LUNARMAGMAOCEAN

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

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 Obiettivo del progetto (Objective)

'The origin of the Moon has been attributed to a giant impact involving a Mars-sized asteroid and the proto-Earth. Energy liberated in this catastrophic event was sufficient to melt a substantial portion of the Moon, a likely cause of a global “Lunar Magma Ocean". This LMO developed early at ca. 4.5 Ga and its crystallization appears to have produced an anorthositic crust and deep cumulate rocks forming the lunar upper mantle, known from their product of partial melting: mare basalts. Direct evidence on how the LMO evolved as it cooled and crystallized remains a major issue. Petrologic models for the solidification of the LMO are derived mainly from thermodynamic phase relationships. The crystallization sequence predicted by these models is olivine, opx ± olivine, olivine cpx ± plag, cpx plag, cpx plag ilmenite. However, the exact composition of late-stage products during the crystallisation of the LMO remains poorly known. The timing for plagioclase accumulation by flotation, that probably occurred at the top of the LMO to form the lunar crust, is undoubtedly related to the density of the magma. It is rather uncertain whether plagioclase has floated throughout its crystallization story or only at precise degree of evolution when a maximum magma density was reached. What occurs during this critical early period of the Moon differentiation sets the stage for all subsequent events. The objective of this project is to perform new experiments in a range of pressure, with sophisticated equipments allowing the control of important parameters such as starting compositions, temperature and oxygen fugacity. New models for how the LMO evolved as it crystallized during its late-stage evolution will be developed and constrained with observations on mare basalts. How do starting composition, pressure, oxygen fugacity and fluids influence this liquid line of descent? How was generated the anorthositic crust and what is the composition of its complementary mantle cumulates?'

Descrizione progetto (Article)

The origin of the Moon has been attributed to a giant impact between a Mars-sized asteroid and the proto-Earth. The heat produced by this event was sufficient to melt a significant part of the Moon, and it produced the Lunar Magma Ocean (LMO). Crystallisation of the LMO appears to have produced an anorthositic crust and deep cumulate rocks forming the lunar upper mantle. The exact composition of late-stage products during the crystallisation of the LMO is not fully known, and how the LMO evolved as it cooled and crystallised remains a topic for debate.

Backed by EU funding, the 'Late-stage evolution of the lunar magma ocean: an experimental study' (LUNARMAGMAOCEAN) project is working to develop new models to explain how the LMO evolved as it crystallised during its late-stage evolution. Progress will be guided with observations on surface lavas (the mare basalts) and data provided by the on going GRAIL spacecraft mission.

Researchers will carry out new laboratory melting experiments using furnaces and presses across a range of pressures and temperatures. The approach is deemed relevant to phase equilibria of lunar silicate magmas. Sophisticated equipment will allow the control of important parameters such as starting compositions, temperature and oxygen fugacity. Using a stepwise experimental technique build on successive experiments and modelling, the complete evolution of the LMO will be reconstructed.

High-pressure experiments have been performed at the Massachusetts Institute of Technology and already offer a clear understanding of the first evolution stages of LMO. The experiments were combined with geochemical modelling to trace the liquid lines of descent of selected bulk Moon compositions. Going forward, LUNARMAGMAOCEAN will proceed with low-pressure experiments at the University of Hannover.

Project members have benefited from collaborative work with experts in the United States who are involved in multidisciplinary approaches to planetology and in spacecraft missions. Continuing project work stands to boost Europe's position in this area of research.