The distribution of Fe and Mg between olivine and lunar basaltic liquids

We have examined the Fe and Mg distribution between coexisting olivine and lunar basaltic liquids produced by equilibrium partial melting of natural lunar samples. In agreement with the findings of Roeder and Emslie (Contrib. Mineral. Petrol. 29, 275–289) on terrestrial compositions, the logarithms of the conventional distribution coefficients, K_ol-L^Fe and K_ol-L^Fe-Mg, are nearly linear functions of inverse temperature; and the exchange coefficient, K_D = K_ol-L^Fe-Mg, is nearly independent of temperature and composition within a given magma group. There are, however, small but significant differences in conventional and exchange distribution coefficients from one magma group to another, e.g. low-Ti vs high-Ti lunar basalts. It is possible to achieve slightly greater precision for the inverse temperature functions by including terms approximating silica activity in the conventional distribution coefficients. The term ($\text{2Si}\text{O}\text{)}{}_{\mathrm{L}}$ is apparently the best simple approximation for silica activity in olivine-saturated liquids based upon data for Fe, Mg, Mn, Ca, Ti and Cr. Pressure has noticeable effects upon Fe and Mg distribution between olivine and liquid only above 5 kbar.The excellent linear correlation of the logarithms of the distribution coefficients with inverse temperature allows calculation of approximate values of $\text{Δ}\text{H}\text{?}{}^0$ for the reactions : 2MgO_L + SiO_2LaiMg₂SiO_4ol and 2FeO_L + SiO_2LaiFe₂SiO_4ol. Values obtained, approx ?26 kcal/mole, are comparable with values of the heats of fusion of forsterite and fayalite calculated by Bradley (Am. J. Sci.260, 550–554) and measured by Orr (J. Am. Chem. Soc. 75, 528–529).The exchange distribution coefficient for Fe and Mg, K_D, is sensitive to large changes in liquid chemistry. Although K_D is explicitly independent of silica activity, K_D apparently changes with silica concentration. This change is a reflection of changes in the mixing properties of Fe and Mg in liquids with different chemistry and hence structure. Regular solution theory predicts that as the mixing properties of an element in a solution change, the most radical changes in activity coefficients occur in the range of dilute concentrations. Therefore, the distribution coefficients for trace elements will also be dependent upon large changes in liquid chemistry, even if corrections for silica and other liquid component activities are applied.