AN EXPERIMENTAL-STUDY OF 2 GARNET PYROXENITE XENOLITHS FROM THE BULLENMERRI AND GNOTUK MAARS OF WESTERN VICTORIA, AUSTRALIA

Experiments were conducted at 6-30 kb and 875-1200-degrees-C on two garnet pyroxenite xenoliths from the Bullenmerri and Gnotuk Maars of western Victoria, Australia. The (garnet + clinopyroxene + plagioclase + spinel) assemblage of DR9734 was stable between 10 and 12.5 kb, and 950 and 1,050-degrees-C. The compositions of its natural mineral phases were most closely approximated in experiments at 12.5 kb and 1,000-1,050-degrees-C. The (garnet + spinel + clinopyroxene + orthopyroxene + amphibole) assemblage of DR10165 was stable at pressures > 8 kb and temperatures > 950-degrees-C. However, differences between natural and experimental mineral compositions indicate that the mineral assemblage of this xenolith persisted metastably after cooling below 950-degrees-C with chemical exchange continuing down to approximately 850-900-degrees-C. When the experimental data for DR9734 and DR10165 are applied to mineralogical data for other mafic and ultramafic xenoliths from the Bullenmerri and Gnotuk Maars, they indicate that previous pressure and temperature estimates for individual xenoliths are 2-3 kb and approximately 50-degrees-C too high. These corrections increase average temperatures for the geotherm beneath western Victoria by about 50-degrees-C over a depth range of 30-45 km and confirm its perturbed (high-temperature) character.