A REE-in-garnet–clinopyroxene thermobarometer for eclogites, granulites and garnet peridotites

Abstract

A REE-in-garnet-clinopyroxene thermobarometer for eclogites, granulites, and garnet peridotites has been developed on the basis of the temperature, pressure and mineral composition dependent partitioning of rare earth elements (REEs) between garnet and clinopyroxene. This new thermobarometer is derived from the garnet-clinopyroxene REE partitioning model of Sun and Liang (2014) that was calibrated against experimentally determined garnet-melt and clinopyroxene-melt partitioning data. It makes use of a group of trace elements that have similar geochemical behaviors at magmatic and subsolidus conditions, and allows one to invert temperature and pressure simultaneously using a least squares method. Application of the REE-in-garnet-clinopyroxene thermobarometer to REE partitioning data from laboratory experiments and field samples (quartz-bearing, graphite-bearing, and diamond-bearing granulites and eclogites; and well-equilibrated mantle eclogite xenoliths) published in the literature validates its reliability at both magmatic and subsolidus conditions. Application of the new thermobarometer to eclogites, garnet granulites and peridotites from various tectonic settings reveals an intriguing observation: temperatures derived from the REE-based thermobarometer are consistently higher than those derived from the widely used Fe-Mg thermometer of Krogh (1988) for samples that experienced cooling, but systematically lower than temperatures derived from the Fe-Mg thermometer for samples from thermally perturbed tectonic settings. The temperature discrepancies are likely due to the relative differences in diffusion rates between trivalent REEs and divalent Fe-Mg in garnet and clinopyroxene. Temperatures derived from the REE-based thermometer are closely related to closure temperatures for samples that experienced cooling, but are likely equilibrium or apparent re-equilibration temperatures at an early stage of heating for samples from thermally perturbed tectonic environments. The REE-in-garnet-clinopyroxene thermobarometer can shed new light on thermal histories of mafic and ultramafic rocks.