Compositional Dependence of the Solubility of Ni, W, and Mo in Silicate Melts: the effects of alkalies

A number of recent investigations have refocused attention on the observation that trace element partitioning between metallic and silicate phases or melts is not only dependent on oxygen fugacity, temperature, pressure, and volatile fugacities, but also on melt composition (Hillgren et al., 1994; Walter and Thibault, 1995; Jana and Walker, 1997; Ertel et al., 1997). To further address this issue, Ertel et al. (1997) studied explicitly the effect of melt composition on the solubility of Ni in haplobasaltic melt (anorthite-diopside 1 atm eutectic) applying the mechanically assisted equilibration technique of Dingwell et al. (1994). By keeping all other experimental conditions constant, Ertel et al. (1997) were able to demonstrate that the addition of quartz and forsterite components had no influence on Ni solubility in the silicate melt, whereas Na-metasilicate component generated an unexpected complex influence on Ni solubility that can be explained by considering the structural role of Ni in melts. This observation suggested that alkali elements might have an important influence on the partitioning behaviour of Ni in silicate melts. To obtain a more fundamental and broad data base for future predictions of metal/silicate partitioning Fortenfant et al. (1998) investigated the influence of other meta-silicate components (Li2SiO3, K2SiO3, and CaSiO3) on the partitioning of Ni. The main result of this study was that additions of Li2SiO3, K2SiO3, and CaSiO3 had a similar initial decreasing effect on the Ni solubility as was observed for Na2SiO3 in previous studies (Ertel et al., 1997). Comparison of the trends observed for the alkali metasilicates lead to the interpretation that the relative stability of network-stabilizing alkali aluminate complexes is K > Na > Li. This observation is consistent with a number of other sources of structural and chemical property data, and correlates well with NBO/T. In continuation of these studies similar experiments were performed for W and Mo for Na2SiO3 (and other metasilicate components are in preparation). For W and Mo, this initial decrease of the equilibrium solubilities determined for Ni was not observed. W and Mo solubilities increase with increasing amounts of Na2SiO3 added starting from pure anorthite-diopside. We infer that this is closely related to the fact that W and Mo are present in higher oxidation states (W: 6+; Mo: 4+ and 6+) than Ni (which exists mainly in the bivalent oxidation state) - and in consequence, differing melt structural properties and behaviour. Therefore, melt structure and thermodynamics are controlling the partitioning behaviour of highly charged MSE in a more sophisticated manner than can be accommodated by the NBO/T factor.