Abundances and significance of platinum group elements in carbonatites from China

The concentrations of the platinum group elements (PGEs: Ir, Ru, Rh, Pt, Pd) were determined in 29 extrusive and intrusive carbonatites from 7 locations in China. The samples were analyzed using an improved Carius tube with high temperature and high pressure. All samples are characterized by low total PGE contents (0.07-2.5 ppb), and only trace magmatic sulfides are observed in thin sections. The carbonatites may reach sulfur saturation because they are known to be derived by small degrees of partial melting (<1%). The PGEs may be compatible in sulfides with very high sulfide-carbonatite melt partition coefficients. Segregation of an immiscible sulfide melt can result in the loss of PGEs in the carbonatites. Compared with the intrusive and silicate-poor extrusive carbonatites, the silicate-rich extrusive carbonatite contains relatively abundant clinopyroxene and titanomagnetite and higher IPGE (it and Ru) contents. This implies that IPGEs may be compatible in these mineral phases. The low IPGE abundances and fractionated patterns in the intrusive and silicate-poor extrusive carbonatites reflect that they may have experienced significant fractionation of sulfides and IPGE-rich mafic silicates and oxides before final emplacement. The high PGE compositions in the silicate-rich extrusive carbonatite may represent mantle materials entrained by the rising carbonatite magma. (C) 2008 Elsevier B.V. All rights reserved.