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The New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide, offering a unique opportunity to investigate key mantle processes. The ophiolite comprises refractory harzburgites, locally overlain by mafic-ultramafic cumulates, and minor lherzolites. Previous geochemical studies indicated that the lherzolites are akin to abyssal-type peridotites, while the harzburgites underwent multiple melting episodes in MOR and supra-subduction zone environments, followed by late stage metasomatism. In this work, Os isotopes, highly siderophile (HSE) and chalcophile element data are reported for the New Caledonia peridotites, in order to constrain the behaviour of these elements in abyssal-type and fore-arc mantle. The variably serpentinised lherzolites (LOI = 6.4–10.7%) yield slightly subchondritic to suprachondritic initial Os isotopic compositions (187Os/188Os i = 0.1273–0.1329) and subchondritic to chondritic Re/Os ratios (0.04–0.11). The gently sloping HSE patterns with increasing depletion towards Au show concentrations in the range of other lherzolites from MOR or continental setting. Sulphur contents are high and variable (202–1268 ppm), and were likely increased during serpentinisation. By contrast, Se/Te ratios and concentrations are within the range of primitive mantle (PM) values, meaning that these elements were not significantly mobilised during serpentinisation. Although displaying homogenous petrographic and geochemical features, the harzburgites are characterised by extremely heterogeneous Re Os and HSE compositions. Type-A harzburgites exhibit subchondritic 187Os/188Os i (0.1203–0.1266) and low Re/Os ratios (0.01–0.04). The strong IPGE-PPGE fractionations (Pd N /Ir N = 0.21–0.56), coupled with positive Pt anomalies and S-Se-Te abundances often below the detection limit, suggest high melt extraction rates, resulting in sulphide consumption and Os Ru metal alloy stabilisation. Type-B harzburgites possess strongly fractionated, Os-Ir-Pt poor (Os = 0.003–0.072 ng/g, Ir = 0.0015–0.079 ng/g) and Pd Re enriched patterns, associated with chondritic to suprachondritic measured 187Os/188Os (0.127–0.153). These characters are uncommon for highly depleted mantle residues. Interaction with an oxidised component does not appear as a viable mechanism to account for the IPGE-depleted patterns of type-B harzburgites, as calculated oxygen fugacities are close to the FMQ buffer (Log ΔFMQ = 0.35 to 0.65). The strikingly uniform mineralogical and geochemical features displayed by both harzburgite sub-types suggest that the different HSE patterns are not linked to their recent evolution, implying that subduction-related processes were superimposed on geochemical heterogeneous mantle domains, which exerted an important control on HSE behaviour during melt extraction and post melting metasomatism. We propose that the HSE characters of the studied peridotites reflect the presence of a highly heterogeneous mantle source with a long term (>1 Ga) evolution, possibly linked to the Zealandia formation. • We present a comprehensive Re Os and HSE study for the New Caledonia peridotites. • Melt percolation and low-moderate melt extraction are inferred for the lherzolites • Based on HSE and Os isotopes, two groups of harzburgites (A-B) can be identified. • The signature of type-A harzburgites results from high degrees of melt extraction. • Type-B harzburgites own atypical HSE patterns, unrelated to their recent evolution [ABSTRACT FROM AUTHOR]

• Multi-element and multi-isotope constraints on the evolution of the subcratonic mantle. • Re-Os and Lu-Hf isotope systematics both record late-Archean metasomatism. • Correlation between silicate modal abundances and Os isotopic composition in Finsch peridotites. Accurately dating the formation and modification of Earth's sub-cratonic mantle still faces many challenges, primarily due to the long and complex history of depletion and subsequent metasomatism of this reservoir. In an attempt to improve this, we carried out the first study on peridotites from the Kaapvaal craton (Finsch Mine) that integrates results from Re-Os, Lu-Hf, Sm-Nd and Sr-isotope systems together with analyses of major-, trace- and platinum-group elements. The Finsch peridotites are well-suited for such a study because certain compositional features reflect they were highly depleted residues of shallow melting (1.5 GPa) at ambient Archean mantle temperatures. Yet, many of them have overabundant orthopyroxene, garnet and clinopyroxene compared to expected modal amounts for residues from partial melting. Finsch peridotites exhibit a wide range of rhenium depletion ages (T RD) from present day to 2.7 Ga, with a prominent mode at 2.5 Ga. This age overlaps well with a Lu-Hf isochron of 2.64 Ga (εHf (t) = +26) which records silico-carbonatitic metasomatism of the refractory residues. This late Archean metasomatism is manifested by positive correlations of Pt/Ir and Pd/Ir with 187Os/188Os ratios and good correlations of modal amounts of silicates, especially garnet, with Os isotope ratios. These correlations suggest that the Highly Siderophile Elements (HSE) and incompatible element reenrichment and modal metasomatism result from one single major metasomatic event at late Archean. Our detailed study of Finsch peridotites highlights the importance of using multiple isotopic systems, to constrain the ages of events defining the evolution of lithospheric mantle. The Re-Os isotope system is very effective in documenting the presence of Archean lithosphere, but only the oldest T RD ages may accurately date or closely approach the age of the last major partial melting event. For a meaningful interpretation of the Re-Os isotope systematics the data must be combined with HSE patterns, trace-element compositions and ideally other isotopic systems, e.g. Lu-Hf. This is highlighted by the widespread evidence in Finsch peridotites of Pt, Pd and Re enrichment through significant Base Metal Sulfide (BMS) addition (mainly in the range of 0.002–0.08 wt%) that systematically shifts the mode of T RD model ages to younger ages. [ABSTRACT FROM AUTHOR]

Reconstructing the building blocks that made Earth and the Moon is critical to constrain their formation and compositional evolution to the present. Neodymium (Nd) isotopes identify these building blocks by fingerprinting nucleosynthetic components. In addition, the 146 Sm- 142 Nd and 147 Sm- 143 Nd decay systems, with half-lives of 103 million years and 108 billion years, respectively, track potential differences in their samarium (Sm)/Nd ratios. The difference in Earth's present-day 142 Nd/ 144 Nd ratio compared with chondrites 1,2 , and in particular enstatite chondrites, is interpreted as nucleosynthetic isotope variation in the protoplanetary disk. This necessitates that chondrite parent bodies have the same Sm/Nd ratio as Earth's precursor materials 2 . Here we show that Earth and the Moon instead had a Sm/Nd ratio approximately 2.4 ± 0.5 per cent higher than the average for chondrites and that the initial 142 Nd/ 144 Nd ratio of Earth's precursor materials is more similar to that of enstatite chondrites than previously proposed 1,2 . The difference in the Sm/Nd ratio between Earth and chondrites probably reflects the mineralogical distribution owing to mixing processes within the inner protoplanetary disk. This observation simplifies lunar differentiation to a single stage from formation to solidification of a lunar magma ocean 3 . This also indicates that no Sm/Nd fractionation occurred between the materials that made Earth and the Moon in the Moon-forming giant impact., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The importance of highly siderophile elements (HSEs; namely, gold, iridium, osmium, palladium, platinum, rhenium, rhodium and ruthenium) in tracking the late accretion stages of planetary formation has long been recognized. However, the precise nature of the Moon's accretional history remains enigmatic. There is a substantial mismatch in the HSE budgets of the Earth and the Moon, with the Earth seeming to have accreted disproportionally more HSEs than the Moon 1 . Several scenarios have been proposed to explain this conundrum, including the delivery of HSEs to the Earth by a few big impactors 1 , the accretion of pebble-sized objects on dynamically cold orbits that enhanced the Earth's gravitational focusing factor 2 , and the 'sawtooth' impact model, with its much reduced impact flux before about 4.10 billion years ago 3 . However, most of these models assume a high impactor-retention ratio (the fraction of impactor mass retained on the target) for the Moon. Here we perform a series of impact simulations to quantify the impactor-retention ratio, followed by a Monte Carlo procedure considering a monotonically decaying impact flux 4 , to compute the impactor mass accreted into the lunar crust and mantle over their histories. We find that the average impactor-retention ratio for the Moon's entire impact history is about three times lower than previously estimated 1,3 . Our results indicate that, to match the HSE budgets of the lunar crust and mantle 5,6 , the retention of HSEs should have started 4.35 billion years ago, when most of the lunar magma ocean was solidified 7,8 . Mass accreted before this time must have lost its HSEs to the lunar core, presumably during lunar mantle crystallization 9 . The combination of a low impactor-retention ratio and a late retention of HSEs in the lunar mantle provides a realistic explanation for the apparent deficit of the Moon's late-accreted mass relative to that of the Earth.