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 ⇌ 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.