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Late-stage evolution of the lunar magma ocean: an experimental study

Final Report Summary - LUNARMAGMAOCEAN (Late-stage evolution of the lunar magma ocean: an experimental study)

The origin of the Moon has been attributed to a giant impact between a Mars-sized asteroid and the Earth. The heat produced by this event was sufficient to melt a significant part of the Moon and produced the "Lunar Magma Ocean" (LMO). Although the Moon has been explored by landed missions and further studied by remote sensing methods, direct evidence on how the lunar magma ocean evolved as it crystallizes are missing. The objective of this project is to perform new experiments in the laboratory using furnaces and presses in a range of pressure and temperature relevant for crystallization processes of lunar silicate magmas. We are developing models to explain how the LMO evolved chemically as it cooled and crystallized during its late-stage evolution and to test/constrain these models with observations. We aim at constraining the liquid line of descent of the magma ocean. These data enable us to understand how the complexities of the anorthositic crust were generated and what is the composition of its complementary mantle cumulates. The other goal is also to gain a better understanding of the cumulate remelting process that is widely agreed upon to form mare basalts and ultramafic glasses. Our study builds on these previous models by producing melt compositions that have evolved by fractional crystallization using the stepwise experimental technique. Fractional crystallization has been simulated by a stepwise approach by making up new bulk compositions having the composition of the last residue with relatively high proportions of liquid. Each new experiment on evolving liquid compositions is performed at decreasing pressure, proportional to the fraction of crystallized minerals in the previous higher-temperature experiments. The first two years of this project (outgoing phase) were spent at the Massachusetts Institute of Technology (Boston, USA). This institution hosts experts dealing with multidisciplinary approaches of Planetology. This project also benefits of strong involvement by researchers from MIT in spacecraft missions such as the GRAIL program led by NASA. High-pressure experiments (>8 kbar) have been performed at the experimental petrology laboratory using piston cylinder devices, under the supervision of Professor Tim Grove. These experiments enable us to have a clear understanding of the first evolution stages of the lunar magma ocean. The experiments were combined with geochemical modelling to trace the liquid lines of descent of selected bulk Moon compositions. The last part of the project was performed at the University of Hannover, Germany. Low-pressure experiments (<5 kbar) have been conducted in internally-heated pressure vessels. Detailed phase equilibria have been obtained for the late-stage evolution of the lunar magma ocean down to 1000°C. The saturation of plagioclase along the evolution of residual melts has been clearly identified for a range of whole Moon composition. These data are used to constrain the origin of the lunar crust.