Rutile/TiO2II Phase Transition Redux
Abstract
There has been considerable disagreement over the high pressure phase diagram for TiO2 (e.g. Akaogi et al., 1992; Olsen et al., 1999). Olsen et al. (1999) used experimental observation of high pressure phase transformations in TiO2 to infer a curved phase boundary between rutile and \alpha-PbO_{2} structured titania (TiO_{2}II), suggesting that TiO_{2}II is restricted to pressures greater than 6 GPa. The rutile/TiO_{2}II phase diagram has subsequently been used to infer the P, T conditions for ultrahigh-pressure metamorphism of diamondiferous gneiss, after the discovery of submicroscopic TiO_{2}II lamellae found in rutile inclusions from garnets (Hwang et al., 2000). We performed phase equilibrium experiments in the multi anvil apparatus to bracket the phase boundary at 700, 1000 and 1200\degC. In addition, growth of TiO_{2}II was measured in experiments at 500 and 600\degC, although at these temperatures growth of rutile was too slow to bracket unambiguously the equilibrium boundary. The starting materials were 50% rutile and 50% TiO_{2}II, and most experiments were fluxed by water. Dry synthesis experiments at 1200\degC were consistent with the phase boundary determined in the fluid-bearing experiments, which suggests that the equilibrium is unaffected by the presence of water. Our bracket of the phase boundary at 700\degC is consistent with the reversed, dry experiments of Akaogi et al. (1992). The new data suggest that the phase boundary is linear, at least between 600 and 1200\degC, in agreement with Akaogi et al. (1992), but in contrast to recent suggestions of Olsen et al. (1999). Growth of the high pressure phase in experiments at 4.3-5.6 GPa and 500\degC show that TiO_{2}II is not restricted to pressures greater than 6 GPa. The results of Olsen et al. (1999) can be explained in terms of kinetics of phase transitions. The thermobarometric implication of TiO_{2}$II in garnet needs reevaluation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.T22E..12W
- Keywords:
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- 3630 Experimental mineralogy and petrology