Ilmenite-magnetite geothermometry and oxygen barometry in granulite and amphibolite facies gneisses from Doubtful Sound,Fiorland, New Zealand

Ilmenite-magnetite composite intergrowths in granulite and amphibolite facies gneisses have been examined petrographically and analysed with an electron probe. A correlation has been found between the oxide intergrowth texture and the temperature and f_o2 as calculated using the Buddington & Lindsley geothermometer. Composites with fine grained, complexly intergrown phases give high temperatures of about 1000°C; those with simple, coarse grained intergrowth textures give lower temperature down to about 500°C; f_o2 is higher in the low temperature intergrowths. It is concluded that Buddington & Lindsley geothermometry and oxygen barometry records the conditions at which diffusion of Ti in ilmenite and magnetite ceases; in metamorphic rocks this is independent of the metamorphic grade indicated by the coexisting silicate minerals. The Doubtful Sound data suggest that, as the metamorphic rocks cooled, f_o2 was buffered under conditions approximating to the Ni/NiO oxygen buffer.