Experimental Crystallization of the Lunar Magma Ocean, Initial Selenotherm and Density Stratification, and Implications for Crust Formation, Overturn and the Bulk Silicate Moon Composition. Issue 5 (24th May 2022)

• Record Type: Journal Article
• Authors: Schmidt, Max W.; Kraettli, Giuliano
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-24
• Subjects: Moon -- magma ocean -- experimental -- lunar crust -- lunar mantle; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007187
• Languages: English
• ISSNs: 2169-9097

Abstract

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/m 3 . Scaled to a full magma ocean, plagioclase appears at 210–120 km depth, mainly as a function of bulk Al2 O3 . 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. Plain Language Summary: Upon its accretion the Moon was likely completely molten, forming a global magma ocean. This study experimentally simulates the crystallization of this magma ocean and constrains the resulting mineral cumulate layers, the initial temperature distribution, and the nature of the original lunar crust. Overall this crust was 2–5 times thicker than observed nowadays, supporting that post‐magma ocean convection has removed the larger part of this in some layers quite dense crust. The down‐slumping crust remelts in the hot deeper lunar mantle, leading to a second generation of magmatism explaining part of the available lunar samples. Key Points: Crystallization of a full Moon magma ocean defines the initial selenotherm Our and previous results yield >95–70 km primordial crust, at least twice the modern value, requiring reprocessing through lunar overturn Slumping dense crust remelts in the cumulative harzburgitic mantle, adding further diversification to a lunar marble‐cake mantle