Marine Paquet
email: mpaquet@ucsd.edu

James M D Day
email: jmdday@ucsd.edu

Diana B Brown

Christopher L Waters

Effective global mixing of the highly siderophile elements into Earth's mantle inferred from oceanic abyssal peridotites

Keywords: +6 supplementary figures) Tables: 1 (+7 supplementary tables) Oceanic mantle, peridotite, highly siderophile elements, Os isotopes, late accretion, mantle convection, heterogeneity

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneities in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187 Os/ 188 Os ratios, in conjunction with mineral chemistry and bulk rock majorand trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm, full spreading rate) to fast (135-150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3-23% melt depletion; 187 Os/ 188 Os from 0.1189 to 0.1336, average = 0.1267±0.0065; 2SD) and Hess Deep abyssal peridotites (15-20% melt depletion; 0.1247±0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at midocean ridges suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ~50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth's mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ~2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

INTRODUCTION

The highly siderophile elements (HSE: Os, Ir, Ru, Pt, Rh, Pd, Re, Au) are key geochemical tools with which to investigate terrestrial accretion and differentiation processes. The HSE are generally compatible during partial melting and are characterized by elevated metal-silicate partition coefficients at mantle pressures (> 10 4 ; e.g., [START_REF] O'neill | Experimental petrochemistry of some highly siderophile elements at high temperatures, and some implications for core formation and the mantle's early history[END_REF][START_REF] Holzheid | Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum[END_REF][START_REF] Ertel | The solubility of rhenium in silicate melts: implications for the geochemical properties of rhenium at high temperatures[END_REF][START_REF] Brenan | Core formation and metal-silicate fractionation of osmium and iridium from gold[END_REF][START_REF] Brenan | Experimental results on fractionation of the highly siderophile elements (HSE) at variable pressures and temperatures during planetary and magmatic differentiation[END_REF]Suer et al., 2021 and references therein). Assuming such high partition coefficients, Earth's core and mantle are not in equilibrium for the HSE, and the mantle has abundances of these elements only ~150 times less abundant than in chondrites; some three orders of magnitude higher than expected [START_REF] Morgan | Ultramafic xenoliths: clues to Earth's late accretionary history[END_REF][START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Morgan | Siderophile elements in Earth's upper mantle and lunar breccias: data synthesis suggests manifestations of the same late influx[END_REF][START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Fischer-Gödde | Rhodium, gold and other highly siderophile elements in orogenic peridotites and peridotite xenoliths[END_REF]Day et al., 2016a;2017a). Three main hypotheses have been suggested to explain this discrepancy: (1) the "late accretion" or "late veneer" hypothesis where addition of "chondritic" impactors occurred after the major phase(s) of core formation (e.g., [START_REF] Turekian | Inhomogeneous accumulation of the Earth from the primitive solar nebula[END_REF][START_REF] Kimura | Distribution of gold and rhenium between nickel-iron and silicate melts: implications for the abundance of siderophile elements on the Earth and Moon[END_REF][START_REF] Chou | Fractionation of siderophile elements in the Earth's upper mantle[END_REF][START_REF] Jagoutz | The abundances of major, minor and trace elements in the earth's mantle as derived from primitive ultramafic nodules[END_REF][START_REF] Wänke | Constitution of terrestrial planets[END_REF][START_REF] Meisel | The osmium isotopic composition of the Earth's primitive upper mantle[END_REF]; (2) mixing or inefficient separation of differentiated outer-core material back into the mantle shortly after core separation [START_REF] Jones | Geochemical constraints on core formation in the Earth[END_REF][START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF], or; (3) lower metal-silicate partition coefficients at higher pressures and temperatures (HP-HT; e.g., [START_REF] Ringwood | Composition of the core and implications for origin of the Earth[END_REF][START_REF] Murthy | Early differentiation of the Earth and the problem of mantle siderophile elements: a new approach[END_REF]. The late accretion model is generally the most popular given its ability to explain both the broadly chondritic 187 Os/ 188 Os as well as the elevated absolute abundances of the HSE that are in chondritic proportions within the bulk silicate Earth (BSE), requiring between ~0.5 and 0.8 wt.% addition of mass to Earth [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF]Day et al., 2016a).

An outstanding question that remains, however, is how evenly distributed the HSE are within the present-day convecting mantle. Late accretion likely occurred through addition of massive impactors [START_REF] Bottke | Stochastic late accretion to Earth, the Moon, and Mars[END_REF], leading to the possibility of heterogeneous distribution of the HSE within Earth's mantle following late accretionary impacts, and enhanced deposition of impactor material within the mantle, perhaps at hemispheric scales, or at the scale of thousands of kilometers. For example, it has been demonstrated that metal-silicate equilibration differs depending on the target latitude of the impactor due to the influence of the planetary rotation on the mixing and settling history, which may generate chemical heterogeneities (e.g., HSE) and isotopic anomalies (e.g., 182 W anomalies) [START_REF] Maas | On the fate of impact-delivered metal in a terrestrial magma ocean[END_REF]. For smaller planetary bodies, such as the probable asteroidal source of eucrite and diogenite meteorites, 4-Vesta, it has been proposed that the inferred patchy distribution of the HSE reflects regional rather than global late accretion effects shortly after planet formation [START_REF] Day | Late accretion as a natural consequence of planetary growth[END_REF]. In contrast, while Earth may have originally had a more heterogeneous distribution of the HSE within the mantle, compositional variations in mantle materials at the present-day are seemingly more consistent with homogeneous distributions through prolonged melting and/or solid-state convection. The evidence from mantle peridotites preserved in ophiolites is that there is more limited heterogeneity at >km length scales than at the scale of meters or less (e.g., [START_REF] O'driscoll | Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland[END_REF][START_REF] Snortum | Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements[END_REF][START_REF] Haller | Meter-Scale Chemical and Isotopic Heterogeneities in the Oceanic Mantle, Leka Ophiolite Complex, Norway[END_REF]. Arguably, however, the ideal test of HSE homogeneity in Earth's mantle comes from the study of oceanic abyssal peridotites. Abyssal peridotites are samples of Earth's present-day convecting mantle sampled within ocean basins, so provide a snap-shot of the degree of presentday HSE homogeneity within the mantle.

Prior attempts to estimate the HSE composition of Earth's mantle have assumed relative initial homogeneity to attain a BSE composition (equivalent to primitive mantle [PM] and referred to as primitive upper mantle [PUM] by [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF], with workers utilizing a range of mantle rock types, including ultramafic massifs and ophiolites (e.g., [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Zhang | Heterogeneous sub-ridge mantle of the Neo-Tethys: Constraints from Re-Os isotope and HSE compositions of the Xigaze ophiolites[END_REF], as well as using abyssal peridotites (e.g., [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF]Day et al., 2017a). These approaches for determining the absolute and relative abundances of the HSE in the bulk silicate Earth, and the extent of heterogeneity have drawbacks. For example, utilizing massif compositions requires effective subtraction of the effects of sometimes complex melt refertilization processes (e.g., [START_REF] Marchesi | Fractionation of highly siderophile elements in refertilized mantle: Implications for the Os isotope composition of basalts[END_REF][START_REF] Becker | Re-Pt-Os isotopic and highly siderophile element behavior in oceanic and continental mantle tectonites[END_REF][START_REF] Lorand | Chalcophile and siderophile elements in mantle rocks: Trace elements controlled by trace minerals[END_REF]. Abyssal peridotites represent residues of ancient and modern depletion events (2% to >16%), which occurred > 0.5 Ga ago for some samples (e.g., [START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Harvey | Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge[END_REF][START_REF] Liu | Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a). Complications with these samples are that melt refertilization (e.g., [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath midocean ridges[END_REF][START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF][START_REF] Reisberg | Osmium isotope constraints on formation and refertilization of the noncratonic continental mantle lithosphere[END_REF] and serpentinization or secondary alteration (e.g., [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Snow | Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: serpentinization is not isochemical[END_REF] processes can affect their compositions. Furthermore, abyssal peridotites are difficult to sample, being either dredged or drilled typically from deep water locations (>3 km) and relatively rare exposures on the ocean floor, such that global coverage has hitherto not been obtained.

Currently, HSE and Os isotope data for abyssal peridotites are mainly from ultraslow to slow spreading ridges (e.g., Gakkel Ridge, Southwest Indian Ridge, Central Indian Ridge, Mid-Atlantic Ridge; [START_REF] Martin | Osmium isotopic characteristics of mantle-derived rocks[END_REF][START_REF] Roy-Barman | 187 Os/ 186 Os ratios of mid-ocean ridge basalts and abyssal peridotites[END_REF][START_REF] Snow | Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites[END_REF][START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Luguet | Laserablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle[END_REF]2003;[START_REF] Standish | Abyssal peridotite osmium isotopic compositions from Cr-spinel[END_REF][START_REF] Alard | In situ Os isotopes in abyssal peridotites bridge the isotopic gap between MORBs and their source mantle[END_REF][START_REF] Harvey | Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge[END_REF][START_REF] Sichel | Geophysical and geochemical evidence for cold upper mantle beneath the equatorial Atlantic Ocean[END_REF][START_REF] Liu | Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean[END_REF]2009;[START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a). More limited data are available on the HSE abundances and Os isotopic composition of mantle beneath intermediate and fast spreading centers (Roy-Barman and [START_REF] Roy-Barman | 187 Os/ 186 Os ratios of mid-ocean ridge basalts and abyssal peridotites[END_REF][START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Rehkämper | Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle[END_REF]. Here we report new bulk rock 187 Os/ 188 Os and HSE abundance data for abyssal peridotites from the Pacific Ocean (Hess Deep along the East Pacific Rise (EPR) and from the Udintsev Fracture Zone (UFZ) along the Pacific Antarctic Ridge (PAR)).

We compare these data with those obtained for abyssal peridotites from ultraslow to slow spreading ridges (e.g., Gakkel Ridge, Southwest Indian Ridge (SWIR), Central Indian Ridge (CIR), Mid-Atlantic Ridge (MAR)). Additionally, new bulk rock major and trace element abundances and mineral chemistry are reported for some samples to examine the relationship to spreading rate and melt depletion in abyssal peridotites, and to assess heterogeneity of mantle composition in terms of HSE abundances and Os isotope ratios and the implications this might have for mantle geodynamics.

SAMPLES AND METHODS

Samples

Dredged abyssal peridotites were analyzed from the fast-spreading East Pacific Rise (EPR), at Hess Deep (denoted AII) from cruise 125, leg 6, aboard the RV Atlantis II in May 1990. Dredged abyssal peridotites were also analyzed from the Pacific Antarctic Ridge (PAR), along two different locations of the Udintsev ridge, denoted as WEST03-MV12 and WEST03-MV13, from the WESTWARD (WEST03MV) cruise aboard the RV Melville in February 1994. Details for this cruise are reported in [START_REF] Castillo | Sr, Nd and Pb isotopic variation along the Pacific-Antarctic risecrest, 53-57 S: implications for the composition and dynamics of the South Pacific upper mantle[END_REF] and [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath midocean ridges[END_REF]. New data are also reported for abyssal peridotites from slower spreading rates (Mid Atlantic Ridge, Central Indian Ridge and Southwest Indian Ridge) from the 1968 CIRCE cruise (Circe97) aboard the RV Argo, the 1970 ANTIPODE cruise (ANTP) aboard the RV Melville, 1984 Protea Cruise (Prot05) aboard the RV Melville, the 1990 PLUME cruise (PLUM05) aboard the RV Thomas Washington, and the 2007 KNOX11RR cruise aboard the RV Roger Revelle. Sample locations are reported in Figure 1 and Table 1.

Mineral compositions

Major-and minor-element mineral compositions were obtained from polished mounts containing olivine, pyroxene and spinel, and as polished thick sections for Hess Deep and PAR samples, using a JEOL JXA-8230 electron probe micro analyzer (EPMA) at the University of Colorado, USA (Department of Geological Sciences). Analyses were made with an accelerating potential of 15 keV and a beam size of 5 μm. Beam currents were 30 nA for olivine and pyroxene and 20 nA for spinel. Both natural and synthetic standards were used to calibrate the EMP and were measured throughout analytical sessions to ensure data quality. Background and peak counting times used were 20-30 s and standard PAP correction procedures were used. Detection limits were ≤ 0.02 wt.% for Si, Al, Mg, Ca, Na, K and P and < 0.04 wt.% for Fe, Ti, Mn, V, Ni and Cr.

Bulk rock major and trace element abundance analyses

Major element compositions were measured by X-ray fluorescence (XRF) at Franklin and Marshall College using a PW 2404 PANalytical XRF vacuum spectrometer following the protocol described in Day et al. (2017a) and references therein. Repeated measurements of basaltic reference material BHVO-2 allow estimations of precision and accuracy, with long-term reproducibility of 0.1% for SiO2, Al2O3, Fe2O3T, MgO and CaO, 0.2% for TiO2, MnO and Na2O, and 0.3% for P2O5 (Table S1).

Trace elements analyses were performed on 100 mg of homogenized bulk rock powder using a Thermo Scientific iCAP Qc ICP-MS at the Scripps Isotope Geochemistry Laboratory (SIGL, University of California San Diego), following the method outlined in [START_REF] Day | Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs[END_REF].

Samples were analyzed with several replicates of powdered peridotites HARZ-01 and PLUM05-49, as well as BHVO-2, BCR-2 and BIR-1a, basaltic reference materials, used as standards to confirm accuracy, with reproducibility on most of trace element abundances better than 10% (Table S2).

Highly siderophile elements and Os isotopic compositions

Osmium isotope and HSE abundance analyses were performed at the SIGL on ~900 mg of homogenized powder from a larger powder aliquot. The samples were precisely weighed and digested in sealed 20 cm borosilicate Carius tubes using a mixture of multiply Teflon distilled 12M HCl (4 mL) and "purged" 15.7M HNO3 (7 mL; expunged of Os using H2O2), with isotopically enriched multi-element spikes ( 99 Ru, 106 Pd, 185 Re, 190 Os, 191 Ir, 194 Pt). Digestions lasted 72h hours in an oven at a maximum temperature of 250°C. Osmium was purified by extracting CCl4 three times from the HCl/HNO3, spike and sample mixture and then back extracting the Os from the CCl4 using HBr [START_REF] Cohen | Separation of osmium from geological materials by solvent extraction for analysis by thermal ionisation mass spectrometry[END_REF], with further purification by micro-distillation [START_REF] Birck | Re-Os isotopic measurements at the femtomole level in natural samples[END_REF]. The other HSE (Re, Pd, Pt, Ru, Ir) were recovered and purified from the residual solutions using anion exchange column chemistry (e.g., [START_REF] Day | Use of hydrofluoric acid desilicification in the determination of highly siderophile element abundances and Re-Pt-Os isotope systematics in mafic-ultramafic rocks[END_REF].

Acquisition of Os isotopic compositions were performed on a Thermo Scientific Triton thermal ionization mass spectrometer in negative ion mode, with HSE abundances calculated from isotopic ratios of Ir, Ru, Pt, Pd and Re measured using a Thermo Scientific iCaP Qc ICP-MS coupled to a Cetac Aridus II desolvating nebulizer. Osmium data were appropriately oxide-, fractionation-, spike-and blank corrected. Precision for 187 Os/ 188 Os, determined by repeated measurements of 35 to 70 pg loads of the UMCP Johnson-Matthey standard, was better than ±0.2 % (2SD; 0.11382 ±0.00012; n = 10). These standard load sizes were smaller than unknown samples, which had >1 ng Os, and that typically ran with signal sizes of 200 K Cps on the largest mass isotope, with stable signals similar to the standards. Rhenium, Pd, Pt, Ir and Ru isotopic ratios were corrected for mass fractionation using the deviation of the standard average run on the day over the natural ratio of the element. External reproducibility on HSE analyses was better than 0.5% for 5 ppb solutions and all reported values are blank corrected. Peridotite standard reference materials (MUH-1, HARZ-01) run during the period of the analytical campaign in the SIGL are reported in Day et al. (2016a) and [START_REF] Snortum | Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements[END_REF] and show good reproducibility and accuracy compared with literature data (e.g., [START_REF] Meisel | Analytical methods for the highly siderophile elements[END_REF]. The total procedural blanks (n = 4) run with the samples had 187 Os/ 188 Os= 0.209 ±0.090, with quantities (in pg) of 0.8 [Re], 7

[Pd], 22 [Pt], 15 [Ru], 3 [Ir] and 0.8 [Os]. These blanks resulted in negligible corrections to samples (<1% in most cases, Table S3).

RESULTS

Sample descriptions and mineral chemistry

Spinel grains in the PAR abyssal peridotites from WEST03MV-12 and WEST03MV-13 span a Mg# range of 58.4 to 70.6 and 68.7 to 72.7, and a Cr# range of 28.7 to 49.6 and 19.1 to 24.9, respectively, similar to plagioclase-free peridotites (e.g., SWIR peridotites, [START_REF] Seyler | Evidence for major-element heterogeneity in the mantle source of abyssal peridotites from the Southwest Indian Ridge (52° to 68°E)[END_REF] global abyssal peridotite database, [START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF]. Spinel grains in Hess Deep (EPR) samples have Mg# between 45.6 and 60.0, and Cr# between 47.3 and 52.0 (e.g., comparable to plagioclaselherzolites from the SWIR; [START_REF] Paquet | Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°-67° E)[END_REF] (Figure S1a). range between 89.7 and90.7, andbetween 90.0 and90.6 in WEST03MV-12 andWEST03MV-13 samples, respectively (Table S4). Orthopyroxenes and clinopyroxenes in PAR samples have similar compositions: between Wo0.01-0.07En0.84-0.89Fs0.08-0.10 and Wo0.32-0.49En0.47-0.62Fs0.05-0.07 for WEST03MV-12 peridotites, and between Wo0.01-0.11En0.81-0.89Fs0.09-0.10 and Wo0.35-0.49En0.46-0.59Fs0.04-0.06 for the WEST03MV-13 peridotites (Figure S1b).

Forsterite contents in olivine grains

WEST03MV-12 and WEST03MV-13 peridotites have Cr2O3 contents that range from 0.66 to 1.08 wt.% and from 0.43 to 0.96 wt.% in orthopyroxenes, and from 0.95 to 1.43 wt.% and from 0.56 to 1.32 wt.% in clinopyroxenes, respectively (Table S4). These values are within the compositions of abyssal peridotites from the global ridge system (e.g., [START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF]. Olivine, orthopyroxene and clinopyroxene grains in the EPR samples were not fresh enough to conduct analyses on.

Bulk rock major and trace element abundances

Hess Deep abyssal peridotites tend to have lower anhydrous-corrected Al2O3, TiO2 and CaO, and higher MgO contents than to those from the Pacific Antarctic Ridge (Figure 2, Table S5). The WEST03MV-13 peridotites generally have lower Al2O3, TiO2 and CaO (and Fe2O3T to a lesser extent) at a given MgO than the WEST03MV-12 peridotites suggesting heterogeneity at the scale of the fracture zone that they were dredged from. All these peridotites show a positive correlation between Al2O3 and CaO, with Hess Deep peridotites being both tightly grouped in terms of composition and having the most refractory (lowest) Al2O3 and CaO contents. Overall, the Pacific abyssal peridotites overlap the trend defined by abyssal peridotites from ridges with slower spreading rates (Day et al., 2017a).

Abyssal peridotites from the UFZ along the PAR are characterized by having subchondritic rare earth element (REE) abundances, and show pronounced LREE-depleted patterns, which are similar to abyssal peridotites from the Mid-Atlantic Ridge, and some samples from the SWIR and Gakkel ridge (Day et al., 2017a) (Figures 3, S2 andTable S6) Overall, incompatible trace element (ITE) and REE abundances for abyssal peridotites from the SWIR are in good agreement with data reported in previous studies (e.g., Day et al, 2017a) (Figures 3, S2 andTable S6).

Abyssal peridotites from the CIR reported in this study show more depleted patterns in the REE, and for the ITE in general with lower Nb contents (and slightly higher Ta abundances), than previously reported samples (e.g., Day et al., 2017a). PLUM05 abyssal peridotite samples from the MAR have HREE similar to those from the same ocean basin in the literature but are more enriched in the LREE and have more pronounced positive Eu anomalies. They also exhibit higher contents in Rb, Ba, Nb and Ta, but lower Zr and Hf abundances than those from Day et al. (2017a).

Most abyssal peridotites show elevated concentrations in fluid mobile elements such as U and Sr which are enriched in seawater (Figure S2).

Highly siderophile element abundances and Os isotopic compositions

Bulk rock Re-Os isotope and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance measurements for Pacific abyssal peridotites and some Southwest Indian Ridge and Central Indian Ridge peridotites are reported in Table 1. PAR samples from the WEST03MV-13 dredge have BSE-like HSE patterns with variable rhenium depletion, whereas samples from the WEST03MV-12 dredge have more variable Re, Pd and Pt abundances (Figure 4), similar to those reported previously (Roy-Barman and [START_REF] Roy-Barman | 187 Os/ 186 Os ratios of mid-ocean ridge basalts and abyssal peridotites[END_REF][START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Rehkämper | Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle[END_REF]. Two of the Hess Deep samples have patterns that are akin to BSE, while the other samples show variable degrees of rhenium depletion relative to BSE. WEST03MV-12 and WEST03MV-13 abyssal peridotites have average Pd/Ir ratios of 1.4 ± 0.6 and 1.4 ± 1.1 (2 SD), and average Ru/Ir ratios of 1.7 ± 0.2 and 1.8 ± 0.4 respectively (Figure 5). Hess Deep samples show average ratios slightly higher but within uncertainties at 1.9 ± 1.1 and 2.0 ± 0.6 for Pd/Ir and Ru/Ir ratios, respectively. Moreover, HSE patterns for the Pacific abyssal peridotites are like those from the Mid-Atlantic Ridge, and some of the abyssal peridotites from the Southwest Indian Ridge (Day et al., 2017a and references therein).

Pacific Antarctic Ridge peridotites show similar ranges and averages in 187 Os/ 188 Os (WEST03MV-12: 0.1264 ± 0.0054; WEST03MV-13: 0.1272 ± 0.0082; all uncertainties are 2 SD), and are slightly more radiogenic, on average, than the samples from Hess Deep (0.1247 ± 0.0030) (Figure 6), with an average value of 0.1264 ± 0.0063 for all Pacific abyssal peridotites from this study. Using the Os abundances in the samples to calculate a weighted mean, we obtained a 187 Os/ 188 Os ratio of 0.1263. These new data overlap the trend defined by abyssal peridotites from slower spreading ridges [START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Rehkämper | Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle[END_REF][START_REF] Luguet | Laserablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a), but are generally more radiogenic than mantle peridotites obtained from Pacific Ocean Islands [START_REF] Snortum | Pacific lithosphere evolution inferred from Aitutaki mantle xenoliths[END_REF], which offer an alternate view of the Pacific oceanic mantle lithosphere.

DISCUSSION

Alteration and melt infiltration

Most abyssal peridotites have experienced serpentinization (<400 °C) and/or sea-floor weathering (~0°C) under both oxidizing and reducing conditions (e.g., [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Bach | Seawater-peridotite interactions: First insights from ODP Leg 209[END_REF][START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15 20′ N, ODP Leg 209): implications for fluid/rock interaction in slow spreading environments[END_REF][START_REF] Klein | Compositional controls on hydrogen generation during serpentinization of ultramafic rocks[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: serpentinization is not isochemical[END_REF]. Samples in this study exhibit evidence for secondary alteration, manifested as loss on ignition (LOI) values of between ~5 and 17%, reflecting the presence of serpentine and other alteration minerals. As noted for abyssal peridotites from a range of spreading ridge environments [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath midocean ridges[END_REF][START_REF] Harvey | Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge[END_REF]Day et al., 2017a), elevated concentrations of fluid mobile elements, occur in many of the samples, consistent with modification by interaction with seawater (e.g., U, Sr, K, P, Na, LREE; e.g., [START_REF] Frisby | Seawater-derived rare earth element addition to abyssal peridotites during serpentinization[END_REF]. However, most major and trace elements appear unaffected by alteration processes.

Prior work has suggested that 187 Os/ 188 Os (e.g., [START_REF] Snow | Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites[END_REF][START_REF] Standish | Abyssal peridotite osmium isotopic compositions from Cr-spinel[END_REF], Re and Pd (e.g., [START_REF] Luguet | Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK area, Atlantic Ocean)[END_REF][START_REF] Harvey | Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge[END_REF] can be modified by serpentinization and seafloor alteration processes but that, in general, abyssal peridotite HSE abundances are typically not strongly affected by such processes (e.g., [START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF]Day et al., 2017a). In agreement with those studies, we find no correlation with LOI and HSE contents and Os isotopic compositions (Figure S3). Indeed, it has been shown that even strongly steatized serpentinite rocks faithfully preserve original HSE abundances (Day et al., 2017b). Overall, we conclude that the bulk rock abyssal peridotite HSE compositions predominantly reflect their high temperature mantle petrogenesis.

Melt refertilization in abyssal peridotites [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath midocean ridges[END_REF][START_REF] Seyler | Asthenospheric metasomatism beneath the mid-ocean ridge: Evidence from depleted abyssal peridotites[END_REF][START_REF] Warren | An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions[END_REF][START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF] can also lead to precipitation of metasomatic sulphides rich in Pd and the other HSE [START_REF] Alard | Non-chondritic distribution of the highly siderophile elements in mantle sulphides[END_REF][START_REF] Luguet | Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK area, Atlantic Ocean)[END_REF]. Evidence for melt refertilization in the sample suite is provided by the enrichments of non-fluid mobile highly incompatible elements in all samples (Figure 3). Enrichments of two or three orders of magnitude in the high field strength elements (HFSE: Nb, Ta, Ti, Zr, Hf) relative to what would be expected for partial melting residues are strong indicators of melt refertilization in Pacific abyssal peridotites (Figure 3), along with some LREE-enriched peridotites, as observed in slower-spreading ridges [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath midocean ridges[END_REF][START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF]Day et al., 2017a). This melt infiltration has been variable between the sample suites, with plagioclase-rich melt infiltration possible in some sample suites (e.g., Hess Deep). These observations match with hand-specimen observations for AII125-6-4D-18, which include minor melt infiltration veins that are cross-cut by serpentine. Moreover, spinel grains in the Pacific abyssal peridotites plot within the lower part of the abyssal peridotite field, with relatively low Cr# at a given Mg#, which is often associated with the presence of pyroxenite-veins in the samples (e.g., [START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF].

Melt infiltration events appear to have limited impact on Pt, Ru, Ir or Os abundances or Os isotopes in Pacific abyssal peridotites. In contrast, samples display variable Re contents, and Pd contents to a lesser extent (Figure 4), which may be attributed to various degrees of refertilization by melts. As shown previously, basaltic melts typically have more radiogenic 187 Os/ 188 Os, but substantially lower Os contents than peridotites, meaning that only high degrees of melt-rock reaction will lead to modification of peridotite HSE compositions (Day et al., 2017a). The generally high Os contents of the samples (~2-9 ppb) means that melt refertilization would only be notable at melt-rock ratios in excess of ten, which is not consistent with immobile incompatible trace element addition in all samples and suggests melt-rock ratios less than one. Even the peridotite with the lowest Os content in the sample set (CIRCE97-HD-2; 0.19 ppb Os) has 187 Os/ 188 Os within the range of the other peridotites, and distinct from more radiogenic MORB melts (e.g., [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]. These lines of evidence suggest that sulfide melt addition is not a significant driver in Pt, Ru, Ir or Os abundances or 187 Os/ 188 Os in the studied abyssal peridotites.

Conversely, melt refertilization can be pervasive and variable in extent from ultraslow to fast spreading ridges in modifying Re and Pd abundances (up to a factor 10 for Re abundances; Figure 4).

Recent versus ancient processes acting on abyssal peridotites

Melt depletion processes can fundamentally modify peridotite compositions (e.g., [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Reisberg | Osmium isotope constraints on formation and refertilization of the noncratonic continental mantle lithosphere[END_REF]. It is possible to estimate the degree of melt depletion experienced by peridotites using a range of methods. For example, non-modal fractional melting models can be used to estimate melt depletion in abyssal peridotites based on REE abundances (here using model parameters outlined in Day et al., 2017a). An increase in melt depletion is associated with lower absolute REE abundances and higher depletion in the LREE relative to the HREE. The REE modeling calculations suggest 3-20% melt extraction for the WEST03MV-12 peridotites, 8-13% for the WEST03MV-13 peridotites, and 15-20% for the EPR peridotites. These estimates are consistent to slightly higher than calculations of melt depletion using Cr# in spinel grains from the same samples (e.g., [START_REF] Batanova | Origin of geochemical heterogeneity in the mantle peridotites from the Bay of Islands ophiolite, Newfoundland, Canada: ion probe study of clinopyroxenes[END_REF][START_REF] Hellebrand | Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites[END_REF]: 13.7-19.0%, 9.6-12.3% and 18.4-19.5% respectively (Table S4). The more BSE-like HSE patterns in the WEST03MV-13 peridotites relate to more limited melt depletions than in other abyssal peridotite samples from the PAR and Hess Deep. The estimated high degrees of melt loss in some of the peridotites (WEST03MV-12 and Hess Deep) are consistent with clinopyroxene being nearly consumed within the mineral assemblage (Ol + Opx + Cpx + Sp) during partial melting. Correspondingly, melt extraction at fast-spreading ridges ranges up to higher values than those calculated for slow and ultraslow-spreading ridges (e.g., [START_REF] Warren | Global variations in abyssal peridotite compositions[END_REF]Day et al., 2017a) (Figure S4).

Melt extraction in the mantle typically leads to the formation of residual peridotites depleted in Re, Pd ± Pt, relative to Ru, Ir and Os, reflecting the extraction of sulfide melt along with silicate melt, or dissolution of sulfides into the melt and formation of refractory platinum group minerals (e.g., [START_REF] Alard | Non-chondritic distribution of the highly siderophile elements in mantle sulphides[END_REF][START_REF] Luguet | Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK area, Atlantic Ocean)[END_REF][START_REF] Ballhaus | Fractionation of the noble metals by physical processes[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Reisberg | Osmium isotope constraints on formation and refertilization of the noncratonic continental mantle lithosphere[END_REF].

Notably, Pacific abyssal peridotites tend to have BSE-like to fractionated HSE patterns with relative depletion in Re, Pd and Pt: all Pacific abyssal peridotites show similar ranges for (Pd/Ir)n and (Pt/Ir)n (WEST03MV-12 dredge: 0.5-1.2 and 0.6 -0.9; WEST03MV-13 dredge: 0.5-1.6 and 0.5-1.0; EPR: 0.9-1.8 and 0.6-1.6 respectively). Pacific Antarctic Ridge samples with low degrees of melt depletion (WEST03MV-13) have BSE-like HSE patterns with variable rhenium depletion, whereas samples with higher degrees of melt depletion (WEST03MV-12) have more variable Re, Pd and Pt abundances (Figure 4). Two of the Hess Deep samples have patterns that are akin to BSE, while the other samples show different degrees of rhenium depletion, suggesting that, in all cases, melt depletion has modified the original HSE abundances. The average total HSE content decreases with decreasing melt depletion degree (Figure S5), suggesting that the variability observed between the different ocean basins in term of HSE abundances can be explained by petrogenetic processes.

Melt depletion in peridotites decreases the incompatible element abundances. Positive correlations exist between Al2O3, as well as the HREE and Y, and 187 Os/ 188 Os for the Pacific abyssal peridotites (Figure 6), as observed for peridotites from slower spreading ridges (e.g., [START_REF] Reisberg | Longevity of sub-continental mantle lithosphere from osmium isotope systematics in orogenic peridotite massifs[END_REF][START_REF] Parkinson | Ancient mantle in a modern arc: Osmium isotopes in Izu-Bonin-Mariana forearc peridotites[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a). Positive relationships also exist between Al2O3 and Re/Ir and Pd/Ir, but do not occur for Pt/Ir, Ru/Ir or Os/Ir, which are essentially invariant with decreasing Al2O3 (Table 1). Rhenium and Pd abundances in abyssal peridotites are depleted during melt loss, whereas Pt, Ru, Ir and Os abundances and 187 Os/ 188 Os are less affected by such processes. Consequently, these correlations reflect variable melt loss in the peridotite suite. The, correlation between 187 Os/ 188 Os and Al2O3 contents in bulk rock abyssal peridotites further implies that some of this melt depletion is also ancient, as 187 Os/ 188 Os in these rocks tracks long-term melt depletion, where rocks with low Al2O3 also have low 187 Re/ 188 Os. Pre-existing heterogeneities in peridotites have been noted in previous 187 Re- 187 Os studies, reporting Re depletion ages (TRD = 1/1.67 × 10-11 × ln{[(0.127 -187 Os/ 188 Ossample)/0.40186]+1}; where TRD ages represent minimum depletion ages, assuming no ingrowth from 187 Re in abyssal peridotites since melt depletion) as ancient as 2 Ga for abyssal peridotites (e.g., [START_REF] Harvey | Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge[END_REF][START_REF] Liu | Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a), as well as for ophiolite peridotites (e.g., [START_REF] Büchl | Formation of podiform chromitite deposits: implications from PGE abundances and Os isotopic compositions of chromites from the Troodos complex, Cyprus[END_REF][START_REF] Schulte | Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile[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][START_REF] O'driscoll | Generations of melt extraction, melt-rock interaction and high-temperature metasomatism preserved in peridotites of the ∼497 Ma Leka Ophiolite Complex, Norway[END_REF] and mantle peridotites from ocean islands [START_REF] Snortum | Pacific lithosphere evolution inferred from Aitutaki mantle xenoliths[END_REF]. In this respect, it is notable that the concept of the TRD model age was originally developed for highly refractory cratonic peridotites, where the estimated degree of melt extraction is ~30 to 50%, leading to nearly Re-free peridotitic residues [START_REF] Walker | Nd, and Pb isotope systematics of southern African peridotite xenoliths: implications for the chemical evolution of subcontinental mantle[END_REF][START_REF] Luguet | Dating mantle peridotites using Re-Os isotopes: The complex message from whole rocks, base metal sulfides, and platinum group minerals[END_REF]Reisberg, 2021, and references therein). Nonetheless, measured 187 Os/ 188 Os below the BSE or even chondritic values in mantle peridotites must reflect long-term rhenium depletion.

Rhenium depletion ages for Pacific abyssal peridotites range up to 1.3 Ga for peridotites from the WEST03MV-12 dredge, up to 1.5 Ga for those from the WEST03MV-13 dredge, and up to 0.9 Ga for Hess Deep (Figure 5). The oldest rhenium depletion ages are therefore in the samples with the lowest degrees of melt depletion. The average Re depletion ages are 0.5 ± 0.3 Ga, 0.6 ± 0.5 Ga and 0.7 ± 0.2 Ga respectively, which are similar to previous estimates from abyssal peridotites [START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a). The distinction between ancient melt depletion and recent melt depletion at the ridge remains one of the most challenging issues to deconvolve in abyssal peridotites. No correlations between 187 Os/ 188 Os or TRD ages with degree of partial melting (or melt depletion, F) are observed, suggesting that separating pre-existing melt depletion from recent-ridge melt depletion cannot be directly established. Nevertheless, Pacific abyssal peridotites with low Pd/Ir also show ancient TRD ages, similar to samples from slower spreading ridges (e.g., [START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a), indicating that abyssal peridotites preserve significant preexisting melt depletion, up to or exceeding 2 Ga, in some cases (Figure 5).

Pacific Ocean mantle composition

Collectively, Pacific abyssal peridotites have similar distributions and variations in the abundances of the HSE, as well as slightly more radiogenic 187 Os/ 188 Os, relative to Pacific mantle xenoliths (Figure 6a) [START_REF] Bizimis | Ancient recycled mantle lithosphere in the Hawaiian plume: osmium-hafnium isotopic evidence from peridotite mantle xenoliths[END_REF][START_REF] Jackson | Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187 Os and 142 Nd isotopic domains and the preservation of Hadean 129 Xe in the modern convecting mantle[END_REF][START_REF] Snortum | Pacific lithosphere evolution inferred from Aitutaki mantle xenoliths[END_REF]. Mantle xenoliths preserve melt depletion ages up to 1.5 Ga in Aitutaki (Cook Islands: Snortum et al., 2019), 1.5 Ga in Savai'i, 1.8 Ga in Tubuai (Samoa andAustral Islands respectively: Jackson et al., 2016) and as ancient as 2 Ga in O'ahu (Hawaii: [START_REF] Bizimis | Ancient recycled mantle lithosphere in the Hawaiian plume: osmium-hafnium isotopic evidence from peridotite mantle xenoliths[END_REF], similar to TRD ages reported for Pacific abyssal peridotites. These observations indicate that the Pacific oceanic mantle records heterogeneous melt-depletion from both ancient and more recent melt depletion events. These melt depletion events can be up to 2 Ga and, in this sense, are similar to observations from global oceanic lithosphere (e.g., [START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Harvey | Ancient melt extraction from the oceanic upper mantle revealed by Re-Os isotopes in abyssal peridotites from the Mid-Atlantic ridge[END_REF][START_REF] Liu | Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a). Even though Pacific abyssal peridotites and mantle xenoliths only provide a glimpse of the Pacific oceanic plate, often at the hand-sample scale, they show evidence for significant heterogeneities at short length scales due to prior melt depletion events up to 2 Ga ago, as well as strong similarities in average HSE abundances and Os isotope compositions to abyssal peridotites from the Atlantic, Indian or Arctic Ocean basins.

Composition of the Bulk Silicate Earth deduced from abyssal peridotites

Average HSE abundances and Os isotope compositions for measured abyssal peridotites from the global mid-ocean ridge system show no correlation with spreading rate (Figure 7 andS6). This observation is important because different melting processes at slow and ultraslow spreading ridges versus fast spreading ridges could conceivably yield differences in modern melt depletion recorded in abyssal peridotites, but this is not the case. The 187 Os/ 188 Os composition of the BSE recalculated with the new Pacific abyssal peridotites is 0.1265 ± 0.0031 (n = 230), and 0.1250 ± 0.0040 for peridotites with > 2 ppb Os. These values are close to those reported by [START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF] for the convecting upper mantle and by Day et al. (2017a) for the depleted MORB mantle (0.1245 and 0.1247 ± 0.0075 respectively). Following a similar approach as Day et al. (2017a), we recalculate the average composition of the BSE using abyssal peridotites with Al2O3 content > 2 wt.% from all mid-ocean ridge systems (Figure 8). We obtain 0.34 ± 0.38 (1 s.d.) ppb [Re], 4.87 ± 2.67 ppb [Pd], 7.29 ± 1.88 ppb [Pt], 7.40 ± 1.89 ppb [Ru], 3.84 ± 1.12 ppb [Ir] and 3.36 ± 1.09 ppb [Os] (Figure 8). Estimates for the Arctic and Indian Oceans give comparable values to those of the global ridge system estimate (Figure 8). Note that the Atlantic Ocean estimate appears slightly depleted, and the Pacific Ocean is slightly enriched in the HSE, compared with the global BSE estimate (Figure 8). These differences can be attributed to variable degrees of ancient and modern partial melting affecting the peridotites. Similar results are obtained when the data are regressed to a BSE Al2O3 value of 4 to 4.5 wt.% (see Day et al., 2017a).

The reevaluated composition for the BSE, including the new Pacific abyssal peridotite data, does not significantly change the average estimates for the BSE previously reported for abyssal and oceanic peridotites [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Chatterjee | 186 Os/ 188 Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales[END_REF]Day et al., 2017a), or for continental peridotites [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Chatterjee | 186 Os/ 188 Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales[END_REF]. Our results for Pacific abyssal peridotites confirm that the BSE estimated from abyssal peridotites does not show high Pd/Ir and Pd enrichment as reported by [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF], and [START_REF] Chatterjee | 186 Os/ 188 Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales[END_REF]. This discrepancy between the two estimates of the BSE compositions is thought to result from Pd enrichment in the continental-derived peridotites used for the calculation due to meltrefertilization (e.g., [START_REF] Aulbach | Distribution and processing of highly siderophile elements in cratonic mantle lithosphere[END_REF][START_REF] Luguet | Highly siderophile element and 187Os signatures in noncratonic basalt-hosted peridotite xenoliths: Unravelling the origin and evolution of the post-Archean lithospheric mantle[END_REF][START_REF] Becker | Re-Pt-Os isotopic and highly siderophile element behavior in oceanic and continental mantle tectonites[END_REF].

Overall, these observations lead to the idea that the BSE shows an abundance range for the most incompatible HSE (Re, Pd) that can exceed one hundred percent, but that more compatible HSE (Pt, Ru, Ir, Os) show less than 30% abundance variation. These variations can be accounted for by melting processes acting on abyssal peridotites over <2 Ga.

Implications of a relatively homogeneous convecting mantle for the HSE

Abyssal peridotites from fast to intermediate spreading ridges reveal few, if any, systematic differences in the distribution and behavior of the HSE, compared with slow to ultraslow spreading ridges, despite generally higher degrees of melt depletion (Figures 7 and8). Additionally, estimates of the 187 Os/ 188 Os isotope composition of the BSE based on abyssal peridotites show little to no difference to oceanic mantle xenoliths (e.g., [START_REF] Chatterjee | 186 Os/ 188 Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales[END_REF][START_REF] Snortum | Pacific lithosphere evolution inferred from Aitutaki mantle xenoliths[END_REF] and samples from peridotite massifs (e.g., [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF]. Taking the available global abyssal peridotite sample set and using Al2O3 > 2 wt.%, HSE and Os isotope compositions argue in favor of present-day convecting mantle composition, with Pt, Ru, Ir and Os abundance variations at <±30% and 187 Os/ 188 Os variations at ~6% (2SD) for the global abyssal peridotite dataset. As noted previously, this variation can be accounted for by recent rather than ancient (>2 Ga) processes.

Compared with abyssal peridotites (e.g., Day et al., 2017a and this study), Archean mantle peridotites from West Greenland (3.8 Ga) and Western Australia (3.46 Ga) have similar broadly chondritic 187 Os/ 188 Os compositions and Os concentrations, arguing in favor of the HSE being added to the Earth, transported and homogenized within the mantle by ~3.8 Ga [START_REF] Bennett | Constraints on mantle evolution from 187 Os/ 188 Os isotopic compositions of Archean ultramafic rocks from southern West Greenland (3.8 Ga) and Western Australia (3.46 Ga)[END_REF][START_REF] Van De Löcht | Earth's oldest mantle peridotites show entire record of late accretion[END_REF]. Homogeneous Ru isotope compositions have been reported in Archean ultramafic rocks younger than 3.5 Ga (Pilbara Craton, Australia: 3.5 -3.2 Ga; Abitibi greenstone belt, Canada: 2.7 Ga; Bushveld, South Africa: 2.05 Ga), as well as in Phanerozoic oceanic and continental mantle domains, and are undistinguishable from the modern terrestrial mantle [START_REF] Bermingham | The ruthenium isotopic composition of the oceanic mantle[END_REF][START_REF] Fischer-Gödde | Ruthenium isotope vestige of Earth's prelate-veneer mantle preserved in Archaean rocks[END_REF]. These lines of evidence all support a relatively homogenized BSE in terms of HSE abundances and Os isotope ratios since at least the Archaean, where variations reflect melting processes within the mantle, rather than heterogeneities incorporated during Earth's accretion.

Based on HSE abundances, it has been proposed that ~0.5 to 0.8% of Earth's present mass was accreted after core formation, if all HSE delivered by these impacts were retained in the mantle, rather than being lost to the core or through inefficient impact retention [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF]Day et al., 2016a). Consequently, late accretion impacts might have been expected to leave graininess in HSE abundances in Earth's mantle. This is because some moderate to large size impactors may not have been capable of completely remelting Earth's mantle. From a geological perspective, Eoarchean ultramafic rocks from Greenland (3.8 -3.7 Ga), and Mesoarchean chromitites from Seqi, Greenland (minimum age of 3 Ga) exhibit 100 Ru excess [START_REF] Fischer-Gödde | Ruthenium isotope vestige of Earth's prelate-veneer mantle preserved in Archaean rocks[END_REF], arguing in favor of a heterogeneous delivery of the HSE to the Earth during late accretion. Similarly, evidence for grainy accretion has been suggested for other bodies, such as Vesta [START_REF] Day | Late accretion as a natural consequence of planetary growth[END_REF]. Numerical impact models also support that delivery of silicate and metal to the Earth by large planetesimals was heterogeneous, leading to projectile material being concentrated within localized domains of Earth's mantle and producing isotopic anomalies in W, Mo and Ru isotopes [START_REF] Marchi | Heterogeneous delivery of silicate and metal to the Earth by large planetesimals[END_REF][START_REF] Maas | On the fate of impact-delivered metal in a terrestrial magma ocean[END_REF].

Geochemical arguments in favor of heterogeneities imparted to the mantle during accretion include positive µ 182 W preserved in most Eoarchean rocks studied to date, which have been interpreted to indicate that late accreted materials were not evenly distributed in Earth's mantle, and not well mixed in the BSE [START_REF] Willbold | The tungsten isotopic composition of the Earth's mantle before the terminal bombardment[END_REF]. Such heterogeneities are not evident in the convecting mantle, with basalts from mid-ocean ridges and ocean islands measured to date lacking positive µ 182 W anomalies (e.g., [START_REF] Mundl | Tungsten-182 heterogeneity in modern ocean island basalts[END_REF]. The HSE have also been argued to have been heterogeneously distributed within the early mantle from observations that Archean (3.5 -3.2 Ga) komatiites from the Barberton greenstone belt (South Africa) and the Pilbara craton (Western Australia) apparently record depleted HSE compositions for their mantle sources relative to late Archean and younger komatiites [START_REF] Maier | Progressive mixing of meteoritic veneer into the early Earth's deep mantle[END_REF]. Arguments in favor of HSE heterogeneities imparted to the mantle by late accretion are not without controversy, however, where it has been argued that komatiites are not faithful recorders of mantle source compositions [START_REF] Waterton | The komatiite-mantle platinum-group element paradox[END_REF].

Because the HSE appear to be homogeneously mixed in the present-day convecting mantle, we favor that whatever HSE input was accreted to the Earth, these heterogeneities have been relatively efficiently mixed into the mantle through convective processes. Anomalies in the HSE, including Ru, as well as Mo and W in Archean crustal rocks can be interpreted to reflect late accretion heterogeneity within the Earth that is well-recorded within isolated crustal and lithospheric fragments. For example, the mantle beneath southwest Greenland had not yet fully equilibrated with late accretion material by 3.7 Ga ago [START_REF] Fischer-Gödde | Ruthenium isotope vestige of Earth's prelate-veneer mantle preserved in Archaean rocks[END_REF]. This is consistent with the average mantle homogenization timescale of ~1.2 Ga calculated from the combined 186 Os/ 188 Os-187 Os/ 188 Os isotopic and Pt/Os and Re/Os variability in peridotites (e.g., [START_REF] Chatterjee | 186 Os/ 188 Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales[END_REF]. Other isotopic systems, such as the short lived 146 Sm-142 Nd system, lead to younger mantle homogenization timescales (~0.4 Ga) as well as a fast mantle stirring rate (e.g., [START_REF] Chatterjee | 186 Os/ 188 Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales[END_REF][START_REF] Hyung | The 142 Nd/ 144 Nd variations in mantle-derived rocks provide constraints on the stirring rate of the mantle from the Hadean to the present[END_REF].

In contrast with lithophile incompatible elements, which are sensitive to resetting by melt refertilization in peridotites, the Re-Os and Pt-Os isotope systems are less susceptible to similar petrogenetic processes. As such, the mixing timescale inferred from Os isotopes in abyssal peridotites of around 0.5-0.7 Ga is approximately consistent with timescales predicted for the whole mantle convection. [START_REF] Hoffman | The destruction of geochemical heterogeneities by differential fluid motions during mantle convection[END_REF] showed that any convecting region of the upper mantle will be well mixed on a horizontal scale of at least 2000 km in 400 Ma, or 8500 km in 1.5 Ga, suggesting that large scale heterogeneities would be destroyed within Earth's lifetime. Our data support this contention. Nonetheless, elemental and isotopic anomalies that are thought to reflect mantle heterogeneities have been recorded in ocean island basalts (e.g., HSE in the Réunion cumulate xenoliths, [START_REF] Peters | Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths[END_REF]; W isotope anomalies in modern flood basalts: [START_REF] Rizo | Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts[END_REF]and modern ocean island basalts: Mundl et al., 2017;[START_REF] Mundl-Petermeier | Temporal evolution of primordial tungsten-182 and 3 He/ 4 He signatures in the Iceland mantle plume[END_REF], 2020;[START_REF] Peters | Combined Lithophile-Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources[END_REF]. These suggest that distinct mantle domains might have been effectively isolated and preserved from the convecting upper mantle through most of the Earth's history (e.g., [START_REF] Allègre | Geodynamic mixing in the mesosphere boundary layer and the origin of oceanic islands[END_REF].

CONCLUSIONS

This global survey of abyssal peridotites from fast to intermediate spreading ridges reveals few if any systematic differences in the distribution and behavior of the HSE, compared with slow to ultraslow spreading ridges, despite variable degrees of melt depletion. Melt refertilization is pervasive and variable in extent from ultraslow to fast spreading ridges and can modify Re and Pd quite significantly. Across ocean basins and independently of spreading rates, the HSE appear relatively homogeneous in the BSE. Using abyssal peridotites with Al2O3 content > 2 wt.% from all mid-ocean ridge systems, including the new Pacific abyssal peridotite data, we recalculated the average composition of the BSE of 0.30 ± 0.33 ppb [Re], 4.94 ± 2.35 ppb [Pd], 7.13 ± 2.16 ppb [Pt], 7.22 ± 3.71 ppb [Ru], 3.79 ± 1.84 ppb [Ir] and 3.77 ± 1.45 ppb [Os]. Variability observed between the different ocean basins, if any, can most likely be explained by partial melting processes. The variations in HSE abundances currently observed within the convecting mantle sampled by abyssal peridotites are primarily due to partial melting and melt refertilization processes over the past 2 Ga rather than to significant HSE heterogeneity of the post-Archean mantle. Preservation of ancient melt depletion heterogeneities in some oceanic peridotites, with osmium TRD ages exceeding 1 Ga and highly depleted Hf isotope signatures measured in clinopyroxene grains [START_REF] Stracke | Abyssal peridotite Hf isotopes identify extreme mantle depletion[END_REF][START_REF] Sanfilippo | Role of ancient, ultra-depleted mantle in Mid-Ocean-Ridge magmatism[END_REF] suggests some refractory domains formed through ancient melt depletion can also be partially preserved in the convecting upper mantle. Tables S1 to S7. Ancillary, major, trace element and the available published HSE and Os isotope dataset for abyssal peridotites. Accuracy and precision for standard reference materials and total procedural blanks are provided for isotope dilution HSE abundance analyses. [START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF]Brandon et al., , 2006;;[START_REF] Luguet | Laserablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle[END_REF][START_REF] Luguet | Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK area, Atlantic Ocean)[END_REF][START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a;Li et al., 2019). Day et al., (2017a). Normalizing values from McDonough & Sun (1995). Gray lines represent 1% melt increments for a non-modal fractional melting model (see Day et al., 2017a for model parameters). An increase of melt depletion is associated with lower absolute REE abundances and higher depletion in the LREE relative to the HREE. [START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a;Li et al., 2019) (d). Normalizing values from Day et al. (2017a). Gray dashed lines correspond to previous studies on Hess Deep abyssal peridotites [START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Rehkämper | Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle[END_REF]. The black solid line in panels a-c corresponds to the Primitive Upper Mantle (PUM) estimate from [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF]. Day et al. (2017a). Literature data are from Snow and Schmidt (1998), [START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Liu | Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean[END_REF], 2009), and Day et al. (2017a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). The dashed and solid lines correspond to the linear regression for the WEST03MV-12 (r 2 = 0.64) and WEST03MV-13 peridotites (r 2 = 0.5), respectively. Literature data for abyssal peridotites are given in the text and can be found in Supplementary Table 7. The green field represents the peridotite xenoliths from the Pacific Ocean [START_REF] Bizimis | Ancient recycled mantle lithosphere in the Hawaiian plume: osmium-hafnium isotopic evidence from peridotite mantle xenoliths[END_REF][START_REF] Jackson | Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187 Os and 142 Nd isotopic domains and the preservation of Hadean 129 Xe in the modern convecting mantle[END_REF][START_REF] Snortum | Pacific lithosphere evolution inferred from Aitutaki mantle xenoliths[END_REF]. The dotted grey lines are the depletion ages for a BSE composition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

FIGURES & FIGURE CAPTIONS

Figure 8. Estimates for the mantle composition for each ocean basin (solid lines), considered to be represented by abyssal peridotites with Al2O3 > 2 wt.% (this study; [START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF]Day et al., 2017a;Li et al., 2019)

  . Samples from the West region span a larger range of REE abundances. Abyssal peridotites from Hess Deep are depleted in the heavy REE, with REE patterns slightly depleted in the light REE (LREE) relative to the HREE, and strong positive anomalies for Eu, suggesting melt impregnation and plagioclase crystallization. A few samples from the PAR, SWIR and CIR also show negative Ce anomalies. Overall, abyssal peridotites from the WEST03MV-13 dredge tend to have higher Nb and Ta abundances, and lower Zr and Hf abundances than those from the WEST03MV-12 dredge. Abyssal peridotites from Hess Deep exhibit depleted patterns in Rb, Ba, Nb, Ta, Zr and Hf compared to other Pacific peridotites.

Figure 1 .

 1 Figure 1. Mid-ocean ridge full spreading rates and the location of abyssal peridotites analyzed for Re-Os isotopes and HSE abundance systematics in this study (EPR, PAR, SWIR, CIR) and prior studies (modified from Rouméjon, 2014). Spreading rates are from DeMets et al. (1990). The black circles represent locations of new HSE and Os isotope data for abyssal peridotites from Hess Deep along the East Pacific Rise (EPR) and from the Udintsev fracture zone along the Pacific Antarctic Ridge (PAR), as well as from the Central and Southwest Indian Ridges. The light gray circles correspond to abyssal peridotite samples from other slower spreading centers[START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF] Brandon et al., , 2006;;[START_REF] Luguet | Laserablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle[END_REF][START_REF] Luguet | Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK area, Atlantic Ocean)[END_REF][START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF] Day et al., 2017a; Li et al., 2019).

Figure 2 .

 2 Figure 2. Variations (in wt.%) of (a) Al2O3 as a function of MgO and (b) CaO as a function of Al2O3 for bulk rock abyssal peridotites from the dredges WEST03MV-12 and WEST03MV-13 (abbreviated WW03-12 and WW03-13 respectively) along the Pacific Antarctic Ridge, Hess Deep region along the East Pacific Rise, the Southwest Indian Ridge, the Central Indian Ridge and the Mid-Atlantic Ridge. Also shown for comparison are abyssal peridotites from the Gakkel ridge, the Southwest Indian Ridge, the Central Indian Ridge and the Mid-Atlantic Ridge for which HSE and Os isotope data are available (see TableS7for references). The star corresponds to the Primitive Mantle estimate fromMcDonough and Sun (1995). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Figure 3 .

 3 Figure 3. Bulk Silicate Earth-normalized rare earth element (REE) concentrations in abyssal peridotites from the dredges WEST03MV-12 (a) and WEST03MV-13 (b) along the Pacific Antarctic Ridge (PAR), Hess Deep region along the East Pacific Rise (EPR) (c), SWIR (d), CIR (e) and MAR (f). Shaded fields are abyssal peridotites from Day et al., (2017a). Normalizing values from McDonough & Sun(1995). Gray lines represent 1% melt increments for a non-modal fractional melting model (seeDay et al., 2017a for model parameters). An increase of melt depletion is associated with lower absolute REE abundances and higher depletion in the LREE relative to the HREE.

Figure 4 .

 4 Figure 4. Bulk Silicate Earth-normalized HSE concentrations in abyssal peridotites from the dredges WEST03MV-12 (a) and WEST03MV-13 (b) along the Pacific Antarctic Ridge (PAR), Hess Deep region along the East Pacific Rise (EPR) (c), SWIR and CIR, together with slowspreading ridge abyssal peridotite literature data as comparison[START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF] Day et al., 2017a; Li et al., 2019) (d). Normalizing values fromDay et al. (2017a). Gray dashed lines correspond to previous studies on Hess Deep abyssal peridotites[START_REF] Snow | Constraints on Earth accretion deduced from noble metals in the oceanic mantle[END_REF][START_REF] Rehkämper | Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle[END_REF]. The black solid line in panels a-c corresponds to the Primitive Upper Mantle (PUM) estimate from[START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF].

Figure 5 .

 5 Figure 5. Time of Re depletion model ages (TRD) versus Pd/Ir. Primitive mantle (also referred to as BSE) Pd/Ir is fromDay et al. (2017a). Literature data are from Snow and Schmidt (1998),[START_REF] Becker | Highly siderophile element composition of the Earth's primitive upper mantle: constraints from new data on peridotite massifs and xenoliths[END_REF][START_REF] Liu | Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean[END_REF], 2009), andDay et al. (2017a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Figure 6 .

 6 Figure 6. Relationship between 187 Os/ 188 Os ratios and (a) Os and (b) Al2O3 contents in abyssal peridotites.The dashed and solid lines correspond to the linear regression for the WEST03MV-12 (r 2 = 0.64) and WEST03MV-13 peridotites (r 2 = 0.5), respectively. Literature data for abyssal peridotites are given in the text and can be found in Supplementary Table7. The green field represents the peridotite xenoliths from the Pacific Ocean[START_REF] Bizimis | Ancient recycled mantle lithosphere in the Hawaiian plume: osmium-hafnium isotopic evidence from peridotite mantle xenoliths[END_REF][START_REF] Jackson | Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187 Os and 142 Nd isotopic domains and the preservation of Hadean 129 Xe in the modern convecting mantle[END_REF][START_REF] Snortum | Pacific lithosphere evolution inferred from Aitutaki mantle xenoliths[END_REF]. The dotted grey lines are the depletion ages for a BSE composition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

  Figure 8. Estimates for the mantle composition for each ocean basin (solid lines), considered to be represented by abyssal peridotites with Al2O3 > 2 wt.% (this study;[START_REF] Brandon | 190 Pt-186 Os and 187 Re-187 Os systematics of abyssal peridotites[END_REF][START_REF] Liu | Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)[END_REF][START_REF] Lassiter | Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle[END_REF] Day et al., 2017a; Li et al., 2019) (a) Pacific Ocean, (b) Atlantic Ocean, (c) Arctic Ocean, (d) Indian Ocean and (e) for the global abyssal peridotite sample suite. Colored fields correspond to the standard deviation of the calculated mantle composition. Dashed lines correspond to the primitive (upper) mantle estimates from Becker et al., (2006) and Day et al. (2017a). The grey field represents the 2 SD uncertainty of the estimate from Day et al. (2017a). Note that abyssal peridotites from the CIR and Hess Deep along the EPR were not included in the calculation for the Indian and Pacific oceans due to their low Al2O3 content. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

TABLE CAPTIONS Table 1 .

 CAPTIONS1 Osmium isotope and highly siderophile element abundance (ppb) data in abyssal peridotites.

ACKNOWLEDGMENTS

This work was largely supported by a University of California San Diego Academic Senate Award, and in part, by the NSF Petrology and Geochemistry program (NSF EAR 1447130 and EAR 1918322 to JMDD). We are grateful to Alex Hangsterfer (SIO Geological Collections) for assistance with sample curation. Dean Poeppe and Garrett Stewart are thanked for assistance with sample preparation. Constructive comments from David van Acken, the Associate Editor, Andreas Stracke, and two anonymous reviewers are gratefully acknowledged.


MD François Lersy

MD, PhD Mathieu Anheim

MD Thibault Willaume

MD Agathe Chammas

PhD Jean-Christophe Brisset

Francois Cotton

Stephane Kremer
email: stephane.kremer@chru-strasbourg.fr

François Cotton

Stéphane Kremer

MD Willaume Thibault

MD, PhD François Cotton

MD, PhD Stéphane Kremer

Stéphane Kremer

MD François Cotton

MD Stéphane Kremer

Cerebral vasculitis of medium-sized vessels as a possible mechanism of brain damage in COVID-19 patients

Keywords: vasculitis, COVID-19, Magnetic Resonance Imaging (MRI) Magnetic Resonance Imaging (MRI), Vascular Disease

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

Introduction

In recent weeks, some cerebral complications related to COVID-19 were documented by brain MRIs [START_REF] Kremer | Brain MRI Findings in Severe COVID-19: A Retrospective Observational Study[END_REF][START_REF] Radmanesh | COVID-19 -associated Diffuse Leukoencephalopathy and Microhemorrhages[END_REF][START_REF] Kremer | Neurologic and neuroimaging findings in COVID-19 patients: A retrospective multicenter study[END_REF][START_REF] Hanafi | COVID-19 Neurologic Complication with CNS Vasculitis-Like Pattern[END_REF][START_REF] Hernández-Fernández | Cerebrovascular disease in patients with COVID-19: neuroimaging, histological and clinical description[END_REF]. Nevertheless, the underlying mechanisms of brain damage remain unclear, probably multifactorial. Among various hypotheses suggested, a possible vasculitis was issued but never confirmed [START_REF] Kremer | Brain MRI Findings in Severe COVID-19: A Retrospective Observational Study[END_REF][START_REF] Radmanesh | COVID-19 -associated Diffuse Leukoencephalopathy and Microhemorrhages[END_REF][START_REF] Kremer | Neurologic and neuroimaging findings in COVID-19 patients: A retrospective multicenter study[END_REF][START_REF] Hanafi | COVID-19 Neurologic Complication with CNS Vasculitis-Like Pattern[END_REF][START_REF] Hernández-Fernández | Cerebrovascular disease in patients with COVID-19: neuroimaging, histological and clinical description[END_REF]. A recent pathologic study [START_REF] Varga | Endothelial cell infection and endotheliitis in COVID-19[END_REF] has shown evidence of direct viral infection of the endothelial cells with endotheliitis. Herein, we aimed to describe brain MRIs focused on the intracranial vessel wall in a population of COVID-19 patients with neurologic manifestations.

Materials & Methods

The study was approved by the ethical committee of Strasbourg University Hospital (CE-2020-37) and was in accordance with the 1964 Helsinki Declaration and its later amendments. Due to the emergency in the context of COVID-19 pandemic responsible for acute respiratory and neurological manifestations, the requirement for patients' written informed consent was waived.

Patient cohort and brain MRI protocols

During the COVID-19 outbreak between March 1 and May 31, 2020, 112 consecutive COVID-19 patients with neurologic manifestations underwent a brain MRI at Strasbourg University hospitals. The diagnosis of COVID-19 was confirmed by the detection of SARS-CoV-2 by reverse transcriptase-polymerase chain reaction (RT-PCR) assays on the nasopharyngeal, throat, or lower respiratory tract swabs. Of these 112 patients, 69 patients were retrospectively included, based on the MRI sequences acquired: unenhanced and contrast-enhanced three-dimensional T1-weighted spin-echo MR imaging with a slice thickness of 1mm or 1.2mm (due to its black blood effect, the latter sequence allows the study of the intracranial vessel wall) [START_REF] Mandell | Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology[END_REF], time-of-flight MR angiography, diffusion weighted-imaging, axial susceptibility-weighted MR and FLAIR sequences. All subjects underwent MR imaging with a 3T MR unit. During the same period, 25 consecutive patients, without suspicion of SARS-CoV-2 infection underwent a brain MRI in an emergency context, for neurologic disorders, with the same imaging protocols, and with similar 3T MR units. Clinical and laboratory data were extracted from the patients' electronic medical records in the Hospital Information System.

MRI interpretation

Two neuroradiologists (S.K and F.L with 20 and 9 years of experience in neuroradiology, respectively) blinded to all patient data (SARS-CoV-2 infection or not), independently reviewed all brain MRIs, and reached a final agreement concerning the diagnosis of cerebral vasculitis. According to expert consensus recommendations [START_REF] Mandell | Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology[END_REF], a vasculitis occurs likely when imaging demonstrates vessel wall thickening with homogeneous and concentric enhancement [START_REF] Mandell | Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology[END_REF][START_REF] Portanova | Intracranial vasa vasorum: insights and implications for imaging[END_REF]. Nonconcentric vessel wall enhancements were considered to be possibly related to atherosclerotic plaque [START_REF] Mandell | Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology[END_REF][START_REF] Portanova | Intracranial vasa vasorum: insights and implications for imaging[END_REF]. This latter pathological condition may also be associated with the development of vasa vasorum, which can mimic vasculitis. Due to the potential pitfall secondary to the development of intracranial vasa vasorum, concentric enhancements localized in the intracranial segments of the internal carotid and vertebral arteries have not been considered [START_REF] Mandell | Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology[END_REF][START_REF] Portanova | Intracranial vasa vasorum: insights and implications for imaging[END_REF].

Statistical analysis

As atherosclerosis may also be associated with concentric enhancements, we described and compared the distribution of the vascular disease risk factors in the two groups (with and without confirmed SARS-CoV-2 infection) to avoid a bias related to their presence. In a second step, in the COVID-19 group, by the same reasoning, the patients with suspected cerebral vasculitis were compared to those with normal vessel wall imaging. Data were described using frequency and proportion (n, %) for categorical variables, using median and range for quantitative data. Categorical data were compared using Fisher exact test. Quantitative data were compared using Student's t-test. A p-value lower than 0.05 was considered significant.

Results

The characteristics of the two populations are summarized in table 1. Among the 69 COVID-19 patients included, 11 (16%), ten men and one woman, with a median age of 69 years (range: 64-79 years) presented arterial vessel wall thickening with homogeneous and concentric enhancement, compatible with cerebral vasculitis (Figure 1). These neuroimaging findings were not found among the 25 patients without SARS-CoV-2 infection, and the difference was statistically significant (p=0.03). They were more men in the group with SARS-CoV-2 infection (p=0.004), without statistical differences concerning the other cardiovascular risk factors (table 1). Among these 11 patients, the vascular involvement was unifocal in 3 (27%) cases and multifocal in 8 (73%) cases. Middle cerebral arteries (9 cases), basilar artery (8 cases), and posterior cerebral arteries (4 cases) were the most frequent vessels involved. Six patients (55%) demonstrated extensive white matter microhemorrhages, 3 (27%) subarachnoid hemorrhage, 4 (36%) acute ischemic stroke (especially watershed cerebral infarctions in three cases, the last patient demonstrated a small vessel infarct), and 2 (18%) brain parenchymal signal abnormalities (Table 2). Only two patients (18%) showed intracranial arterial narrowing (Figure 1). Of these 11 patients, 6 underwent a lumbar puncture (the median between the onset symptoms of COVID-19, mostly respiratory, to lumbar puncture was 28 days, range 8-44 days), which presented increased inflammation markers for three of them, but SARS-CoV-2 RNA was never detected. These patients' demographic and clinical characteristics, their neuroimaging findings, and cerebrospinal fluid analysis are summarized in Table 2. As previously mentioned, the presence of vasa vasorum can mimic vasculitis, that is why the intracranial segments of the internal carotid and vertebral arteries have not been taken into account. Vasa vasorum may develop with pathologic conditions such as intracranial atherosclerotic plaques. As above-mentioned, to avoid a bias related to the presence of vascular disease risk factors, among the COVID-19 patients, the two populations, with and without suspicion of cerebral vasculitis, were compared (table 3). The COVID-19 patients with normal vessel wall imaging were younger (p=0.003) without differences concerning stroke history or other risk factors for vascular disease.

Discussion

Our study reinforces the assumption of cerebral vasculitis as one of the mechanisms at the origin of brain damage related to COVID-19. This seems possible, as angiotensin-converting enzyme 2 (ACE2), the main receptor for SARS-CoV-2, is expressed, among others, by endothelial cells [START_REF] Varga | Endothelial cell infection and endotheliitis in COVID-19[END_REF]. A recent pathologic study has shown for the first time that SARS-CoV-2 can infect endothelial cells with diffuse endothelial inflammation [START_REF] Varga | Endothelial cell infection and endotheliitis in COVID-19[END_REF]. This may recall what was previously described with the varicella-zoster virus (VZV), which is associated with a direct viral vascular involvement and cerebral vasculitis, either following a primary infection (varicella) or viral reactivation (herpes zoster) [START_REF] Nagel | The varicella zoster virus vasculopathies: clinical, CSF, imaging, and virologic features[END_REF][START_REF] Nagel | Virus vasculopathy and stroke: an underrecognized cause and treatment target[END_REF]. Indeed, its DNA and antigens were detected in cerebral arteries from patients with VZV vasculopathy [START_REF] Nagel | The varicella zoster virus vasculopathies: clinical, CSF, imaging, and virologic features[END_REF]. Nine of our eleven COVID-19 patients suspected to have cerebral vasculitis demonstrated common cerebral vasculitis complications, such as acute ischemic stroke, subarachnoid or cerebral hemorrhages. Six of them showed extensive white matter microhemorrhages with an atypical involvement of the corpus callosum. This pattern was recently described on brain MRIs in critically ill COVID-19 patients (1,2). However, the underlying pathophysiological mechanisms leading to diffuse microbleeds in the COVID-19 pandemic context remain unknown, and several hypotheses were proposed, notably those concerning the potential role of hypoxemia or vasculitis [START_REF] Kremer | Brain MRI Findings in Severe COVID-19: A Retrospective Observational Study[END_REF][START_REF] Radmanesh | COVID-19 -associated Diffuse Leukoencephalopathy and Microhemorrhages[END_REF]. Only two of our eleven COVID-19 patients suspected of cerebral vasculitis showed evidence of intracranial arterial narrowing, probably because small arteries injury could not be studied so easily as for medium-sized vessels. Moreover, although the presence of angiographic abnormalities (including vascular irregularities, stenoses, aneurysms, and occlusions) is helpful in diagnosing cerebral vasculitis, these are usually described if larger vessels are involved, and a normal angiography does not rule out the diagnosis of vasculitis [START_REF] Suthiphosuwan | Tumefactive Primary Central Nervous System Vasculitis: Imaging Findings of a Rare and Underrecognized Neuroinflammatory Disease[END_REF]. Concerning the cerebrospinal fluid analysis, only six patients suspected of cerebral vasculitis had a lumbar puncture, and SARS-CoV-2 RNA was never detected by RT-PCR, nevertheless, its absence does not exclude the diagnosis. First, it should be kept in mind that the lumbar punctures were performed late after SARS-CoV-2 infection (median of 28 days after the onset of respiratory symptoms). This long delay could explain our samples' negativity since the viral clearance was already significant at this time. Second, in VZV vasculopathy, search for VZV DNA in cerebrospinal fluid is frequently negative [START_REF] Nagel | The varicella zoster virus vasculopathies: clinical, CSF, imaging, and virologic features[END_REF], and the detection of VZV antibody seems to be more sensitive [START_REF] Nagel | The varicella zoster virus vasculopathies: clinical, CSF, imaging, and virologic features[END_REF]. Unfortunately, search for SARS-CoV-2 antibodies in cerebrospinal fluid was not done in our study. Third, the mechanism leading to cerebral blood vessels inflammation is not necessarily a direct viral infection but may involve other phenomena, such as immune-mediated disorders. Of the six patients who had a lumbar puncture, five displayed identical oligoclonal bands in cerebrospinal fluid and serum (type IV), in agreement with previous reports [START_REF] Bellon | Cerebrospinal fluid features in SARS-CoV-2 RT-PCR positive patients[END_REF][START_REF] Neumann | Cerebrospinal fluid findings in COVID-19 patients with neurological symptoms[END_REF]. In this context, they were probably secondary to the systemic infection with passive diffusion into the cerebrospinal fluid [START_REF] Trbojevic-Cepe | Detection of Oligoclonal Ig Bands: Clinical Significance and Trends in Methodological Improvement[END_REF]. Our study's main limitation is the lack of neuropathological confirmation since none of our patients had a brain biopsy. Nevertheless, even if cerebral vasculitis related to COVID-19 has never been highlighted, clinical and histologic evidence of vasculitis has also been demonstrated in other organs, especially in the skin [START_REF] Caputo | A generalized purpuric eruption with histopathologic features of leucocytoclastic vasculitis in a patient severely ill with COVID-19[END_REF][START_REF] Mayor-Ibarguren | Cutaneous small vessel vasculitis secondary to COVID-19 infection: a case report[END_REF][START_REF] Dominguez-Santas | Cutaneous small-vessel vasculitis associated with novel 2019 coronavirus SARS-CoV-2 infection (COVID-19)[END_REF]. Moreover, during the COVID-19 outbreak, the high incidence of Kawasaki disease [START_REF] Verdoni | An outbreak of severe Kawasaki-like disease at the Italian epicentre of the SARS-CoV-2 epidemic: an observational cohort study[END_REF][START_REF] Ouldali | Emergence of Kawasaki disease related to SARS-CoV-2 infection in an epicentre of the French COVID-19 epidemic: a time-series analysis[END_REF], an example of vasculitis affecting medium and small-sized vessels [START_REF] Jennette | revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides[END_REF], seems to bolster this hypothesis.

Conclusion

Cerebral vasculitis of medium-sized vessels seems to be one of the mechanisms at the origin of brain damage related to COVID-19.

Figure:

Figure 1 :

 1 Figure:Figure1: 69-year old man with pathological wakefulness after sedation. Axial unenhanced T1weighted spin-echo (A), axial (B, E), and coronal (C, D) contrast-enhanced T1-weighted spin-echo, 3D TOF MR angiography (F). Basilar artery wall enhancement (B, C). Left middle cerebral artery (D), and right posterior cerebral artery (E) with concentric wall enhancement. Posterior cerebral arteries narrowing (F).

Table 1 :

 1 Characteristics of the two populations

		COVID-19 patients	Patients without SARS-	p-values
		(N=69)	CoV-2 infection (N=25)
	Sex	46 men / 23 women	8 men / 17 women	0.004
	Age (median, range) (years)	65 (21-86)	65 (27-91)	0.66
	History of stroke	4 (6%)	4 (16%)	0.2
	Diabetes	17 (25%)	6 (24%)	1
	High blood pressure	33 (48%)	7 (28%)	0.1
	Hyperlipidemia	25 (43%)	7 (28%)	0.62
	Smoking	6 (9%)	4 (16%)	0.47
	Obesity	24 (35%)	7 (28%)	0.62
	Vessel wall imaging compatible	11 (16%)	0	0.03
	with cerebral vasculitis

Table 2 :

 2 Description of the patients with suspected cerebral vasculitis

Acknowledgments: none

Sources of Funding: none

Disclosures


Plotine Jardat

Ludovic Calandreau

Vitor Hugo

Bessa Ferreira

Chloé Gouyet

Céline Parias

Fabrice Reigner

Léa Lansade

How does Pet-Directed Speech affect horses?

Why

  Pet-Directed Speech (PDS) ↔ Infant-Directed Speech (IDS) ≠ Adult-directed Speech (ADS) High, varying pitch Slow rate of speech, repeated words, simple syntax and semantics  In infants, non-human primates and dogs: PDS helps to attract horses' attention and has an arousing effect on them  H2: PDS influences the valence of emotions felt by horses, making them more positive Methods Pre-registered videos of women speaking in PDS and ADS Horses were more attentive to PDS than ADS  Horses' heart rates increased more during PDS than ADS 8 PDS helps to attract horses' attention and has an arousing effect on them H1✔ Horses were not more relaxed or less alert, they did not show less signs of fear, during PDS than ADS  Test not fearful for horses  Lack of indicators for positive emotions in horses 10 No demonstrated influence of PDS on the valence of emotions felt by horses H2 ? PDS is arousing and attention-drawing for horses → can be used during human-horse interactions  Broadcasting pre-registered videos of humans allows to study horses' perception of us