KINETIC CONTROLS ON THE PARTITIONING OF TRACE ELEMENTS BETWEEN SILICATE AND SULFIDE LIQUIDS

Abstract

Mass transfer between sulfide and silicate liquids is often discussed by reference to a mass ratio between hypothetical liquid reservoirs at equilibrium, defined as R. In an open system not at equilibrium the mass of silicate melt from which a growing sulfide droplet appears to have scavenged chalcophile elements will depend on diffusivities D-l and D-Fs of the element and of the FeS component in the silicate melt. respectively, on the degree of supersaturation of the melt with FeS C-Fes on the radius a of the droplet and on the velocity v with which the droplet is advected through the melt. so that R = 1/C-Fes (D-l + 1/2 root aDy/D-Fes + 1/2 root aD(Fes)V). Chalcophile elements are expected to shot,, inter-element variations in D spanning several orders if magnitude in some melts, leading to large inter-element variations in effective R in a given sulfide body. These variations in effective R will lead to spatial separations between zones of maximum enrichment of chalcophile elements produced during fractional segregation of sulfide in layered intrusions. Whereas it has proven impossible to model the generation of spatially offset stratiform horizons in the Australian Munni Munni layered intrusion using equilibrium partition coefficients, the present kinetic model reproduces the observed patterns with great fidelity.