Effects of Fe-Ca-Mg substitutions on the equation-of-state of pyrope-rich garnet from ab initio modeling and experiments: Insights and implications for the upper mantle

We report systematic first-principles results of structural properties and compression behavior based on density functional theory (DFT) and an exchange-correlation functional for solids, of Al-bearing garnets of general compositions in the pyrope-almandine-grossular solid solution system. The combination of DFT and a simple solid solution model is able to produce a compositional dependence of the compression curve consistent with trends observed in experimental studies. Using end-member properties extrapolated from our computations and perturbing an extant thermodynamic model we observe only marginal effects on the bulk sound velocity of pyrolite and MORB along relevant geothermal paths. However, this could hide important effects on the elemental partitioning between garnet and other major phases which should be further investigated both experimentally and computationally. We also present simulations of the effect of combined Fe and Ca substitutions for Mg on the elastic tensor of Al-bearing garnets, our simplified modeling shows only partial agreement with the trends observed in experiments. Therefore, further computational investigations, especially of the effect of Fe-Mg substitution on the tensor, are needed.