Search for: All records

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.