2016 Volume 111 Issue 3 Pages 226-240
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 ⇌ AlVISiIV, 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:
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.