Sulfide melt-silicate melt distribution coefficients for noble metals and other chalcophile elements as deduced from MORB: Implications for partial melting

Abstract

Analyses of chalcophile elements in coexisting sulfide and glass of basalts from the FAMOUS area of the mid-Atlantic ridge have been obtained by combined instrumental and radiochemical neutron activation analysis and directly coupled plasma spectrometry of hand-picked separates. In one sulfiderich sample, 526-1, both the glass and sulfides were analyzed for the noble metals. The sulfide meltsilicate melt Nernst partition coefficients determined for Au and Ir are 1.5–1.9 × 10 4 and 1.2–1.6 × 10 4, respectively. That for Pd is estimated to be 3.5 × 10 4. Partition coefficients for Ni are within the range 500–900, those for Co are between 26–51 and a single determination for Cu yields a value of 1383. The distribution coefficients for Ni, and possibly Cu and Co, are shown to be compositionally dependent. The data have been used to model the behavior of chalcophile elements during generation of MORB in its source region. The model assumes that all the platinum group elements, Se, Cu and S in the mantle reside in sulfides and that sulfide-saturated melt in the mantle contains 800 ppm S. Starting with a bulk upper mantle composition based on data from mantle xenoliths, 20–23% batch partial melting yields a silicate melt and a residual mantle sulfide that, with the exception of Ir, have compositions similar to sample 526-1 glass and sulfide, respectively. This implies that sulfide similar to that in 526-1 is left as a residue of partial melting to produce MORB and that primitive MORB compositions are determined by their equilibration with that sulfide. In contrast to MORB glass, the relative abundances of the platinum group elements, Se, Cu, and S in boninites closely resemble those of the sulfide that separated from MORB. This supports a model which holds that while the relative PGE abundances of boninite are controlled by mantle sulfide, no sulfide residue remains after the generation of boninite magma. That the modelling fails to account for Ir abundances supports the possibility that the distribution of Ir is controlled by a phase(s) in addition to sulfide.