Magma Ocean Crystallization and its Consequence on the Thermochemical Evolution of the Moon

Kurzfassung

The early Moon was covered by a global magma ocean that is assumed to have undergone fractional solidification during cooling - as evidenced by the anorthositic crust. With complete fractionation of the lunar magma ocean (LMO), the solid mantle consists of layers with different compositions, densities, and melting temperatures. After solidification, the layers mix due to solid-state convection, causing some of the mantle material to melt and rise to the surface. The melted material is the origin of the secondary crust, which consists of Mg-suite rocks (ages at 4348 ± 25 Ma [1]) and the mare basalts (ages from 4.0 to 1.2 Ga [2]) that are still found on the lunar surface. Estimates indicate that the Mg-suite represents 6-30% of the total crustal volume [3], while mare basalts constitute only 1-2% [4]. The amount and timing of secondary melt production can therefore be used to constrain the modeling of melting processes in the lunar mantle and improve our understanding of the magmatic evolution of the Moon. The temporal evolution of crust formation has been studied in previous work by e.g [5] and [6]. The former neglect the formation of a fractionated mantle and assume a simple homogeneous mantle composition - those models typically overestimate the amount of secondary crust. [6], in turn, used the layered mantle structure of [7]. Here, the layers differ in density and concentration of heat-producing elements. This latter study [6] only considers the melting of ilmenite bearing cumulates (IBC), which crystallize below the crust during the last stage of magma ocean solidification. A more recent study by [1] investigates the origin of the Mg-suite but neglects the formation of the mare basalts. In their geodynamical model, they use a layered mantle composition and set the initial temperature profile at the solidus temperature of peridotite.