A BAROMETER FOR GARNET AMPHIBOLITES AND GARNET GRANULITES

Abstract

New barometer based on the equilibrium: CaFe 2 Al 2 Si 3 O 12+2 \hspace{0.17em} TiO 2=2 \hspace{0.17em} FeTiO 3+ CaAl 2 Si 2 O 8+ SiO 2 garnet ( Gr 1 Alm 2) \hspace{0.17em} rutile \hspace{0.17em} Ilmeoite \hspace{0.17em} anortbite \hspace{0.17em} quartz has been calibrated with experiments conducted in the piston-cylinder apparatus. Reversed equilibria have been obtained using well-calibrated 2-54 cm NaCl furnace assemblies, Ag80Pd20capsules withfO2 buffered at or near iron-wustite. The equilibrium is located between 10.6–10.8,12.0–12.2, 13.2–13.4 and 14.2–14.4 kb, at 800, 900, 1000, and 1100°C, respectively. The barometer is applicable in both garnet-bearing amphibolites and granulites. Its greatest potential is in garnet amphibolites wherein multi-variant amphibole-bearing mineral assemblages do not define pressure and few, if any, well-calibrated barometers are available. Application of the garnet-rutile-ilmenite-plagioclase-quartz barometer in amphibolite and granulite terranes yields geologically reasonable pressures that are in agreement with other well-calibrated barometers in those terranes where comparisons can be made.