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

Toshisuke Kawasaki
Geochemical Journal, Vol. 33, No. 1, P. 33-58, 1999

ABSTRACT

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 (ΔGlo, WFeMgOl and WFeMgCpx; 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 (WFeMgOl, M1=WFeMgOl, M2=WFeMgOl and WFeMgCpx, M1=WFeMgCpx, M2=WFeMgCpx). The free energy change of reaction at the standard state for the Mg-Ca partitioning between olivine and clinopyroxene, ΔG2o, was estimated from Adams and Bishop's (1982) data. Volume changes of Reactions (3) and (4) (ΔV3o and ΔV4o) 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 (WCaFeOl, M2; Mukhopadhyay and Lindsley, 1983) and forsterite-monticellite (WFo and WMo; Adams and Bishop, 1985). Asymmetric parameters WEn and WDi 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, DCa (=XCaOl, M2/XCaCpx, M2) reaches a maximum at intermediate values of Fe/(Fe+Mg). The position of the maximum DCa shifts towards the Mg-rich side with increasing temperatures. The CaO content of clinopyroxene significantly affects KD [=(XMgOlXFeCpx)/(XFeOlXMgCpx)]. The KD increases drastically for a slight decrease in CaO content of clinopyroxene in the Mg-rich system at low temperatures, whereas a small increase in KD is found in the Fe-rich system. At high temperatures, KD changes only slightly when XCa decreases in the system.

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