A REE-in-two-pyroxene thermometer for mafic and ultramafic rocks

Abstract

A REE-in-two-pyroxene thermometer for mafic and ultramafic rocks has been developed on the basis of the temperature and pyroxene composition dependent rare earth element (REE) partitioning between coexisting orthopyroxene and clinopyroxene. This trace element based two-pyroxene thermometer is built on two parameterized lattice strain models for REE partitioning between pyroxene and basaltic melt that were independently calibrated using data from pyroxene-melt partitioning experiments (Sun and Liang, 2012; Yao et al., 2012). By treating REE and Y as a group in temperature calculations, one can reduce analytical uncertainties in trace element analysis through least squares inversion of orthopyroxene–clinopyroxene partitioning data, and identify and exclude variations induced by secondary or metasomatic processes. For convenience, a simple robust regression program is provided for temperature inversion. Application of the REE-in-two-pyroxene thermometer to well-equilibrated spinel lherzolite and harzburgite xenoliths from the continental lithosphere demonstrates the internal consistency of the two pyroxene-melt REE partitioning models and the trace and major element based pyroxene thermometers: temperatures derived from the REE-in-two-pyroxene thermometer agree very well with temperatures calculated using major element based pyroxene thermometers. Applications of the REE-in-two-pyroxene thermometer to abyssal peridotites and mafic cumulates reveal a significant difference in the calculated temperatures between the REE and major element based pyroxene thermometers: temperatures derived from the REE-in-two-pyroxene thermometer are consistently higher than those derived from the major element based pyroxene thermometers. This discrepancy in temperature likely results from a difference in diffusion rate and hence closure temperature between the 2+ and 3+ cations in the pyroxenes. An independently calibrated REE-in-two-pyroxene thermometer may offer new insight into the thermal history of mafic and ultramafic rocks from the upper mantle and lower crust.