A method for estimating the activity of titania in magmatic liquids from the compositions of coexisting rhombohedral and cubic iron-titanium oxides

摘要

A method is described for estimating the activity of titania (TiO_2 in a magmatic liquid from the compositions of coexisting cubic oxide (spinel) and rhombohedral oxide (ihnenite). These estimates are derived from the thermody-namic models of Ghiorso and Evans (Am J Sci 308: 957-1039, 2008; see also Sack and Ghiorso in Contrib Mineral Petrol 106:474-505, 1991a; Am Mineral 76:827-847, 1991b) and may be computed self consistently along with temperature and oxygen fugacity for an assumed pressure. The method is applied to a collection of 729 naturally occurring oxide pairs from rhyolites and dacites. For this suite of oxides, values of titania activity relative to rutile saturation range from 0.3 to 0.9. Genetically related groups of oxide pairs display activity-temperature trends with negative slopes at higher activities (0.6-0.9) or positive slopes at lower activities (0.3-0.7). Thermodynamic analysis supports the assumption of two-oxide, liquid equilibrium for the former group, but suggests that such an interpretation for oxide sequences with positive activity-temperature trends may be problematic. Application of the estimation method to oxide pairs from the Shiveluch Volcano and the Bishop Tuff reveals that the former are consistent with having equilibrated with known matrix glass compositions, whereas the latter pairs are inconsistent with equilibration with preemptive liquids trapped in quartz inclusions.