| Summary: | Carbonatites are intrusive and extrusive rocks with content of carbonate minerals > 50% predominantly derived from upper mantle. They represent a potential economical source for platinum- group elements as can be seen on actively mined sites like Phalaborwa in South Africa or Ipanema in Brazil. The first complete dataset for highly siderophile element (HSE) abundances along with their 187 Os/188 Os compositions for carbonatites, silicocarbonatites and associated alkaline rocks (pyroxenite, syenite, monzogabbro and tonalite) from two Neoproterozoic (~ 800 Ma) suites from Samalpatti and Sevattur, Tamil Nadu region in south India is presented. The data were obtained by a standard methods in involving decomposition of samples in Carius Tubes, Os separation by CHCl3 following microdistillation and Ir, Ru, Pt, Pd isolation by anion exchange chromatography. The data show that carbonatites from Samalpatti and Sevattur are characterized by very low HSE contents, lower than other mantle-derived mafic melts such as basalts or komatiites. Suprachondritic OsN/IrN ratios might suggest that carbonatites are able to concentrate not negligible amount of Os. Due to high, non-uniform 187 Os/188 Os ratios, we suggest that the source of carbonatitic melts was largely heterogeneous with high contribution of crustal...
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