ACTIVITIES OF NICKEL, COBALT, AND MANGANESE SILICATES IN MAGMATIC LIQUIDS AND APPLICATIONS TO OLIVINE LIQUID AND TO SILICATE METAL PARTITIONING

The activities of NiSi0.5O2, COSi0.5O2, and MnSi0.5O2 in magmatic liqu ids have been calibrated from experimentally determined distributions of Ni, Co, and Mn between olivine and silicate melt. Chemical potentia ls of these species in silicate liquids are extracted from an expressi on for the chemical potentials of minor components in olivine and are described by an extension of the regular solution model of GHIORSO and SACK (1994). The extended liquid mixing model, together with the mixi ng model for multicomponent olivine, retrieves olivine/liquid D-M(ol/l iq) for Ni, Co, and Mn with average absolute values of deviations equa l to 14%, 13%, and 14%, respectively. Partitioning of Ca between olivi ne and liquid, using previously calibrated chemical potentials of CaMg SiO4 in olivine and CaSiO3 in silicate melt, is reproduced with an ave rage relative deviation of 18%. Extrapolation of these models successf ully predicts experimentally determined partitioning of Ni and Co betw een olivine and ultramafic liquids to >2000 degrees C and 100 kbar, th ereby casting doubt on the importance of pressure-induced coordination changes to the geochemistry of Ni and Co under these conditions. Calc ulated partitioning of Ni and Co between silicate liquid and molten me tal indicates that these elements remain markedly siderophile to tempe ratures greater than 3000 K. Calculations suggest that the siderophile behavior of Ni and Co may be reduced by increasing pressure. Forward modeling of phase equilibria and Ni partitioning during crystallizatio n of mafic melts from Hawaii suggests that the Ni/MgO systematics of H awaiian basaltic glasses are consistent with a parental liquid having 17% MgO and 760 ppm Ni. Activity coefficients of NiSi0.5O2, CoSi0.5O2, and MnSi0.5O2 in natural magmas are generally greater than unity, owi ng in a large part to positive energetic interactions between transiti on metals and SiO2. The primary determining compositional variable for these liquid activity coefficients and partition coefficients is mola r silica content. The effects of changing temperature and liquid compo sition on olivine/ liquid partitioning of Ni, Co, and Mn are comparabl e and complementary, leading to more compatible behavior at lower temp eratures and in more evolved liquids. Activity coefficients are greate st for NiSi0.5O2, and smaller for CoSi0.5O2 and MnSi0.5O2. Composition al variation of mineral/liquid partitioning and of activity coefficien ts owing to interactions with SiO2 are likely to be important for many other trace elements.