TITANIUM SOLUBILITY IN COEXISTING GARNET AND CLINOPYROXENE AT VERY HIGH PRESSURE: THE SIGNIFICANCE OF EXSOLVED RUTILE IN GARNET

Abstract

Exsolution microstructures including ilmenite±garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the Sulu ultrahigh-pressure (UHP) terrane. In order to understand the phase relations and Ti solubility in both garnet and clinopyroxene in a natural TiO2-bearing system, several experiments at 5–15 GPa, 1000–1400°C were carried out using the multianvil high-pressure apparatus. The Hujianlin ilmenite-rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material, because it closely approaches a composition lying in the TiO2–CaO–MgO–FeO–Al2O3–SiO2 system. Except for minor melt in one experiment at 1400°C and 5 GPa, other run products contain majoritic garnet+clinopyroxene±ilmenite (or rutile) and exhibit neoblastic texture. With increasing pressure, Ti and Ca, Mg and Si contents of neoblastic garnet increase with decreasing Al. The principal coupled substitutions are Ca2+Ti4+→2Al3+ and Si4+Mg2+→2Al3+ responding to majorite component increase. Titanium solubility (0.8–4.5 wt% as TiO2) in garnet and GrtTi/CpxTi ratio have a pronounced positive correlation with pressure between 5 and 15 GPa. On the other hand, the coexisting clinopyroxene contains low Ti (0.17–0.53 wt% as TiO2), and shows no significant pressure effect. Rutile exsolution in garnet is coupled to that of pyroxene exsolution; both are exsolved from majoritic garnet on decompression. Therefore, the amount of such exsolved lamellae is a potential indicator of high-pressure metamorphism in exhumed rocks, whereas the TiO2 content of clinopyroxene coexisting with garnet is not sensitive to pressure change.