| Abstract |
Recent observations by lunar explorations have shown that the lunar highland crust is highly anorhositic in composition and is ~45-60 km thick. The Moon has been thought to have undergone a global magma ocean stage very early in its history and the anorthositic crust was formed by accumulation of anorthite crystallized in lunar magma ocean (LMO).
The bulk composition of the Moon has been estimated by previous studies from geochemical and geophysical data. There are, however, large disparities among the estimates, because of the lack of direct chemical and structural information of the lunar interior right after the solidification of the magma ocean. The initial composition of the LMO, particularly FeO and refractory elements (Al2O3 and CaO), largely affects physical properties of the melts as well as the phase relation of anorthite crystallization, and thus dynamics of the cooling LMO.
I have developed a fractional crystallization model of the LMO with thermo-dynamical calculations using MELTS/pMELTS [Ghiorso & Sack, 1995; Ghiorso et al., 2000] and investigated the conditions for the effective floatation of anorthite in the LMO to reproduce the observed critical features of the lunar crust to constrain the FeO and refractory element contents.
The estimated initial composition of the LMO must satisfy the following observation: (a) anorthite crystallized and floated to accumulate in differentiated magma ocean, (b) the amount of anorthite was abundant enough to form the anorthositic crust of the average thickness of ~45 km, (c) the major composition of minerals, such as Mg# (= molar [MgO/(MgO+FeO)] ×100) of mafic minerals and An content of plagioclase should be consistent with the observations, and (d) the trace element concentrations should be also consistent with observation.
The results showed that the initial FeO content should be more abundant than that of the bulk silicate Earth (BSE), and the degree of enrichment of refractory elements should be less than 1.4 times of BSE. The initial composition of LMO estimated in this study may imply that the Moon formed by the impactor and the primitive crust of proto-Earth that was enriched in Al2O3 and FeO. |
