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

抄録

Experimental data have been analyzed thermodynamically over temperature and pressure ranges from 900 to 1550°C and 0.8 to 7.5 GPa, using a two-site regular solution model for the equilibria between olivine and clinopyroxene in the CaO-FeO-MgO-SiO₂ system: (1)CaMgSi₂O₆+1/2FeFeSiO₄=CaFeSi₂O₆+1/2 MgMgSiO₄CpxOlCpxOl(2)CaMgSi₂O₆+MgMgSiO₄=MgMgSi₂O₆+CaMgSiO₄CpxOlCpxOl(3)CaFeSi₂O₆+FeFeSiO₄=FeFeSi₂O₆+CaFeSiO₄CpxOlCpxOl(4)1/2MgMgSi₂O₆+1/2FeFeSiO₄=1/2FeFeSi₂O₆+1/2MgMgSiO₄CpxOlCpxOl Calibrations were achieved using thermodynamic data on the Fe-Mg partitioning between olivine and clinopyroxene (ΔG_l^o, 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 a ual to each other for both olivine and clinoroxene (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, ΔG₂^o, was estimated from Adams and Bishop's (1982) data. Volume changes of Reactions (3) and (4) (ΔV₃^o and ΔV₄^o) 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^OlX_Fe^Cpx)/(X_Fe^OlX_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.