MSA ROEBLING MEDAL LECTURE: A SIMPLE MODEL FOR TRACE ELEMENT PARTITIONING INTO MAGMATIC SULFIDES

We have measured the partitioning of a large number of elements Cu, In, Tl, Pb, Ag, Zn, Cr, Co, Ni, Sb, Mn, Ti, Nb, Ta and Cd between FeS-rich sulfide liquids and anhydrous basaltic melts at 1.5GPa and temperatures of 1300-1700 C. It is not strictly necessary to measure or control oxygen and sulfur fugacities because there are simple relationships between the FeO contents of the silicate melts and the sulfide-silicate partition coefficients for the individual trace elements. These relationships can be represented as follows:

log D_M = A - 0.5nlog[FeO] (1)

where A is a constant related to the free energy of Fe-M exchange, n is related to the valency of M and [FeO] is the FeO content (strictly FeO activity) of the silicate melt. We find that the dilution of FeS in the sulfide by Ni and Cu, common in nature, can be accounted for by a simple correction.

Empirically n is close to the valency for most elements, but there are marked deviations for strongly lithophile (e.g Mn, Nb, Ti) and strongly chalcophile (e.g Cu, Ag) elements. On plots of logD vs log[FeO] the straight line relationship becomes a curve and in the cases of the most lithophile elements (e.g. Nb) becomes a “U”-shape. The reason is that at very low FeO contents of silicate melt the sulfide is oxygen-free and partition coefficient of M declines with increasing FeO with approximately the “correct” slope of valence n. As FeO concentration of the silicate melt increases, however, the oxygen content of the sulfide increases and strongly lithophile elements start to partition more readily into this phase. At high oxygen contents of sulfide (high FeO contents of the silicate) the interactions of lithophile elements with oxygen dominate and D into sulfide begins to increasewith increasing FeO. Hence the “U”-shape. The oxygen-effect can be accounted for most elegantly with a 1-parameter extension to equation (1).

We have quantified the effects of temperature and silicate melt composition on partitioning for most elements. This enables us to determine the compositions of sulfides in different magmatic settings and to quantify the amount of sulfide precipitated in, for example MORB differentiation.