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-SiO
2 system: (1)CaMgSi
2O
6+1/2FeFeSiO
4=CaFeSi
2O
6+1/2 MgMgSiO
4CpxOlCpxOl(2)CaMgSi
2O
6+MgMgSiO
4=MgMgSi
2O
6+CaMgSiO
4CpxOlCpxOl(3)CaFeSi
2O
6+FeFeSiO
4=FeFeSi
2O
6+CaFeSiO
4CpxOlCpxOl(4)1/2MgMgSi
2O
6+1/2FeFeSiO
4=1/2FeFeSi
2O
6+1/2MgMgSiO
4CpxOlCpxOl Calibrations were achieved using thermodynamic data on the Fe-Mg partitioning between olivine and clinopyroxene (Δ
Glo,
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, Δ
G2o, was estimated from Adams and Bishop's (1982) data. Volume changes of Reactions (3) and (4) (Δ
V3o and Δ
V4o) 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 CaMgSi
2O
6-Mg
2Si
2O
6 clinopyroxene (Newton
et al., 1979). Kawasaki's (1998) model is available for the Fe-Mg site occupancy of olivine. The effect of Ca
2+ 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.
View full abstract