Garnet stability in the deep lunar mantle: Constraints on the physics and chemistry of the interior of the Moon

Abstract

High-pressure, high-temperature experiments have been conducted at deep lunar mantle conditions to constrain the garnet stability field. Using the Taylor Whole Moon composition, garnet is found to be stable at pressures above 3 GPa and temperatures below 1700°C, yielding a smaller stability field than previously suggested on the basis of thermodynamic calculations. Experimental data are used to model equilibrium crystallization in a ‘two-stage’ model of lunar magma ocean (LMO) crystallization starting from a fully molten Moon. In the first stage, isothermal (1600°C) equilibrium crystallization of the LMO would produce garnet-bearing lherzolite cumulates (containing up to ∼20 wt.% garnet) in the lowermost lunar mantle. Garnet in the deep lunar mantle would significantly decrease the Al2O3 content of the residual LMO and impact HREE/MREE fractionation. Numerical modeling of the second stage (residual LMO fractional crystallization) shows a delay in plagioclase saturation compared to models of single-stage fractional crystallization of a whole-Moon LMO of the Taylor Whole Moon composition, thinning the anorthositic crust from 95 km to 75 km. To reach the upper limit of current estimates of the average lunar crustal thickness (∼45 km), the two-stage scenario needs to be accompanied by a total of 10% liquid trapped in cumulates and 70% efficiency of plagioclase flotation. We also conduct trace element evolution modeling and reproduce a REE pattern identical to high K, REE, and P (KREEP) compositions after 99.8% solidification, when starting with a CI chondritic REE abundance. The density of a garnet-bearing deep lunar mantle is significantly higher than the density of olivine/orthopyroxene mixtures without garnet. The present-day lowest mantle in the Moon could therefore be characterized by chemical interactions between the earliest (garnet-bearing) and latest (ilmenite-bearing) products of LMO crystallization.