Controls on PGE fractionation in the Emeishan picrites and basalts: Constraints from integrated lithophile–siderophile elements and Sr–Nd isotopes

Picrites from the Emeishan large igneous province (ELIP) may be divided into those that show LREE enrichment and those that are characterized by LREE depletion. All picrite types have low 87Sr/86Sr ratios (<0.7057) but variable εNd, suggesting that crustal contamination has been negligible, and source mantle variation is responsible for their chemical diversity. In contrast, coeval basalts show strong signatures of crustal contamination. Calculated mantle potential temperatures are 240 °C higher than that in ambient mantle, supporting a plume origin for the province. A depleted plume source and ancient sub-continental lithospheric mantle are inferred to be the most important end-members involved in the generation of the Emeishan picrites. Pt and Pd values of the picrites show no evidence of depletion; Pt/Pd ratios are highly variable and show no correlation with picrite type. PGE analyses of both picrites and coeval basalts confirm that Os, Ir, and Ru (IPGE) have behaved compatibly and Pt, Pd, and Rh have behaved incompatibly during Cr-spinel and olivine crystallization. PGE-depleted basalts are present in the ELIP, but the most fractionated basalts tend not to show significant depletion in Pt and Pd. This suggests that fractional crystallization alone was not sufficient to induce sulfide saturation in the magmas, and that contamination involving crustal sulfur was an important driving mechanism. Significant depletion of Ir relative to Ru in some of the basalts regardless of the sulfide saturation state of the parental magma suggests that Ir-dominant alloys segregated prior to magma eruption. Depletions of IPGE relative to Pt and Pd in the Emeishan picrites are consistent with the notion that IPGE alloys are stable in the mantle during partial melting.