Abstract

The available experimental data for the equilibrium reactions that characterize either the enstatite-diopside join or the Al-concentration in pyroxenes coexisting with an Al-rich phase are reduced to three or four thermodynamic constants, entropy, enthalpy, and change in volume (two parameters for the En/Di join). Used together with a set of partition coefficients derived from natural assemblages, these values allow independent estimates of pressure and temperature to be made for any single pyroxene phase in equilibrium with a second pyroxene and with either spinel or garnet; otherwise, minimum temperatures and maximum pressures are obtained. This method has been checked for consistency both internally (similar estimates for coexisting pyroxenes in equilibrium) and externally (agreement with independent observations).

The geothermometers and geobarometers are applied to xenoliths from “kimberlites” of the Navajo Indian Reservation and to ophiolites from Western Newfoundland, for both of which partial recrystallization into hydrous phases often prevented the use of any other technique. This method is also applied to determine pressure-temperature paths for partially reequilibrated textures, to derive pyroxene geotherms from xenocrysts, to detect disequilibrium between coexisting pyroxenes, and to reveal actual equilibration facies of complex para-geneses. These studies support (1) a remarkable shortening of Newfoundland ophiolitic sequences, originally 100 km thick, (2) a shallow origin for the Navajo Reservation “kimberlites,” and (3) an underthrusting of oceanic-type lithosphere beneath the western United States’ crust.

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