Experimental Crystallization of the Lunar Magma Ocean, Initial Selenotherm and Density Stratification, and Implications for Crust Formation, Overturn and the Bulk Silicate Moon Composition

Abstract

Eleven isobaric experimental series simulate the fractional crystallization of 1,150 km deep lunar magma ocean. Crystallization begins at 1,850°C with olivine (to 32 per cent solidified, pcs), followed at 1,600°C by olivine + opx ± Cr-spinel (to 62 pcs), at 1,210°C cpx + plagioclase ± olivine ± Ti-spinel (to 97 pcs) and at 1,060°C quartz + cpx + plagioclase + Ti-spinel, leaving 1.8 wt% residual magma that crystallizes minor K-feldspar and apatite in addition. Melt compositions remain near 45 wt% SiO2, while FeO increases from 11 to 26 wt%, TiO2 peaks at 4 wt% at Ti-spinel saturation. The available experimental liquid lines of descent yield an overall fractional crystallization sequence of olivine→opx→cpx + plagioclase→quartz→FeTi-oxide. Plagioclase appears concomitantly with cpx, a result of the low magma ocean floor pressures (≤1 GPa) after 66%–76% of olivine + opx-fractionation. A few wt% of FeTi-oxides form mostly once the quartz + plagioclase + cpx-cotectic is reached, cumulate densities remain ≤3,740 kg/m3. Scaled to a full magma ocean, plagioclase appears at 210–120 km depth, mainly as a function of bulk Al2O3. As buoyancy driven plagioclase-cpx separation is likely limited, these depths may correspond to the primordial lunar crustal thickness. Allowing for complete plagioclase flotation to the quartz + plagioclase + cpx + FeTi-oxide ± olivine cotectic yields 95–70 km primordial crust of anorthosite and quartz-gabbro, far in excess of the 35–50 km observed. This supports an overturn of primordial layers, remelting of dense gabbroic cumulates in the harzburgitic cumulate mantle leading to further mixing and differentiation. We posit that such complex density induced convection led to a lunar marble cake mantle with primitive and fairly evolved reprocessed cumulates next to each other.