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The siderophile elements, which include Re, Pt, Os, and W, directly constrain the accretionary history of Earth. The largely chondritic 186,187Os/188Os ratios of Earth’s mantle, coupled with excesses in siderophile element abundances, provide nearly incontrovertible evidence that some meteoritic addition continued after core formation was complete. Osmium and W isotope systematics of plume-derived mafic-ultramafic rocks reveal the complex chemical evolution of their deep mantle sources. In the upper mantle, Re-Os dating of whole-rock xenoliths and sulfide inclusions in diamonds hosted by kimberlites indicate both ancient melt depletion and subsequent modification of the mantle lithosphere beneath the earliest continents, with Re-Os ages of eclogitic diamonds possibly recording the transition to a sustained plate tectonic regime on Earth. 

In order to gain further insights into the origin of platinum-group minerals (PGM) and the source of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) in the chromite deposits associated with the Urals ophiolitetype massifs, we carried out a mineralogical, HSE abundance, and Pt-Re-Os isotope study of chromitites and PGM from the Verkh-Neivinsk and Alapaevsk dunite-harzburgite massifs in the Middle Urals. The chromitites are characterized by negatively-sloped Bulk Silicate Earth (BSE)-normalized HSE patterns, consistent with the predominance of PGM of the Ir-group platinum-group elements (PGE), i.e., Os-Ru-Ir alloys and Ru–Os sulfides, over the PGM of the Pd-group PGE, i.e., stibiopalladinite and geversite. These two groups of PGM are interpreted to represent the primary and secondary mineral assemblages, respectively. The observed HSE patterns in the studied chromitites are typical of those formed in supra-subduction zone (SSZ) settings. The near-chondritic average initial γ187Os and μ186Os values in the chromitites and PGM of the Verkh-Neivinsk massif indicate that its HSE budget was derived from the convecting upper mantle source that evolved with time-integrated nearchondritic Re/Os and Pt/Os ratios. These features are also typical of the sources of most Archean and Proterozoic mafic-ultramafic rocks worldwide. In contrast to the Verkh-Neivinsk massif rocks, the Alapaevsk massif chromitites show radiogenic initial γ187Os values indicating evolution of the mantle source of these rocks with a supra-chondritic time-integrated Re/Os ratio. This long-term enrichment in Re relative to Os could be the result of interaction of the source peridotites with 187Os-enriched melts derived from ancient recycled oceanic crust. 

In order to further evaluate the timing and possible mechanisms responsible for the transition from both positive and negative to no 142Nd and 182W anomalies in the Archean mantle, we obtained 142,143Nd, 176Hf, 186,187Os, 182W isotope and lithophile trace and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data for mantle plume-derived ~2.0 Ga picrites and an associated differentiated mafic–ultramafic sill from the Onega Basin on the Fennoscandian Shield. The Onega Basin picrites share striking chemical similarities with the modern Kilauea picrites, featuring enrichments in light rare earth elements (LREE; La/SmN = 1.5 ± 0.2), depletions in heavy rare earth elements (HREE; Gd/YbN = 2.2 ± 0.1), positive high field strength element (HFSE) anomalies (Hf/Hf* = 1.2 ± 0.1, Nb/Nb* = 1.6 ± 0.1), and modern bulk silicate Earth (BSE)-like average W/Th = 0.20 ± 0.08 (2SD). Model calculations indicate that the parental picritic magmas were likely derived from 3 % equilibrium batch melting of a LREE-depleted garnet lherzolite PREMA-type mantle source containing a component of recycled oceanic crust. The 147Sm-143Nd, 176Lu-176Hf, and 187Re-187Os mineral-whole-rock isochron ages constrain precisely the timing of the Onega Basin lava emplacement at ~1974 Ma. The corresponding initial ε143Nd = +3.0 ± 0.5 and ε176Hf = +2.8 ± 1.2 values indicate evolution of the Onega mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The lower ε176Hf relative to ε143Nd further implies decoupling of the two lithophile element isotope systems in the source. The initial μ186Os and γ187Os values are suprachondritic at +4.9 ± 2.1 and +2.9 ± 0.4, respectively, indicating evolution of the Onega mantle plume source with time-integrated slightly suprachondritic Pt/Os and Re/Os ratios. The μ142Nd = –1.1 ± 3.3 (2SD) and μ182W = 0.0 ± 4.8 (2SD) obtained for the Onega picritic magmas are unresolvable from the modern BSE values, implying that their mantle source had 142Nd and 182W compositions similar to those of the BSE. When considered together, the trace element systematics, suprachondritic Pt/Os and Re/Os ratios, and Hf-Nd isotopic decoupling are best explained in terms of incorporation into the Onega mantle plume source of 10–20 % recycled komatiite-basalt crust aged in the mantle for 1 to 2 Ga. These results provide new evidence that 142Nd and 182W anomalies that were common in the Archean mantle were effectively homogenized by 2.0 Ga ago on the scale of the mantle domains sampled by the Onega Basin magmas. This may have occurred due to the enhanced convective whole-mantle mass and heat transfer facilitated by processes of oceanic crust recycling that may have operated since at least the early Archean. 

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10-15% higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable 182W values ranging from modern bulk silicate Earth-like to strongly negative values. The 182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest 182W value (-53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/RA). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative 182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary.