Distribution of Mg2+ between olivine and silicate melt,and its implications regarding melt structure

Consideration of experimental data on the distribution of Mg²⁺ between olivine and silicate liquid clearly demonstrates that the distribution coefficient (K_Mg) is dependent upon variations in temperature, pressure and melt composition, largely because these variables control the solubility of Mg²⁺ in the melt phase. Attempts to minimize composition dependence of K_Mg, utilizing various activity-composition models for silicate melts, have been partially successful. Composition-related effects do not appear to be large, however, for melts of restricted range in composition (e.g., tholeiitic or lunar basalts) as long as the contents of alkalis and the alkali/alumina ratio are relatively small (on a molar basis). For such melts, K_Mg may be used as a reliable geothermometer. By analogy, these conclusions can be extended to the distribution of other divalent cation such as Fe²⁺, Mn²⁺, Ni²⁺ and Co²⁺.