10:45 AM - 11:00 AM
[PPS05-07] Composition of the hydrous lunar magma ocean and the crystallization process at the crust formation
Keywords:hydrous lunar magma ocean, anorthosite, crust formation
To constrain the initial composition of the hydrous LMO, the calculation with the pMELTS and rhyilite-MELTS algorithm was performed to demonstrain the crystallization process.The initial water content were set to 10, 100, 1000, 2000 ppm water. The initial FeO, Al2O3 contents were set to 0.8 to 2.0 times those in the BSE (Bulk Silicate Earth) composition (McDonough and Sun, 1995, EPSL). In this stuidy, four different conditions among near maximum fractionation and near perfect equilibrium crystallization are simulated with adopting the parameter, suspended crystal fraction limit, “Xscfl” proposed by Sakai et al. (2014, Icarus). The calculation results were investigated with three conditions as follows : (1) the crustal thickness formed with anorthite, (2) the Mg# (= Mg/(Mg+Fe)) of the mafic minerals crystallized with anorthite, (3) the possibility of differentiation of anorthite.
The result of this research is tracking the entire process of the crystallization of initially hydrous LMO with thermodynamic caluculation. As the initial water content increased, the crystallization amount of olivine increased. The composition of melt at the time of plagioclase crystallization is poor in SiO2 and the viscosity is smaller as the water content is higher. The CIPW normative calculation with the liquid composition at the time of plagioclase crystallization, the crustal rock which applied among anorthositic norite to gabbro norite in Clementine Classiification (Wieczorek et al., 2006, RiMG) . This suggests that the lunar crustal rocks that are consistent with observations can be created even under hydrated conditions.In this study, the crustal thickness and the Mg# of the mafic minerals crystallized with anorthite do not dependent on the initial water contents.
In this study, from the condition of liquid viscosity at the time of plagioclase crystallization and Mg# of mafic mineral, the minimum value and the maximum value of the initial FeO amount were restricted according to the initial water content. The larger the initial water content, the larger the minimum value of the initial FeO content and the smaller the maximum value. It is suggested that the initial FeO content under hydrated conditions was appropriate between 1.0 and 2.0 times BSE in this study. Depending on the constraints of lunar crust thickness, initial compositions with initial Al2O3 content is less than BSE should be more appropriate and the conditions which are close to the equilibrium crystallization should also be appropriate.