ON THE CHALCOPHILE CHARACTER OF MOLYBDENUM - DETERMINATION OF SULFIDE SILICATE PARTITION-COEFFICIENTS OF MO AND W

Partition coefficients for Mo and W between Fe-S liquids and silicate melt D(sul/sil) were experimentally determined at 1200-degrees-C. The composition of silicates was basaltic while sulfide compositions were variable, ranging from FeS saturated with metallic Fe to sulfides free of Fe metal. Both Mo and W partition coefficients show a systematic decrease with decreasing Fe content of sulfides, varying from about 200 at Fe70S30 to 60 at Fe50S50 for Mo and similarly from 3 to 1.5 for W. A dramatic decrease in both partition coefficients was found for systems with excess S. Values as low as 3 for Mo and 0.3 for W were measured. Experiments with nickel-rich sulfides yield similarly low or even lower partition coefficients. Thermodynamic calculations indicate that the activity coefficient of MoS2 in sulfide strongly increases with decreasing Fe content of the sulfides. From the experimental partition coefficients, activity coefficients for Mo oxide in silicate were calculated. By assuming MoO3 as stable species in molten silicate an activity coefficient gamma-MoO3 of 0.0003 is obtained, compared to 30 for MoO2 which is the more likely stable Mo-species in molten silicate. Molybdenum has been previously used as "Leitelement" (index element) for sulfide separation during geological processes. However, the small Mo partition coefficients for S-rich systems imply that sulfide removal under upper-mantle conditions (or in more oxidized crustal reservoirs) will produce only small Mo depletions in the residual silicate magma. Therefore Mo abundances are not indicative for sulfide fractionation. During planetary differentiation the behaviour of Mo is determined by its siderophile character. The Mo/W ratio in a planetary mantle is not a good indicator of sulfide fractionation, but instead reflects metal segregation.