9:45 AM - 10:00 AM
[PPS08-04] Evaluating the Influence of Initial Composition and Crystal Separation Efficiency in the Lunar Magma Ocean on the Formation of the Moon's Internal Structure.
Keywords:Lunar Magma Ocean, Overturn, Internal Structure, Differenciation
Introduction: The formation and evolution of the Moon's internal structure remain key challenges in planetary science. Recent high-resolution rock data from missions like SLIM have provided insights, but integrating these observations into a comprehensive understanding of the Moon's origin remains complex. A key factor is the role of the Lunar Magma Ocean (LMO) solidification and subsequent mantle overturn, which are critical in determining mineral distribution and the evolution of Mg# (molar Mg / (Mg + Fe)) through the mantle. This study aims to contribute to a unified model of the Moon's internal structure formation, capable of explaining both current and future observational data, by evaluating the effects of initial composition and crystallization efficiency on these processes.
Methods: This study evaluates how initial composition and crystallization efficiency within the LMO influence the lunar mantle’s structure. Three initial compositions were used: the Bulk Silicate Earth (BSE) composition, a modified BSE with 1.5x FeO, and the composition from Elkins-Tanton et al. (2011) with a lower Mg/Si ratio. The LMO solidification model, based on Sakai et al. (2014), and the post-overturn structure, modeled from Elkins-Tanton et al. (2011), assessed the impact of crystallization efficiency and composition on the mantle’s structure. Thermodynamic equilibrium calculations with the MELTS program determined mineral types, compositions, and densities of precipitated minerals and residual magma during solidification. By varying crystallization efficiency, we developed a model simulating the LMO solidification from maximum fractional to equilibrium crystallization. The internal structure after overturn was reconstructed using a density-based reconfiguration method for the deeper layers, assuming adiabatic conditions with depth-dependent temperature and pressure variations.
Results: As observed in previous LMO models, all compositions initially form mafic minerals dominated by olivine. Mg# decreases with depth, with a sharp drop within the last 300 km. At the onset of feldspar crystallization, marking crust formation, the Mg# for the BSE composition is around 70, while it decreases to about 60 for both the modified BSE with 1.5x FeO and Elkins-Tanton compositions. Our results closely reproduce the internal mantle structure observed in Elkins-Tanton's model, especially for maximal crystallization efficiency. Significant differences in mantle structure before overturn were observed across compositions and crystallization efficiencies. The Elkins-Tanton composition leads to relatively more orthopyroxene (opx) crystallization due to the lower Mg/Si ratio, favoring orthopyroxene stability.
Discussions & Conclusions: The timing of opx crystallization in the LMO solidification sequence depends on composition and crystallization efficiency. Low Mg/Si ratios (~0.9) and near-equilibrium crystallization promote earlier opx crystallization, supporting the coexistence of different LMO solidification stages after overturn (Elkins-Tanton et al., 2011). Higher Mg/Si ratios (~1.1–1.2) and near-maximal fractional crystallization delay opx formation, leading to a simpler mantle dominated by high Mg# olivine. In such cases, olivine and orthopyroxene layers coexist at the deepest and shallowest parts, not intermediate layers. This study highlights the importance of considering initial composition and crystallization efficiency when evaluating lunar rock data. Careful evaluation of these parameters is crucial for an accurate lunar formation model.
Methods: This study evaluates how initial composition and crystallization efficiency within the LMO influence the lunar mantle’s structure. Three initial compositions were used: the Bulk Silicate Earth (BSE) composition, a modified BSE with 1.5x FeO, and the composition from Elkins-Tanton et al. (2011) with a lower Mg/Si ratio. The LMO solidification model, based on Sakai et al. (2014), and the post-overturn structure, modeled from Elkins-Tanton et al. (2011), assessed the impact of crystallization efficiency and composition on the mantle’s structure. Thermodynamic equilibrium calculations with the MELTS program determined mineral types, compositions, and densities of precipitated minerals and residual magma during solidification. By varying crystallization efficiency, we developed a model simulating the LMO solidification from maximum fractional to equilibrium crystallization. The internal structure after overturn was reconstructed using a density-based reconfiguration method for the deeper layers, assuming adiabatic conditions with depth-dependent temperature and pressure variations.
Results: As observed in previous LMO models, all compositions initially form mafic minerals dominated by olivine. Mg# decreases with depth, with a sharp drop within the last 300 km. At the onset of feldspar crystallization, marking crust formation, the Mg# for the BSE composition is around 70, while it decreases to about 60 for both the modified BSE with 1.5x FeO and Elkins-Tanton compositions. Our results closely reproduce the internal mantle structure observed in Elkins-Tanton's model, especially for maximal crystallization efficiency. Significant differences in mantle structure before overturn were observed across compositions and crystallization efficiencies. The Elkins-Tanton composition leads to relatively more orthopyroxene (opx) crystallization due to the lower Mg/Si ratio, favoring orthopyroxene stability.
Discussions & Conclusions: The timing of opx crystallization in the LMO solidification sequence depends on composition and crystallization efficiency. Low Mg/Si ratios (~0.9) and near-equilibrium crystallization promote earlier opx crystallization, supporting the coexistence of different LMO solidification stages after overturn (Elkins-Tanton et al., 2011). Higher Mg/Si ratios (~1.1–1.2) and near-maximal fractional crystallization delay opx formation, leading to a simpler mantle dominated by high Mg# olivine. In such cases, olivine and orthopyroxene layers coexist at the deepest and shallowest parts, not intermediate layers. This study highlights the importance of considering initial composition and crystallization efficiency when evaluating lunar rock data. Careful evaluation of these parameters is crucial for an accurate lunar formation model.