The Initial Lunar Mantle Structure Constrained by Thermodynamic Simulation

The initial lunar internal structure formed by the crystallization of the Lunar Magma Ocean (LMO) is the beginning of its subsequent evolution, and the crystallization process is governed by two parameters : the initial depth and the composition of the LMO. Due to the lack of rock samples directly from the deep interior of the Moon, current studies on the evolution of LMO mainly rely on experimental and computational simulations. The LMO evolution models were evaluated by comparing the thickness of the lunar crust formed through LMO differentiation with detected results. The latest Gravity Recovery and Interior Laboratory (GRAIL) mission suggests that the lunar crust's thickness is 34 similar to 43km, lower than 70km estimated from Apollos' data, which challenges all the former models on the LMO evolution. In this paper, we adopt and modify the FXMOTR package to simulate the crystallization process of LMOs with different initial depths and compositions. We quantify the effect of the initial depth and composition on the thickness of the lunar crust, and combine the results of studies on the partitioning of trace elements in the lunar interior to calculate the changes in the lunar mantle's trace element composition through crystallization and compare the rare-earth elements ( REE) composition of the residual melt with the urKREEP, primeval KREEP component (incompatible K, REE, and P rich). Results from our simulation show that an LMO extent to its core-mantle boundary with composition as Lunar Primitive Upper Mantle (LPUM) can form a crust with thickness close to the GRAIL data and a reasonable urKREEP layer. There is about 2. 5% garnet crystallized after olivine during equilibrium crystallization remaining in the lunar lower mantle. Based on this model, we simulate the whole lunar internal structure on composition and density, and discuss its mantle overturn process.