Chemical Disequilibria in the Source of Oceanic Basalts: Insights from Grain-Scale Models

We have developed a 2D numerical model of multi-phase coarsening, diffusive trace element partitioning and near-fractional melting. Application of the model to decompression melting in the source of oceanic basalts (OIB and MORB) has indicated that trace element partitioning occurs far from equilibrium. For example, while Sm/Yb in OIB erupted on thick lithosphere ( 4) is elevated compared to MORB ( 1), it is lower than that expected for OIB from equilibrium fractional melting ( 10). A hypothetical enriched component incorporated into low-degree melts in the OIB source exacerbates this problem. On the other hand, observed MORB and OIB REE chemistry can be fitted by disequilibrium melting models with reasonable values for grain size, source temperature, and models of lithospheric thickness. A problem with our current model formulation is that the reacting phase assemblage is based on equilibrium thermodynamic calculations. Thus, we have begun improving the model formulation by developing a numerical approach for non-equilibrium mass transport, phase transformation, and coarsening in the framework of classical irreversible thermodynamics. The current version is functional for ad hoc multivariate thermodynamic potentials. However, the main task in our developments is now the integration of an existing thermodynamic model for mantle assemblages and melt (as in MELTS software) into the non-equilibrium code. The resulting self-consistent grain-scale model is expected to self-consistently simulate microchemical evolution during melting, solid-state phase transformation, and grain coarsening in the mantle. We will attempt to apply the model to decompression melting of ocean basalt sources to corroborate results previously obtained with simpler models and elucidate additional insights on the extent of disequilibrium and thermochemical state of the mantle.