Constraining Modal Error in Ultramafic Thermodynamic Solution Models: Validating Interpretations of Seismic Wave Speed

Abstract:

Mantle seismic wave speed is dependent on pressure, temperature, and composition. Therefore interpreting mantle seismic wave speeds at a given depth in terms of composition and temperature requires the use of mineral moduli alongside thermodynamic modelling of whole-rock compositions. Solution models are calibrated on laboratory experiments, but rarely validated against mantle xenoliths over a wide range of compositions and pressure-temperature (P-T) conditions. To constrain modal error for mantle solution models, we compile hundreds of ultramafic xenoliths and mantle melting experiments with measured mineral compositions, modal proportions, and whole-rock composition. Our database uses modal-proportions calculated by least squares fitting of bulk compositions as well as EBSD image analysis. With this database, we calculate the equilibrium mineral assemblages for all available ultramafic solution models (e.g. Stixrude & Lithgow-Bertelloni, 2011; Jennings & Holland, 2015; Holland et al., 2018) using the whole-rock compositions at P-T conditions predicted for the xenoliths from thermobarometry or known from experiments. For spinel-facies peridotites or peridotites lacking a barometer, we calculate modes over a grid of potential P-T conditions and choose the best fitting P-T condition. To consider metastability, we utilize various cutoff equilibration temperatures. In addition, we compare the seismic wave speeds predicted by direct calculations of the database samples at the P-T conditions with seismic wave speeds calculated for the outputs of each solution model to see how the various modal errors change geophysical interpretation.