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Subsolidus phase relations in the system MgO-ZnO-SiO2 and Mg-Zn partitioning between olivine, pyroxene and spinel

J.-P. Li1, H.StC. O’Neill and J. Kornprobst2
1
Visiting fellow (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China)
2 Departement des Sciences de la Terre, Universite Blasie Pascal, France

Although zinc is often a chalcophile element in the Earth’s crust, under upper mantle conditions, Zn occurs in oxides and silicates. For example, the Zn content in mantle clinopyroxene and garnet may reach 150 ppm; that in mantle spinel, 400-1300 ppm. The concentration of Zn into spinel reflects the unusual preference of the Zn2+ cation for tetahedral co-ordination, compared with other M2+ cations of similar ionic radius (Mg2+, Fe2+, Ni2+, etc). The difference in crystal-chemical behaviour suggests that the partitioning of Zn and Mg between spinel and other phases in which the M2+ cations are in octahedral co-ordination may prove useful for geothermometry and geobarometry. Therefore, experiments have been carried out in the systems MgO-ZnO-SiO2 and MgO-ZnO-SiO2-Cr2O3 at temperature of 1100-1550°C and pressure of 5-40 kb in a piston-cylinder apparatus.

The subsolidus phase relations in the system MgO-ZnO-SiO2 at 10 kb and 1200°C has been determined (Figure 1).

 

Figure 1: Subsolidus phase relations in the system MgO-ZnO-SiO2 at 10 kb and 1200°C. Filled circles represent the starting compositions.

The Mg-Zn distribution coefficient (Kd) between Ol, Px and Sp have been experimentally measured as a function of temperature and pressure. The results show that (1) Kdol-sp and Kdpx-sp decrease and Kdol-px increases slightly with increasing temperature; (2) the pressure effect on these Kd is small at pressure 0-40 kb. The results confirm that Zn partitions strongly in spinel relative to olivine and pyroxene, but there is little preference between olivine and pyroxene at mantle conditions. The compositional effect on the Mg-Zn partitioning between mantle olivine, pyroxene and spinel is being determined and the thermodynamic properties of Mg-Zn olivine, Mg-Zn pyroxene and Mg-Zn spinel will be obtained.