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 TiVIAlIV ⇌ TiVIAlIV, quasi- chemically equivalent to Ti ⇌ Si, is predominant rather than the coupled substitution MVITiVI ⇌ AlVIAlVI (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 NSi + NTi = 3 is not an evidence of TiIV ⇌ SiIV. 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 N2Ti/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.