Magnesium Isotopic Compositions of the Moon and Achondrites: Implications for their Origin and Evolution

The main objectives of this thesis are to estimate Mg isotopic compositions of the Moon and achondrites, to understand the behavior of Mg isotopes during magmatic differentiation processes in different planetary bodies, and to evaluate the extent of Mg isotopic heterogeneity in the solar system. In order to achieve these goals, Mg isotopes have been measured for 47 well-characterized lunar samples and 22 differentiated meteorites by MC-ICPMS. The limited Mg isotopic variations among mare and highland regolith, mare breccias, and highland impact-melt rocks reflect negligible Mg isotope fractionation during lunar surface processes (e.g. solar wind, cosmic rays, micrometeorite bombardments, meteorite impacts, etc.). However, the significant Mg isotopic variation (~0.628 / for δ26Mg) between high- and low-Ti basalts suggests the source heterogeneity produced during the lunar magmatic differentiation. High abundance of ilmenite with lighter Mg isotopic composition than coexisting olivine and pyroxene may cause lighter Mg isotopic composition of high-Ti basalts. The δ26Mg values of differentiated meteorites, including 7 types of achondrites and pallasites, range from -0.318 / to -0.183 /. The significant variation of Mg isotopic compositions of these meteorites versus their major chemical compositions suggests that the isotopic variation in achondrites is caused by different mineralogical sources produced during the magmatic differentiation of their parent bodies. Overall, the average Mg isotopic compositions of the Moon (δ26Mg = 0.259 ± 0.162 /) and achondrites (δ26Mg = -0.260 ± 0.046 /) estimated in this study are identical to those of the Earth (δ26Mg = -0.25 ± 0.07 /) and chondrites (δ26Mg = -0.28 ± 0.06 /), indicating a homogeneous Mg isotopic distribution in the solar system. Magnesium is a moderately refractory element, and unlike Fe and Si cannot be fractionated during the planetary core formation. Therefore, homogeneous distribution of its stables isotopes implies the lack of the separation and sorting of chondrules objects in protoplanetary disk during the solar system formation. It also suggests negligible Mg isotopic fractionation by volatilization during the Moon-forming giant impact.