Platinum-group elements in Deccan mafic rocks: a comparison of suites differentiated by Ir content

A suite of 18 Deccan mafic rocks including 14 basalts and four dolerite dykes from seven different localities in the Maharashtra region of the western Deccan volcanic province were analyzed for Ir, Pt, Pd and Au by radiochemical neutron activation analysis (Ir, Pd and Au) and lead fire assay (Pt, Pd and Au) with ICP–MS finish. To provide additional petrogenetic insight, major elements, a suite of trace elements including rare earth elements (REEs), and Sr and Nd isotopic compositions were also determined. Based on the distribution of Ir values, two subsets of samples were distinguished. One includes 14 samples with average Ir concentration of 0.024 ppb, and the other, a much smaller group of four samples, averages 0.21 ppb Ir or some 10 times the average concentration of the larger group. These two subsets are differentiated by other compositional properties in addition to noble metal contents. These include lower REE, higher transition element (Ni, Cr) and S contents, large negative Nb–Ta anomalies in mantle normalized profiles, more radiogenic Sr and very negative εNd isotopic compositions in the Ir-rich group. In addition, Pd, Pt and Au averages are lower in the Ir-rich group. geochemical features are interpreted to reflect processes of magma generation in a mantle environment and magma contamination in a crustal or lithospheric mantle setting. The critical mantle variable is percentage partial melting of source peridotite. Relatively higher percentage partial melting is required to generate the Ir-rich magmas. Based mainly on Sr and Nd isotopic evidence and Nb–Ta abundances, the Ir-rich group is considered more strongly modified by crustal and/or lithospheric mantle assimilation than the main group of melts. onal crystallization appears to have increased Pd, Au and Cu contents of the main group of magmas relative to the Ir-rich magmas, and is probably a significant factor distinguishing these Deccan rocks as having higher concentrations of these elements than other continental flood basalts (CFBs).