THERMOBAROMETERS BASED ON REE PARTITIONING BETWEEN MANTLE MINERALS

Distributions of REE between mantle minerals depend on temperature, pressure and mineral compositions. We have developed thermobarometers based on REE partitioning between the following mineral pairs for ultramafic and mafic rocks: orthopyroxene-clinoopyroxene, garnet-clinopyroxene, plagioclase-clinopyroxene, and plagioclase-orthopyroxene. These trace element based two-mineral thermobarometers are built on parameterized lattice strain models for mineral-melt REE partitioning that we independently calibrated using published data from mineral-melt trace element partitioning experiments. By treating REE as a group, we have demonstrated that one can reduce analytical uncertainties in trace element analysis through least squares inversion of mineral-mineral REE partitioning data, and identify and exclude variations induced by secondary processes. Through numerical simulations of diffusive redistribution of REE in cooling bi-mineralic systems, we have demonstrated that the temperatures derived from mineral-mineral REE partitioning are the closure temperature of REE in the bi-mineralic systems.

For well-equilibrated peridotites and eclogites, the temperatures derived from the REE-based thermobarometers agree very well with those derived from the major element-based thermometers, which independently justifies our REE-based thermobarometers. However, systematic differences between the REE-derived and major element-derived temperatures appear in mafic and ultramafic rocks that experienced fast cooling or significant metasomatic processes: for abyssal peridotites and mafic cumulates that experienced fast cooling processes, the REE-derived temperatures are systematically higher than the major element-derived values; for garnet peridotites and mantle eclogites from metasomatized lithospheric mantle, the REE-derived temperatures are systematically lower than the major element-derived values. Because REE diffuse slower than divalent cations in these mantle minerals, the REE-based thermometer may record early thermal events, whereas the major element-based thermometers may record more recent thermal perturbations. Independently calibrated REE-in-two-mineral thermobarometers may offer new insights into the thermal history of mafic and ultramafic rocks.