Ti–in–garnet thermometer for ultrahigh–temperature granulites

Abstract

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 °C. We confirm that the combined substitution Ti^VIAl^IV ⇌ Al^VISi^IV, quasi–chemically equivalent to Ti ⇌ Si, is predominant rather than the coupled substitution M^VITi^VI ⇌ Al^VIAl^VI (M: Ca, Mg, Fe). The chemical formula of Ca– and Ti–poor garnet under ultrahigh–temperature metamorphic conditions can be expressed as M₃Al₂Si_3−xTi_xO₁₂, which indicates that the relation N_Si + N_Ti = 3 is not an evidence of Ti^IV ⇌ 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:

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.