Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province, SW China

Abstract

The 260 Ma Emeishan continental flood basalts (ECFB) occur in SW China and northern Vietnam. A 550-m-thick sequence of ECFB in Guizhou Province (SW China) is composed of 12 basaltic lava flows, each ranging from 10 to 140 m in thickness. All samples belong to the high-Ti basalts (TiO 2 = 1.9-4.4 wt.% and Ti/Y = 400-700). Relative to the upper flows (Flows 9-12), those from the lower part of the section (Flows 1-8) are depleted in Nb, Ta, P and Sr and enriched in Zr, La and Th, consistent with higher degree of crustal contamination. Overall, the concentrations of Os, Ir and Ru range from 0.01 to 0.23 ppb and are low relative to those of Rh, Pt and Pd, which range from 0.2 to 1.1, 6 to 17 and 5 to 18 ppb, respectively. Different lava flows have different primitive mantle-normalized PGE patterns, and samples from the same flow show similar patterns. Fractionations between PPGE and IPGE are mainly controlled by olivine and chromite fractionation. The negative correlations between Pd/Ir and Pt/Pd, and Ni/Pd and Pd/Cr are consistent with crystal fractionation. Flows 6-8 and 10-12 show obvious negative Ru anomalies, explained by fractionation of laurite (Ru, Os, Ir)S 2 or Os-Ir-Ru alloys in the primary magma, or by coprecipitation of laurite with chromite or olivine. Flows 9-12 are Os-poor and have higher initial 187Os/ 188Os ratios than Flows 1-8, indicating it is easy to alter the Os isotopic compositions of basaltic magmas with lower Os concentrations. © 2007 Elsevier B.V. All rights reserved.