Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle

Abstract

The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt, and Pd were determined in 18 mantle-derived basalts from a variety of tectonic settings and six komatiites from three locations. All analyses were performed using isotope dilution, Carius tube digestion, and the precise technique of multiple collector inductively coupled plasma mass spectrometry. Multiple analyses of two samples indicate external reproducibilities, based upon separate dissolutions, of approximately 2–9% in the ppt to ppb concentration range. Mid-ocean ridge basalts from the Kolbeinsey Ridge, tholeiites from Iceland and alkali basalts from the Cameroon Line define three individual sample suites that are characterized by distinct major, trace, and platinum-group element systematics. All three-sample suites display correlations of the PGE with MgO, Ni, and Cr. The new analytical results are employed to constrain the geochemical behavior of the PGE during the formation and differentiation of mantle–derived melts. The PGE are inferred to be compatible in sulfides during partial melting with sulfide-silicate melt partition coefficients of ∼1 × 10 4. The fractionated PGE patterns of mantle melts are a consequence of the incompatibility of Pd in nonsulfide phases, whereas Ir and Ru must be compatible in at least one other mantle phase. Model calculations indicate that PGE alloys or spinel may be responsible for the higher compatibility of the latter elements during partial melting. It is further demonstrated that the shape of the melting regime has a profound effect on the PGE systematics of mantle magmas. The systematic trends of the three sample suites in plots of PGE against Ni and Cr are the result of magma differentiation processes that involve fractional crystallization of silicate minerals and the concurrent segregation of an immiscible sulfide liquid. The behavior of the PGE during magma fractionation indicates that the segregated sulfides probably equilibrate with >90% of the silicate magma and that PGE scavenging by sulfides is best described by a combination of batch and fractional equilibrium partitioning.