Quantifying trace element disequilibria in mantle xenoliths and abyssal peridotites

We apply a tool based on element distribution between orthopyroxene and clinopyroxene for quantifying rare earth element (REE) disequilibria in ultramafic rocks in the subsolidus state. We present case studies of the REE contents of mineral cores in mantle xenoliths and abyssal peridotites using in situ analytical tools. Even when only mineral cores are measured (to avoid enriched rims), equilibrium is not always achieved on the mineral scale. Mineral cores in mantle xenoliths are closer to equilibrium than those in abyssal peridotites even though mantle xenoliths are known to be light REE-contaminated from the host lava. In the case of the abyssal peridotites, 13 out of 14 are out of equilibrium with the least metasomatized most in disequilibrium and the most metasomatized closest to equilibrium. We discuss hypotheses for these observations, but regardless of what caused the disequilibria, this tool allows one to “see through” the effects of secondary processes, such as infiltration by fluid inclusions via cracks and diffusive exchange between minerals and melts/fluids along grain boundaries. The ease of making in situ REE measurements makes this tool formidable in identifying different generations of clinopyroxenes in ultramafic lithologies. Such data will complement the interpretation of isotopic and petrographic studies of continental and oceanic lithospheric mantle.