A 436 Willis and Goldstein ' Trace Element Partitioning

Abstract

The effect of the minor elements P, S and C on the distribution coefficients of It, Ge, Ga, Au, Cu, and Cr between the solid and liquid phases in Fe-Ni alloys was measured experimentally. Distribution coefficients for It, Ga, and Au increase with increasing P content while the distribution coefficients for It, Ge, and Ni increase with increasing C and S contents. For Ge, the distribution coefficient exceeds 1.0 when the P or C content of the solid exceeds 0.4 or 0.9 wt% respectively, or the S content of the liquid exceeds 7 wt%. The measured distribution coefficients were used in a solidification model of a parent body core. The model assumes fractional crystallization in which the distribution Interpretation of the elemental trends in the iron meteorites is model dependent. The simplest model is one of fractional crystallization. In this model, it is assumed that liquid metal is thoroughly mixed, that no mixing occurs in the solid, and that distribution coefficients are constants. With these assumptions, the equation relating the concentration of an element, X, is an increment of freezing solid, •, to the fraction of material solidified, f, is S o 1 Cx = Cx. x. (1) o where C. is the concentration of element X in the initial melt [Rayleigh, 1896]. The concentration coefficients vary with composition and predicts of a second element, Y, in a solidifying increthe trace element distributions in the ramjot iron ment can be related to the first by eliminating f meteorite groups. The model correctly predicts from simultaneous equations, the observed trends for P vs. Ni and Au vs. Ni in the iron meteorite groups considered. Although the calculated trends for Ir vs. Ni and for Ge vs. Ni are relatively satisfactory for groups IVA and IVB, they fail to produce the observed trends for groups IIAB and IIIAB. Interelement effects of S and C on the distribution coefficients may explain these inconsistencies.

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