Solubility of TiO 2 in garnet and orthopyroxene: Ti thermometer for ultrahigh-temperature granulites
We report the TiO 2 solubility in garnet and orthopyroxene coexisting with rutile calibrated from experimental data at pressures 7–20 kbar and temperatures 850–1300°C in two Antarctic granulite systems.The Ti would substitute for the tetrahedral Si, whereas we could not find positive evidence of the coupled substitution of M-Ti (M is Mg or Fe) for Al-Al in octahedral sites as well as the interstitial substitution of -Ti ( is avacancy) for M-M in triangular-dodecahedral sites of garnet at ultrahigh-temperature (UHT) metamorphicconditions.The TiO 2 content of garnet increases with temperature and decreases with pressure. In orthopyroxene Ti substitutes for atoms in the tetrahedral and octahedralsites.The Ti content of orthopyroxene increases with temperature and increases with pressure.Titanium solubility in garnet and orthopyroxene is not so sensitive to pressure change as compared with temperature dependence given by following empirical equations: ln (X Grt;IV Ti /X Grt;IV Si )= -15366/T +5.962 and ln ( X Opx;IV Ti /X Opx;IV Si )=-11367=T +3.107 where X is the mole fraction of Ti or Si on the tetrahedral sites and temperature is given in Kelvin.