Effect of pressure on cation partitioning between immiscible liquids in the system TiO2-SiO2

Abstract

Liquidus phase relations in the system Si02-TiO2 have been determined at 3.0 GPa to investigate the effect of pressure on the partitioning of Si and Ti between coexisting immiscible liquids. Experiments on oxide mixtures containing 10, 50, 90, and 100 mol% TiO2 were performed in multi-anvil and piston-cylinder apparatus at 2173–2740 K. At 1 atm, immiscible liquids form because the liquid structures are dissimilar: tetravalent cations occur in primarily fourfold (Si02-rich liquid) vs. sixfold (TiO2-rich liquid) coordination by oxygen. At 3.0 GPa, immiscible liquids also coexist, but the phase relations and liquid compositions are significantly different. The eutectic liquid is now Ti02-rich (78.4 vs. 8.1 mol% TiO2 at 1 atm). Above the liquidus, the coexisting immiscible liquids are enriched in Si02. The Si02 content of the TiO2-rich liquid has increased by more than a factor of 3, while the TiO2-content of the SiO2-rich liquid has decreased by 12. The persistence of a wide solvus implies that pressure does not cause liquid structure assimilation. The observed shift in immiscible liquid compositions indicated that pressure has raised the activity of TiO2 in the liquid dominated by tetravalent cations in fourfold coordination and lowered the activity of SiO2 in a liquid dominated by tetravalent cations in sixfold coordination. The decrease in TiO2 content of the SiO2-rich liquid with pressure is probably related to the positive volume of mixing observed in 1 atm glasses in this composition range. The enhanced solubility of SiO2 in the TiO2-rich liquid with pressure may be due to a pressure-induced increase in the coordination state of Si.