	dunite (vein)	16 OD 1A	0.337	0.904	3.145	0.087	0.134	0.13324	0.13303	0.00065
	16 OD 1B-d						4.353	0.094	0.104	0.13135	0.13119	0.00061
	16 OD 31	Wadi Tayin	dunite (band)		0.116	0.908	2.147	0.910	2.043	0.12814	0.12491	0.00215
	Basal peridotites
	16 OD 13C	Nakhl-Rustaq	harzburgite	16 OD 13B	0.637	0.909	4.415	0.415	0.453	0.12607	0.12536	0.00054
	16 OD 13C-d						4.076	0.379	0.448	0.12647	0.12577	0.00058
	16 OL 25	Wadi Tayin	harzburgite		0.724	0.905	6.289	0.081	0.062	0.13464	0.13454	0.00088
	16 OL 26B	Wadi Tayin	harzburgite	16 OL 26A	0.724	0.910	3.214	0.022	0.032	0.12151	0.12146	0.00148
	16 OL 27B	Wadi Tayin	harzburgite	16 OL 27A	0.784	0.910	4.843	0.314	0.312	0.12522	0.12472	0.00107
	16 OL 27C	Wadi Tayin	harzburgite		0.784	0.913	4.097	0.046	0.053	0.11314	0.11306	0.00121
	16 OL 28B	Wadi Tayin	harzburgite		0.860	0.909	4.481	0.168	0.180	0.11659	0.11630	0.00113
	16 OL 29	Wadi Tayin	harzburgite		0.872	0.906	5.187	0.169	0.157	0.12256	0.12231	0.00100
	16 OL 24	Wadi Tayin	harzburgite		0.878	0.913	5.475	0.221	0.194	0.12335	0.12305	0.00097
	16 OD 13B	Nakhl-Rustaq	dunite (band)	16 OD 13C	0.637	0.923	7.169	0.158	0.106	0.12155	0.12139	0.00054
	16 OD 15E	Nakhl-Rustaq	dunite (band)		0.934	0.911	2.291	0.106	0.223	0.11688	0.11653	0.00070
	16 OD 18B	Sumail	dunite (band)		0.969	0.908	6.134	0.018	0.014	0.12800	0.12798	0.00060
	16 OD 19A	Sumail	pure dunite (band)		0.946	0.910	0.774	0.066	0.413	0.11773	0.11707	0.00120
	16 OL 26A	Wadi Tayin	dunite (band)	16 OL 26B	0.724	0.909	4.460	0.014	0.015	0.12227	0.12225	0.00112
	16 OL 27A	Wadi Tayin	dunite (band)	16 OL 27B	0.784	0.912	4.998	0.079	0.077	0.12560	0.12548	0.00102
	14 11E	Haylayn	pure dunite (band)		0.990	0.922	1.474	0.157	0.512	0.12573	0.12492	0.00053

* For 10 OM 5C sample, ( 187 Os/ 188 Os)95 Ma was calculated using Re compositions of its duplicate. Mg# = Mg/(Mg+Fe)at; Hzb = harzburgite; Dun = dunite.

  -Os alloys, laurites (RuS 2 ) and other PGM are probably the main hosts of Os in dunites and other sulfide-poor mantle peridotites (reviewed in Lorand and Luguet (2016) and O'Driscoll and González-Jiménez (2016)). In addition to PGM, experimental data (summarized in Brenan et al. (2016)) indicate that Ir and Ru are compatible in Cr-rich spinel, which constitutes ~1-3% of the dunite (Table A.2), and

Acknowledgements

We are very grateful to C. Zimmermann for her helpful assistance during Re-Os chemistry and isotope analyses by N-TIMS and MC-ICP-MS. We also thank C. Parmentier for her help with chemistry, D. Cividini and A. Schumacher for their technical support during MC-ICP-MS sessions, the SARM staff for whole-rock major element analyses, C. Martin (Géosciences Montpellier) and L. Causse (AETE-ISO platform, OSU-OREME/Université de Montpellier) for their assistance during whole-rock trace element chemical preparation and analysis by quadrupole ICP-MS, C. Prigent for providing her samples from the Haylayn massif and E. Grellier for her petrographic observations during her Master. Informal discussions and contributions during fieldwork from F. Boudier, S. Singh and D. Levy were also highly appreciated. We thank B. O'Driscoll and H. Becker for their thorough and constructive reviews.

Financial support was provided by the TelluS Program of CNRS/INSU. We are finally highly grateful to the Ministry of Commerce and Industry, Sultanate of Oman, for its hospitality and its permission to undertake field sampling in Oman.

David Jousselin: Conceptualization, Investigation, Writing -Review & Editing, Supervision, Funding acquisition Marguerite Godard: Validation, Writing -Review & Editing, Supervision Claire Aupart: Formal analysis, Investigation, Writing -Review & Editing

Author Contributions

CRediT author statement

Competing Interests statement

None of the authors have competing interests.


Luc Museur

Eduard Feldbach

Andrei Kanaev

Defect-related photoluminescence of hexagonal boron nitride

Photoluminescence of polycrystalline hexagonal boron nitride (hBN) was measured by means of time-and energy-resolved spectroscopy methods. The observed bands are related to DAP transitions, impurities and structural defects. The excitation of samples by highenergy photons above 5.4 eV enables a phenomenon of photostimulated luminescence (PSL), which is due to distantly trapped CB electrons and VB holes. These trapped charges are metastable and their reexcitation with low-energy photons results in anti-Stockes photoluminescence. The comparison of photoluminescence excitation spectra and PSL excitation spectra allows band analysis that supports the hypothesis of Frenkel-like exciton in hBN with a large binding energy.

I. Introduction.

Optical and fluorescence properties of hexagonal boron nitride deserved particular interest during last decade since the first observation of an intense far-UV exciton emission 1,2 making this material a candidate for use in new light-emitting devices. A successful synthesis of high-purity single crystal samples initially at high-P,T 1,2 and later at atmospheric conditions 2 enabled this achievement. Accordingly, understanding of the electronic band and exciton structure of hBN becomes even more important issue as it serves a basic system for single-wall 3,4 and multi-wall boron nitride nanotubes [5][6][7][8][9] .

Despites of many efforts in the past, the electronic properties of hBN remain largely unknown. They have been earlier studied by luminescence 1,[10][11][12][13][14][15][16][17] , optical reflectance and absorption [18][19][20][21] , photoconductivity 22 , x-ray emission [23][24][25] , inelastic x-ray scattering [26][27][28] , x-ray absorption 25,29,30 , or electron energy loss [31][32][33] spectroscopy. After all, a large spread of band gap energies reported in literature, ranging from 3.1 to 7.1 eV 14 , is currently explained by sample quality and related to experimental methods used. Recently, arguing on generally high luminosity of the free exciton luminescence, Watanabe et al 1 assumed that hBN is a direct band gap material. They have measured the band gap energy of 5.971 eV and inferred an exciton binding energy of 0.149 eV that corresponds to the Wannier exciton model. However this result is in a large disagreement with the most recent theoretical calculations using the so-called all electrons GW approximation 34,35 . They predict hBN to be an indirect band gap material with a band gap energy of 5.95 eV and a lowest direct interband transitions at 6.47 eV 34 . Moreover, Arnaud et al. 34 has deduced a huge exciton binding energy of 0.72 eV and predicted that the low-lying exciton in hBN belongs to the Frenkel type. The intense free exciton luminescence observed in single crystal hBN is explained by a large oscillator strength of merged excitonic transitions 34 .

In view of many disagreements further confrontation between experiment and theory will continue. In these conditions, more experimental data concerning electronic and related optical properties of hBN are highly required. In particularly, energy-and time-resolved photoluminescence methods provide valuable information about excitonic and interband transitions. Moreover, in polycrystalline samples energy transfer to impurities and defects may inhibit these intrinsic transitions and strongly affect the fluorescence spectra. However, the energy transfer is specific to excitation energy and the relevant excited states can be experimentally resolved.

In the present article we report on detailed analysis of the defect-related intraband luminescence of hBN. Important information about intrinsic properties of hBN is obtained by combining two experimental approaches: time-and energy-resolved photoluminescence and photostimulated luminescence. The discussion is organized as follow. The experiment is described in section II and experimental results are presented on the section III. In section IV-A we discuss the nature of the observed luminescence bands. Finally, in section IV-B, careful comparison between photoluminescence excitation (PLE) and photostimulated luminescence (PSL) excitation spectra brings more precision to exciton and bandgap transitions of hexagonal BN.

II. Experiment.

The samples were prepared from commercial hexagonal BN powders (Alfa 99.5%) compacted in pellets of size 8x8x1 mm 3 at hydrostatic pressure of 0.6 GPa. The grit size of the hBN powder has been estimated by means of granulometry and transmission electron microscopy (JEM 100C JEOL). It ranged from 0.3 to 10 µm with an average particle size of 3.1 µm corresponding to the maximum in the mass distribution curve. The samples were then heated at 800 K under vacuum for 12 hours to avoid organic impurities and traces of water.

The luminescence properties of the samples were studied using VUV synchrotronradiation (SR) source of DORIS storage ring at HASYLAB (DESY, Hamburg). The facility of the SUPERLUMI station used in the experiments is described in details elsewhere 36 . Briefly, samples were cooled down to 8K and irradiated by monochromatized SR (

III. Results.

Under photoexcitation above 6 eV the luminescence spectra of hBN are composed of several luminescence bands (Figure 1). Basically, one broadband and one structured emissions between 3 and 4 eV, and two relatively narrow near bandgap emissions at 5.3 eV and 5.5 eV were distinguished. Two high-energy emissions were discussed in details in our recent publication 17 . They were assigned respectively to quasi donor-acceptor pairs (q-DAP)

(5.3 eV band) and bound excitons (5.5 eV band). Below we complete the assignment of lowenergy luminescence.

A -Luminescence around 4 eV in hBN powder samples.

The hBN powder sample is a strongly luminescent material, which luminescence spectra depend on excitation energy. The low-temperature luminescence spectra of hBN samples recorded with different excitation energies varying from 4 eV to 6.5 eV are presented in Figure 1. Under excitation below 5.0 eV a strong structured UV emission is observed. Four peaks, labeled (α), (β), (γ) and (δ), are clearly visible. A multiple Gaussian fit procedure results in the peak energies of 4.099 eV (α ), 3.912 eV (β), 3.731 eV (γ) and 3.539 eV (δ). These four peaks are equally spaced in energy by ω g = 186 ± 1.4 meV. At room temperature these peaks are broadened, but no appreciable shift was detected. When the excitation energy exceeds 5.0 eV a very broad band (∆E~1 eV) centered at 3.9 eV appears and superimposes with the structured emission (Figure 1.c and d). However, the peaks of the structured emission can always been observed on the top of the broad band.

At excitation energy of 4.27 eV only the structured UV emission can be observed. The luminescence decay curves of its four peaks (α)-(δ) are shown in Figure 2 by curve (ε). At this excitation energy the broad band dominates the luminescence spectra. Although the probed energy E lum = 3.91 eV is in coincidence with the peak (β) of the structured emission, the decay curve appears to be multi-exponential and longer lifetime than that of the peak (β).

The time-resolved photoluminescence method allows separating several contributions of superposed luminescence bands if their respective lifetimes are noticeably different. This is the case of the structured and continuous broadband emissions around 4 eV. Similar luminescence spectra to those depicted in Figure 1 have been recently

reported on photoluminescence and cathodoluminescence experiments of commercial hBN powder 10,[39][40][41][42][43][44][45][46][47][48] . However, in these publications, no distinctions were made between the broadband and structured emissions. We now assign the structured UV emission to impurities (probably, C) and the broadband emission to deep donor-acceptor pair (DAP) of strongly localized center. These assignments will be discussed below in section IV-A.

B -Photostimulated luminescence in hBN powder samples.

The photostimulated luminescence arises from the trapping of free charge carriers in distant lattice sites with subsequent recombination of carriers released from these traps by a visible light 38 . The charge separation and trapping can result from different excitation processes listed below and depicted in Figure 4:

o o exc h e υ h D A D A D A + → + + + → + + + - + - + - (1) o o o exc e υ h D A D A D A + → + + → + + - + + - (2) o o o exc h υ h D A D A D A + → + + → + + + - + - (3) o o exc υ h D A D A + → + + + - (4)
The band-to-band transitions (Equation 1) or impurity-band ionization (Equations 2-3) are the most likely contributions to PSL. In these cases at least one of photoexcited charge carriers (e -or h + ) becomes free and can migrate away from the point of excitation before being trapped by the acceptor or donor defects. Spatially closely trapped charges then annihilate giving rise to phosphorescence. In contrast, at rather large distance their lifetime becomes infinite and their recombination can only be possible after reactivation by light or thermally.

The Figure 5 The observed phosphorescence might also be related to the "dark" (dipole forbidden) exciton states theoretically predicted in hBN by Wirtz et al. 35 . The authors suggested a coupling between the "white" (dipole allowed) and "dark" exciton states, which makes them appear in the absorption spectra of hBN monocrystals reported by Watanabe et al 1 .

However, we disregard their contribution in PSL experiments since the phosphorescence is already observed at 1 5. = exc E eV (Figure 5), which is well below the exciton energy in hBN. Moreover, PSL is related to the states with infinite lifetime (excited above 5.5 eV), which depopulation is only triggered by light. In contrast, the hypothetic "dark" states become accessible when the crystalline symmetry is broken through defects or limited sample quality: this is our case, since no free exciton emission has been observed. Accordingly, the singlet and triplet exciton are expected to be strongly coupled in our polycrystalline hBN samples and their lifetime is short (sub-nanosecond). In this case no long-lived exciton states are expected.

The PSL excitation spectrum is represented by open circles in Figure 6. The full measured spectrum is shown in the insert of the figure for indicative purposes. In the following, we will discuss the PSL phenomenon specific to excitation energies ranging from 5 to 7 eV. The signal shows up appreciably at 50 5.

> exc E
eV in correlation with the appearance of a long component of the phosphorescence decay (Figure 5). . This emission was discussed in details in our previous publication 17 and has been assigned to the radiative recombination of the so called quasi donor acceptor pair (q-DAP).

IV. Discussion. A -Luminescence of polycrystalline hBN samples

The structured UV luminescence ( Figure 1.a and b) has been reported in the past by numerous groups, either in cathodoluminescence 10,43,46,47 or photoluminescence experiments 10, 14, 39-42, 44, 45, 47, 48 . Some authors have assigned it to intrinsic luminescence of hBN molecular layers 47 or to phonons assisted band edge luminescence 42 . However, recent experiments 1 and theoretical calculations 34 indicate the hBN bandgap energy in the range from 6 to 6.5 eV that disables these interpretations. Others authors 45, 49, 50 claimed that the structured luminescence is due to transition between the conduction band and acceptor carbon atom at substitutional N site. Recently, experimental evidence of the correlation between carbon and oxygen contents in hBN samples and intensity of the structured luminescence has been given 51 .

In a recent publication 39 the structured luminescence of hBN has been attributed to defects or impurities. The peaks (β), (γ) and (δ) in Figure1.a were ascribed to phonon replicas of the zero phonon line (α) involving TO phonons (ω TO = 169 meV). Due to the low sample temperature in present experiments, the luminescence peaks become well resolved, allowing more precise determination of their spectral positions than in room-temperature experiments 39, 42 . The obtained phonon energy of ω g = 186 meV, which falls between the known energy of LO (199 meV) and degenerated TO/LO (169 meV) phonons at the Γ point 18,19,26,52 , corresponds more probably to a local phonon mode around the impurity involved in the luminescence process. Moreover, the pronounced red shoulder observed on each phonon replica peak shows that multi-phonon processes play a major role in the energy relaxation of the impurity. The coupling between the defect and the lattice is weak as it is shown by the Huang-Rhys factor S = 1.3 obtained from the normalized peak intensities. This assignment is consistent with fast luminescence decay of the peaks (α), (β), (γ) and (δ) (figure 2).

The photoluminescence excitation (PLE) spectrum shown in Figure 3.b supports our interpretation of the defect related luminescence. Indeed, it reflects features characteristic of such transitions. The luminescence intensity steeply increases and then decreases progressively with the excitation energy. The sharp onset in the PLE spectrum at 4.09 eV corresponds to the minimum energy required to excite the impurity. As in the case of weak electron-phonon coupling, this absorption onset coincides with the zero-phonon luminescence peak (α). Interestingly, two phonons replicas separated by ω e =170 meV, that is close to the energy of the TO/LO degenerated phonons 26 , can be observed in the PLE spectra. They are indicated by arrows in Figure 3.a. We see that the energies of phonons involved in the luminescence (ω g ) and excitation (ω e ) processes are different. This results from the electronic state of the defect, which is not the same in both cases. Consequently, lattice deformations around the defect are different that affects the local phonon frequencies.

The fact that e g ω > ω signifies a stronger matrix interaction with the impurity in the excited state.

We do not evidence the impurity involved in the luminescence process in current experiments. However, we can guess about the influence of carbon with more or less confidence. Recent experiments reveal that the structured luminescence strongly shows up in hBN samples contaminated by carbon and oxygen 51 . This result is consistent with our previous results 39 and is supported by our complementary photoluminescence experiments carried out with a pyrolytic BN sample (pBN). These experiments will be described in details in a forthcoming article 53 . The pBN is a high purity material, free of carbon compound, obtained by gas-phase reactions between BCl 3 and NH 3 at 2300 K and deposition on a Si substrate (CVD method). Actually, pBN sample exhibits no structured emission around 4 eV whatever the excitation energy is. Its excitation by energy photon above 4.5 eV uniquely results in a continuous emission band similar to that observed in hBN 53 . Moreover, the PLE spectrum of hBN (Figure 3) is rather complicated with a first edge at 4.09 eV and a second one at 5.2 eV. This may indicate that the defect involved in the radiative process has at least two excited levels separated by 1.1 eV positioned within the bandgap. Since, the substitutional carbon impurity at nitrogen site (C N ) is supposed to introduce two energy levels split by ~0.8 eV in the energy range 3.2 -4.9 eV below the conduction band of hBN, 50 its participation in the luminescence process is most likely.

We discuss now the nature of the broad luminescence band observed in hBN powder sample under photoexcitation above 5 eV. Due to its long and multi exponential decay (curve (ε) in figure 2), we assign this luminescence to radiative recombination of deep donor and acceptor pairs (DAP):

DAP hυ + + + + → D A D A - o o (5)
We remark that this deep DAP is different from that related to the 5.3-eV emission (q-DAP 17 ).

As Figure 3.c shows, the correspondent PLE spectrum of the long-lived luminescence component is dominated by the excitonic peak at 5.95 eV. This fact indicates that the energy transfer to the DAP recombination channel via free excitons is efficient:

0 0 - D A D A h + → + + → + + exciton exciton υ hBN exc (6)
This excitonic peak is preceded by a weak shoulder for excitation energies ranging from 5 eV to 5.5 eV. We have assigned this shoulder to a direct excitation of the deep DAP (Equation 4). The PSL spectrum displayed in Figure 6 shows that no noticeable photostimulated luminescence can be observed following hBN irradiation between 5 and 5.5 eV that indicates no efficient population of distant traps. In contrast to the direct ionization of the donor or acceptor (Equations 2-3), the mechanism (4) does not lead to the charge injection into CB (e - ) or VB (h + ) and does not contribute significantly to PSL.

A very large bandwidth of the deep DAP band (~1 eV) and its nearly symmetrical shape, suggest that at least one of defects involved in the emission is strongly coupled to the lattice 54 . This assumption is supported by a comparison of the luminescence spectra obtained at room and low (9 K) temperatures: when the temperature is increased from 9 K to 300 K (Figure1.c) thermally activated quenching is intensified. The intensity of the broad DAP band then drops and the band shifts to the blue. Blue shifts induced by the rises of the temperature has been reported for deep, so-called "self activated", DAP luminescent bands of several semiconductors: ZnS 55, 56 , GaAs 57 or GaN 58 . The blue shift is generally observed when localized complex centers with strong electron-phonon coupling are involved in the luminescence process. The configurational coordinate (CC) model, which takes into account the interaction of such localized center with matrix, predicts a linear shift of the band position with temperature 56 . The relevant blue shift results from thermal occupation of vibrational levels associated with the excited state and thermal quenching is due to radiationless recombination of + -h e pairs.

The acceptor complex center involved in such emissions is usually formed by an acceptor type vacancy adjacent to a donor impurity atom, [55][56][57][58] and it is often called "Acenter". The emission results from the electron transition from a relatively distant donor to the donor-vacancy acceptor complex 59 . According to theoretical calculations most stables defects in hBN are supposed to be boron vacancy V B for n-type conditions and nitrogen vacancy V N for p-type conditions. To the best of our knowledge there are no calculations concerning vacancy-impurity complexes in hBN. Nevertheless, results obtained in cBN shows that

B -Photo-stimulated luminescence and band gap transitions of hBN

As we have already mentioned, the PSL excitation onset cannot be a measure of bandgap energy. However in combination with PLE spectra, PSL excitation spectra can provide valuable information concerning exciton and band edge energy positions. Below we will discuss the PSL excitation in the framework of relevant processes depicted in Figure 4 Several interesting details can be remarked from the comparison of PLE and PSL excitation spectra displayed on Figure 6.

Several free exciton transitions in hBN merge in the dominant peak of the PLE spectrum (Figure 3c) in the range of photon energies between 5.8 and 6.0 eV. 34 One remarkable feature in Figure 6 is the non-contribution of these excitons to the PSL excitation.

In fact, dissociation of excitons is required for a storage of distant charges, which results in PSL. This dissociation is only possible if the gain in energy due to the charge localization is greater than the exciton binding energy. Using this reasoning, we have previously set up the lower limit to the exciton binding energy: 4 0. > e D

eV 17 . Two factors additionally contribute to the inhibition of exciton dissociation: its short lifetime and limited spatial extent. We have not observed free exciton luminescence in our hBN polycrystalline sample: the free excitons are rapidly bound to defects where a part of them recombine radiatively. The lifetime of free excitons in hBN is then small that reduces their dissociation probability. In a similar way, small exciton radius decreases the dissociation probability. The non-contribution of the free exciton of hBN to the PSL processes may be an indication of its small radius and tight binding energy.

The PSL excitation spectrum in Figure 6 follows below 5.7 eV the exponential growth of the PLE spectrum of q-DAP luminescence at 5.3 eV 17 . The q-DAP states apparently contribute to the PSL signal that complements the general schema in Figure 4 by the type IV transition corresponding to the direct q-DAP excitation (Equation 4). This fact seems surprising since no free charge carriers are created in such process required for a distant charge trapping. Indeed it can be understood if we consider the q-DAP transition energy as a function of the pair distance. Closer charge pairs possess a higher transition energy and distant pairs a lower. The charge diffusion within the manifold of q-DAP levels below ionization can therefore take place toward more distant and hence long-lived states, which contribute to PSL. The existence of the exchange between q-DAP traps excited according to Equation 4 was suggested in our previous article 17 .

In the same publication we have reported a change in the q-DAP population mechanism at 5.7 eV. At this energy the q-DAP ionization takes place following Equations ( 2)-( 3) and results in the regime crossover from Raman to photoluminescence. This can be ionization of either acceptor or donor states whatever is higher lying. Higher-energy photons between 5.7 and 6.0 eV efficiently produce hBN excitons, however they do not appreciably contribute to PSL. This is seen in Figure 6 in both PLE (E lum = 5.3 eV) and PSL spectra.

However, another sharp PSL excitation maximum is observed in the high-energy wing of the exciton absorption band at 6.1 eV. We can relate a high efficiency of the distant charge traps population at this energy to the ionization of acceptor or donor states whatever is lower lying.

Resuming, we attribute two spectral features observed at

i or i g D i or i g A E E E E E E - = - = + - (7)
Keeping in mind this assignment, we can estimate the bandgap energy of hBN. Indeed, the energy of the q-DAP luminescence qDAP E = 5.3 eV is given by the relation

+ -- - = D A g qDAP E E E E (8)
Combining Equations ( 7) and (8) we then obtain:

eV E E E E qDAP i i g 5 . 6 1 2 ≈ - + = (9)
Alternatively, we can assign the spectral feature observed at In framework of this hypothesis the dissociation of lower-energy excitons is energetically forbidden. Indeed, the energy gain from the charge localization on donor or acceptor is

1 i ex E E - , where 2 i ex E E ≡
is the exciton energy. For the efficient charges separation, this gain in energy has to be higher than the exciton binding energy

ex g E E -
. This allows the energy balance equation relative to the process threshold:

eV E E E E E E E i i g i ex ex g 5 6 2 1 2 1 . = - = - = - (10)
Interesting, both alternatives in identification of the spectral feature at Our estimation of the hBN bandgap energy is in good agreement with that obtained from allelectron GW calculations recently performed by Arnaud et al. 34 . Moreover, taking the exciton transitions at 5.8 eV, we obtain the exciton binding energy of 0.7 eV. This disagrees with the Wannier-type exciton suggested by Watanabe et al. 1 and supports the Arnaud's prediction of the Frenkel-type exciton with strong binding energy 34 .

The shape analysis of PLE and PSL excitation spectra supports the obtained value of hBN bandgap energy. On Figure 7 we have plotted the PLE spectra of the bound excitons luminescence ( eV E lum 5 5.

=

) together with the PLE spectra of broad DAP luminescence ( eV E lum 73 3.

=

). The PLE ( eV E lum 5 5.

=

) peak at 5.8 eV fits well the position of the strongest excitonic transitions predicted theoretically, 34,60,61 as represented by vertical bar in Figure 7. As well the plateau of the PLE spectra in the energy range between 6.1 and 6.5 eV is related to low-intensity excitonic transitions converging to the dissociation limit.

Interestingly, both PLE spectra coincide in the high-energy spectra range above 6.1 eV and follow the PSL spectrum until 6.5 eV. Above 6.5 eV the PSL grows faster while the intensity of the PLE spectra decreases. The PSL is expected to increase at Our final remark concerns the shape of PLE spectrum. From a general point of view, as long as the hBN can be considered as optically thin material, the shape of the PLE spectrum is expected to follow the intrinsic absorption spectrum. At higher absorbance, a saturation or even a decrease of PLE intensity appears as an intensification of non radiative recombination processes. Interestingly, we observe the indication of saturation effect around 6.5 eV in good agreement with our bandgap estimation.

V. Conclusion.

Photoluminescence of hexagonal boron nitride has been studied by means of the timeand energy-resolved photoluminescence spectroscopy methods. Depending on the excitation energy several luminescence bands have been observed. (i) A strongly structured band in the energy range between 4.1 and 3.3 eV is assigned to phonon replicas of an impurity luminescence. (ii) A very broad band ( 1 Ẽ ∆ eV) centered at 3.9 eV is assigned to DAP transitions involving a strongly localized acceptor complex center. Moreover, the intensity ratio between these two emissions strongly depends on excitation photon energy.

(iii) q-DAP are responsible for the 5.3 eV emission and (iv) emission of excitons bound to defects is observed at 5.5 eV.

The excitation of samples by high-energy photons above 5.4 eV enables another phenomenon called photostimulated luminescence, which is due to distantly trapped photoinduced charges. PSL is observed in hBN following interband and acceptor/donor-band transitions. Moreover, we show that in contrast to DAP, PSL can also result from a direct q-DAP excitation. The comparison of photoluminescence excitation and PSL spectra allows Theoretical oscillator strengths 34,60,61 of free exciton transitions in hBN are shown by straight vertical bars.

  vacuum (~10 -9 mbar). The measurements of luminescence spectra were carried out using a visible 0.275-m triple-grating ARC monochromator equipped with a CCD detector or a photomultiplier operating in the photon-counting mode. The pulse structure of SR (130 ps, 5 MHz repetition rate) enables time-resolved luminescence analysis at time-scale of 200 ns with sub-nanosecond temporal resolution. Spectra were recorded within a time gate τ ∆ delayed after the SR excitation pulse. Typically two time gates have been used simultaneously: a fast one of ns 4 decay curves were measured at fixed excitation and luminescence photon energies. The recorded spectra were corrected for the primary monochromator reflectivity and SR current. The photostimulated luminescence (PSL) excitation spectra of pd-hBN samples were measured at the BL 52 beamline of MAX-lab synchrotron (Lund Sweden). The experiment built up by the Tartu group is described in ref 37, 38 . At each excitation energy exc E the sample was irradiated by a fixed number of UV or VUV photons (2.5•10 6 counts) in the energy region of exc E = 5-15 eV. After completing the dose, the irradiation was stopped and the phosphorescence decay was measured. When the phosphorescence intensity drops until almost zero (PM noise level) that typically happens after few minutes, the sample was activated by a bright spectroscopic source at hν=2.0±0.5 eV through a double-prisms monochromator DMR-4 and the time dependence of PSL intensity was recorded. The integral PSL intensity was taken as a measure of the number of recombined electron-hole pairs.

Figure 3

 3 .a shows the total PLE spectrum measured at E lum = 3.73 eV. The last corresponds to the peak (γ) of the structured emission and is close to the maximum of the broadband emission. Two principal features can be distinguished in this spectrum: (i) a sharp onset at 4.09 eV followed by a decrease of the luminescence intensity until nearly zero at E exc = 4.9 eV, and (ii) a strong peak at 5.95 eV preceded by a weak structured footer that begins at 5.0 eV. The PLE spectra obtained with short and long time windows are displayed in Figure3.b and c. The long time-window of ns 200 lived excited states, which apparently are responsible for the broad emission band (Figure3.c). The peak maximum at 5.95 eV strongly contributes to excitation of this band. This peak is blue-shifted with respect to that of the exciton absorption at 5.82 eV in hBN single crystal1 . Nevertheless, a weak luminescence at 5.9 eV is also reported in hBN single crystal at low temperatures and assigned to another exciton series. The Stokes shift with respect to the excitation peak at 5.95 eV could results from inhomogeneities of the local field or dislocations such as stacking faults1 . Therefore we tentatively assign the peak at 5.95 eV to the excitation of higher exciton series. This excitonic peak is preceded by a weak and unstructured footer ranging from 5 eV to 5.5 eV and is followed by a broad continuum until 7.6 eV. Conversely, the PLE spectra obtained with the short time window ( from both short and long decay luminescence components (Figure3.b). Nevertheless, a comparison with the PLE spectra of the long component (Figure3.c) allows firmly assigning the structured emission with the onset at 4.09 eV to a short-lived excited state. Moreover, we note a second weak excitation onset of the structured emission at E exc = 5.2 eV.

  shows the phosphorescence decay curves ( decay curves are characterized by an almost mono-exponential decay with characteristic time of dotted line in Figure 5 shows the PM dark noise). However, at 60 . 5 ≥ exc E eV another extremely long-lived component of low intensity appears which looks like a plateau in Figure 5. We ascribe this long component to the recombination of charges trapped at large distance tending to infinity.

  attains a maximum. At this energy the slope suddenly changes and the growth of the signal becomes much slower. Another local maximum of the PSL spectrum appears at for excitation energies above 6.2 eV the PSL signal increases progressively until 7 eV where a plateau is reached.For the sake of the discussion we have superimposed in Figure6the PLE spectrum of the near band edge luminescence at eV

  acceptors. Such center in association with the shallow B C or N O donor may be responsible for DAP transitions at 3.9eV observed in hBN polycrystalline samples.

  spectrum to the direct ionization of donor and acceptor levels (or vice versa) involved in the q-DAP luminescence (processes II and III of Figure4and Equations 2-

  dissociation of excitons on the high-lying acceptor or donor state of energy

  4, the process I efficiently contributes to the distant traps population. This difference in shapes between the PLE and PSL spectra is therefore explained by interband transitions and supports well our bandgap energy estimation

Figure 1

 1 Figure 1 Luminescence spectra of polycrystalline hBN. The excitation energy is indicated on each spectrum. The phonons replicas are labeled in (a). The star in (c) and (d) indicates the second order of the 5.5 eV band.

  lum = 4.09 eV (E exc = 4.27 eV) β) E lum = 3.91 eV (E exc = 4.27 eV) γ) E lum = 3.73 eV (E exc = 4.27 eV) δ) E lum = 3.54 eV (E exc = 4.27 eV) ε) E lum = 3.91 eV (E exc = 5.96 eV)

Figure 2 .

 2 Figure 2. Luminescence decay curves at different photon energies. Curves α, β, γ and δ correspond to the peaks of the structured emission. The curve ε corresponds to the broad emission maximum.

Figure 3 .Figure 5 .Figure 6 .Figure 7 .

 3567 Figure 3 . PLE spectra (E lum = 3.72 eV) of polycrystalline hBN (a) and its short-(b) and longlived (c) components.

bandgap energy estimation of 6.5 eV and supports the hypothesis of Frenkel-like exciton in hBN with large binding energy of 0.7 eV. These conclusions support most recent theoretical calculations. 34,35

Acknowledgments

This work has been supported by the IHP-Contract HPRI-CT-1999-00040 of the European Commission. The authors are grateful to G. Stryganyuk for assistance in conducting experiments at HASYLAB (synchrotron DESY) and to P. Jaffrennou and F.

Ducastelle for helpful discussions.