The effects of an initially layered composition on the thermochemical evolution of the lunar mantle

Kurzfassung

Due to the Moon-forming impact the Moon was initially covered by a global magma ocean. Cooling and subsequent fractional solidification led to an initially layered lunar mantle composition. The aim of our work is to investigate how this mantle composition affects the convection and the subsequent melting of the lunar mantle. In our work we use the mantle convection code GAIA [1] to model the thermochemical evolution of the Moon. We consider a compositionally heterogeneous mantle and, for comparison, a homogeneous lunar mantle scenario. For both scenarios, a petrological model [2] provides the initial density, melting curves, density change due to depletion, and, in case of a heterogeneous lunar mantle, the initial temperature profile and the initial layered structure. Our models use a 2D half cylinder geometry. We employ an Arrhenius law to calculate the temperature and depth dependent viscosity, and we account for core cooling, radioactive decay, and mechanical mixing. The mantle composition is tracked via a particle-in-cell method [3], where tracer particles carry information about material properties (density, melting temperature, degree of depletion, amount of heat producing elements). We account for latent heat consumption during melting and consider both the increase of solidus and the changes in density of the residual material due to mantle depletion. We calculate the thickness of the secondary crust produced during the evolution and require that successful models fit present-day estimates of 0.4 - 12.4 km. This range accounts for both the basaltic lava flows (<1% of today's crust [4]) and the Mg-suite rocks (6% to 30 %). Our results show that a model with a homogeneous initial mantle composition either produces too much crust to match today’s estimates, or produces melt too late to match the ages of the oldest basalts. The heterogeneous models match the secondary crust estimates, but stop producing melt too early to account for the youngest lunar basalts. Acknowledgements: Irene Bernt and Sabrina Schwinger were supported by the German Research Foundation (Deutsche Forschungsgemeinschaft) SFB-TRR170, (subprojects C4 and A5). References [1] Hüttig C. et al. (2013) PEPI [2] Schwinger S. and Breuer D. (2021) PEPI [3] Plesa A.C. et al. (2013) IGI Global [4] Head J.W. (1976) Rev. Geophys.