PARTITIONING OF SIDEROPHILE ELEMENTS (W, MO, AS, AG, GE, GA, AND SN) AND SI IN THE FE-S SYSTEM AND THEIR FRACTIONATION IN IRON METEORITES

Abstract

The partitioning of some moderately siderophile elements and Si between solid metal and liquid metal-sulfide in the Fe-S system at 1000 to 1400°C and low pressure has been investigated at minor abundances using EPMA, and the effect of S content of the liquid on molar partition coefficient [D(x)] quantified by linear regression. The parameters (C) and (β) for the Jones and Malvin (1990) equation ln D(x) = -β ln(1-2αXS) + C are: W 0.14(26), 2.07(20); Mo -0.52(53), 1.25(44); As -1.30(14), 1.19(15); Ag -2.05(12), -1.77(9); Ge -0.57(8), 1.79(5); Ga -0.18(6), 1.40(6); Sn -1.82(8), 0.40(5), and Si -0.78(41), 2.57(22), respectively, where α = 1.05 and 1.09 for Fe-S and Fe-Ni-S systems, respectively, and XS is the mole fraction of S in the liquid; the parameters for W, Ge and Ga are based on this study and literature results. The fractionation patterns for Ir, W, Au, As, Ge, and Ga in group IIIAB iron meteorites are reproduced quantitatively using a non-equilibrium batch crystallization model with 4 mol% crystallization steps. The Ge fit required correction for the effect of P on D(Ge). The modeled batch size is interpreted to relate to the molar proportion of the crystallized IIIAB asteroidal core that maintained communication with the metal-sulfide liquid. This chemical model is consistent with core evolution by either inward or outward concentric crystallization.