HR: 08:45h
AN: V31F-04    [Abstracts]
TI: Geothermometers and Geobarometers for Spinel Peridotite Phase Assemblages
AU: * MacGregor, I
EM: macgregori@si.edu
AB: Synthesis of spinel peridotite phase assemblages that cover a wide range of natural compositions allow for the empirical determination of a number of geothermometers and geobarometers for exchange reactions between and among olivine-orthopyroxene-clinopyroxene-spinel phase assemblages. Geothermometers are derived for the reactions that govern the solubility of Al2O3 in ortho- and clino-proxenes, and Mg-Fe exchange reactions between coexisting silicate and spinel, and silicate-silicate phase assemblages. The distribution of both Al2O3 and Mg-Fe between/among coexisting phase(s) is governed by temperature and the Cr2O3 content of the coexisting spinel. The geothermometers are given as multiple linear least square fits to temperature, composition and distribution coefficients. The data show that Ca solubility and the Mg-Fe exchange between coexisting pyroxenes is both temperature and pressure dependent. Linear least square fits to the data provide useful geobarometers for samples where temperature is independently determined. The geothermometers and geobarometers are used to estimate temperatures and pressures for selected suites of spinel peridotities from ultramafic massifs, ocean floor peridotites, xenoliths assemblages in continental alkali olivine basalt eruptions, kimberlite pipes, and spinel-pyroxene reaction corona encircling garnets in garnet peridotites. The estimated temperature and pressures all fall in the spinel peridotite stability field and allow useful insights into the tectonic environments, and processes that govern the formation of the different suites. Systematic differences in estimated temperatures between Al2O3 and Mg-Fe exchange reactions for the same assemblages help illustrate the petrogenetic history of the different suites.
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005