1Max W. Schmidt,1Giuliano Kraettli
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2022JE007187]
1ETH, Zuerich, Switzerland
Published by arrangement with John Wiley & Sons
Eleven isobaric experimental series simulate the fractional crystallization of a 1150 km deep lunar magma ocean. Crystallization begins at 1850 oC with olivine (to 32 per cent solidified, pcs), followed at 1600 oC by olivine+opx±Cr-spinel (to 62 pcs), at 1210 oC cpx+plagioclase±olivine±Ti-spinel (to 97 pcs) and at 1060 oC 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 yields 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, cumulates densities remain ≤3740 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, re-melting 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.