A long-lived lunar magma ocean

Kurzfassung

Soon after its formation, the Moon underwent an episode of large-scale mantle melting, referred to as the lunar magma ocean (LMO). The LMO extended from the surface down to a depth between a few hundred and about one thousand kilometers [1]. The first 80% of the volume of the LMO likely solidified very rapidly through radiative cooling to cold space at the liquid surface, or efficient conductive cooling through a very thin quenched crust. Afterwards, buoyant plagioclase crystals started to solidify and accumulate at the LMO’s surface, building an insulating flotation crust sampled in today’s anorthositic highlands. According to thermal models, the complete crystallization of the LMO beneath a growing plagioclase crust can take up to ten million years [1]. This protracted timescale is compatible with onset of solid-state convection in the cumulates [2]. Secondary melts due to convection in the cumulates will then be extracted into the overlying LMO, where their higher temperature acts as an additional heat flux in the thermal balance of the LMO, referred to as “heat piping”. In this work we coupled a thermal model of the LMO solidifying beneath a growing flotation crust and a dynamic model of convection in the solid cumulates underlying the LMO. The latter allows us to account for the heat piping effect, which has been neglected in previous studies. By varying the reference viscosity of the solid cumulates, which has a first order influence on the intensity of the convection and in turn on the quantity of secondary melts produced, we show that the lifetime of the LMO can be extended by a few hundreds of millions of years in comparison with cases where heat piping is ignored. To reconcile the extended lifetime of the LMO with the 4.37 Ga model age for the final LMO crystallisation product (termed urKREEP) [3], the Moon must have formed earlier than currently thought. [1] Elkins-Tanton L. et al., EPSL, 2011 [2] Maurice M. et al., JGR: Planets, 2017 [3] Gaffney A. and Borg L., Geochim. Cosmochim. Acta, 2014