SiO₂ solubility in rutile at high temperature and high pressure

Abstract

Silicon-bearing rutile has been found in chromitite from the Luobusa (罗布莎) ophiolite, Tibet. However, the extent of SiO₂ solubility in rutile and the nature of its origin are still unclear. At high pressure, SiO₂ takes a rutile structure with Si in 6-fold coordination. Thus, high pressures may enhance its solubility in rutile because of possible isovalent exchange in the octahedral site. In this study, we report new experimental results on SiO₂ solubility in rutile up to 23 GPa and 2 000 °C. Starting materials were mixtures of powdered pure rutile and pure quartz, with compositions of (Ti_0.5Si_0.5)O₂, (Ti_0.93Si_0.07)O₂, and (Ti_0.75Si_0.25)O₂. The mixtures were loaded into either platinum capsules (for a 10/5 assembly) or rhenium capsules (for an 8/3 assembly). The experiments were carried out using multi-anvil high-pressure apparatus with a rhenium resistance heater. Sample temperatures were measured with a W5%Re-W26%Re thermocouple and were controlled within ±1 °C of the set temperature. TiO₂-rich and SiO₂-rich phases were produced in all the quenched samples. Microprobe analyses of the phases show that the solubility of SiO₂ in rutile increases with increasing pressure, from 1.5 wt.% SiO₂ at 10 GPa to 3.8 wt.% SiO₂ at 23 GPa at a temperature of 1 800 °C. The solubility also increases with increasing temperature from 0.5 wt.% SiO₂ at 1 500° to 4.5 wt.% SiO₂ at 2 000° at a pressure of 18 GPa. On the other hand, the solubility of TiO₂ in coesite or stishovite is very limited, with an average of 0.6 wt.% TiO₂ over the experimental P-T ranges. Temperature has a much larger effect on the solubility of SiO₂ in rutile than pressure. At high pressure, the melting point of SiO₂ is definitely higher than that of TiO₂ and the eutectic point moves towards SiO₂ in the TiO₂-SiO₂ system. Lower oxygen fugacity decreases the solubility of SiO₂ in rutile, whereas water has little effect on the solubility. Our experimental data are extremely useful for determining the depth of origin of the SiO₂-bearing rutile found in nature.

Keywords

rutile / stishovite / coesite / polymorph / high temperature and high pressure / experiment

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