Thermodynamic analysis of partitioning of Ca, Fe and Mg between olivine and clinopyroxene

Abstract

Calibrations were achieved using thermodynamic data on the Fe-Mg partitioning between olivine and clinopyroxene (ΔG10, W(FeMg)(Ol) and W(FeMg)(Cpx); Kawasaki and Ito, 1994), where the Fe-Mg interactions in M1 and M2 sites were assumed to be equal to each other for both olivine and clinopyroxene (W(FeMg)(Ol, M1) = W(FeMg)(Ol, M2) = W(FeMg)(Ol) and W(FeMg)(Cpx, M1) = W(FeMg)(Cpx, M2) = W(FeMg)(Cpx). The free energy change of reaction at the standard state for the Mg-Ca partitioning between olivine and clinopyroxene, ΔG20, was estimated from Adams and Bishop's (1982) data. Volume changes of Reactions (3) and (4) (ΔV30) and ΔV40) were calculated from unit cell volume data of olivine (Akimoto and Fujisawa, 1968; Mukhopadhyay and Lindsley, 1983; Adams and Bishop, 1985) and clinopyroxene (Newton et al., 1979; Hugh-Jones et al., 1994). Mixing parameters of Ca-Fe and Mg-Ca olivines were evaluated from solvi of kirschsteinite-fayalite (W(CaFe)(Ol, M2); Mukhopadhyay and Lindsley, 1983) and forsterite-monticellite (W(Fo) and W(Mo); Adams and Bishop, 1985). Asymmetric parameters W(En) and W(Di) were estimated from calorimetric data on CaMgSi2O6-Mg2Si2O6 clinopyroxene (Newton et al., 1979). Kawasaki's (1998) model is available for the Fe-Mg site occupancy of olivine. The effect of Ca2+ on the Fe-Mg occupancy in clinopyroxene (McCallister et al., 1976) was empirically formulated. The present results show that the Ca-Fe-Mg distribution between olivine and clinopyroxene is highly sensitive to compositional variation and is temperature-dependent. The Ca partition coefficient, D(Ca) (=X(Ca)(Ol, M2)/ X(Ca)(Cpx, M2)), reaches a maximum at intermediate values of Fe/(Fe+Mg). The position of the maximum D(Ca) shifts towards the Mg-rich side with increasing temperatures. The CaO content of clinopyroxene significantly affects K(D) [=(X(Mg)(Ol)X(Fe)(Cpx))/(X(Fe)(Ol)X(Mg)(Cpx))]. The K(D) increases drastically for a slight decrease in CaO content of clinopyroxene in the Mg-rich system at low temperatures, whereas a small increase in K(D) is found in the Fe-rich system. At high temperatures, K(D) changes only slightly when X(Ca) decreases in the system.