FORMATION, EQUILIBRATION, AND EVOLUTION OF LUNAR METEORITES AND METAMORPHIC ROCKS (Invited Presentation)

Lunar meteorites and metamorphic rocks record structural and compositional information at the micron to nanometer scale that can be used to understand broader geologic processes. These grain boundary processes likely control much of the formation and evolution of meteorites and metamorphic rocks, from mineral crystallization and equilibration to serving as a record of impact and tectonic histories, respectively. This talk will focus on understanding fundamental metamorphic and meteorite forming processes by analyzing microstructures, compositions, and textures.

Lunar meteorite Northwest Africa (NWA5000) is a polymict leucogabbronorite breccia that is unique in its high concentrations (2%) of metal. Mineralogically, NWA5000 contains >200µm anorthite (An96) fragments that are interpreted to be unmelted, refractory lunar highlands material surrounded by a ‘melt-rock’ texture of subophitic anorthite (An92), subhedral olivine, and interstitial augite and pigeonite. It is likely that NWA5000 formed via a series of impacts, first a large metal rich asteroid melted the lunar highlands, concentrating the metal, followed by a second impact, which brecciated the melt deposit, prior to the meteorite being ejected from the surface of the Moon to the Earth.

Under increasing pressure and temperature conditions, metamorphic rocks recrystallize such that their mineralogy and mineral compositions reflect the lowest energy state of the system. However, the atomic scale processes by which minerals recrystallize and equilibrate is not well understood. Garnet specifically, is an important mineral, as its composition reflects changing equilibration conditions in the crust, leading to its utility in interpreting the tectono-metamorphic evolution of terranes. Microstructural data on garnet-bearing metapelites reveals that garnet crystallizes with one of three crystal directions parallel to the foliation of the rock. Atomic-scale crystal structure models show that the crystal structure of garnet and muscovite (specifically the spacing and orientation of Al and Si atoms) is similar enough that garnet could crystallize by templating on the crystal structure of muscovite. This is then interpreted as an epitaxial relationship which reduces garnet’s energy barrier to nucleation.