Ti-in-garnet thermometer for ultrahigh-temperature granulites

We propose a Ti-in-garnet thermometer for ultrahigh-temperature granulites calibrated from experimentally reversed data of the TiO2 solubility in garnet coexisting with orthopyroxene, rutile and quartz at pressures 7-23 kbar and temperatures 850-1300 degrees C. We confirm that the combined substitution (TiAlIV)-Al-VI reversible arrow (AlSiIV)-Si-VI, quasi-chemically equivalent to Ti reversible arrow Si, is predominant rather than the coupled substitution (MTiVI)-Ti-VI reversible arrow (AlAlVI)-Al-VI (M: Ca, Mg, Fe). The chemical formula of Ca-and Ti-poor garnet under ultrahigh-temperature metamorphic conditions can be expressed as M3Al2Si3-xTixO12, which indicates that the relation N-Si + N-Ti = 3 is not an evidence of Ti-IV reversible arrow Si-IV. The TiO2 content of garnet increases with temperature and pressure, though the pressure dependence is small, as is given by the following equation: 17777 + 0.964T + 139.5P = T ln N-Ti(2) /NAlNSi , where N is the number of cation per formula unit based on a 12-oxygen atom normalisation. Temperature T and pressure P are given in Kelvin and kbar, respectively. The present thermometer is useful to estimate metamorphic conditions. This new thermometer is applied to natural garnet in geologically and petrologically well-characterised Antarctic ultrahigh-temperature granulites. The resultant pressure and temperature estimates are consistent with those reported from granulites of these areas.