Titanium solubility in coexisting garnet and clinopyroxene at very high pressure: the significance of exsolved rutile in garnet

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, 1000-1400degreesC 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 1400degreesC 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+ --> 2Al(3+) and Si4+Mg2+ --> 2Al(3+) responding to majorite component increase. Titanium solubility (0.8-4.5 wt% as TiO2) in garnet and Grt(Ti)/Cpx(Ti) 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. (C) 2003 Elsevier B.V. All rights reserved.