Complex cooling histories of lunar troctolite 76535 and Stillwater orthopyroxenite SC-936

Data from the Clementine and Prospector missions have shown that a simple model of lunar crustal stratigraphy in which an upper crust of ferroan anorthosite is underlain by a noritic lower crust is inadequate. The Moon shows a strong asymmetry in crustal thickness, composition, and distribution of KREEP and mare basalt. Some asymmetry may be primary, some may be a consequence of impacts focused on the nearside that have redistributed the upper layers, and some may be related to mantle overturn. In an effort to reconstruct the primary lunar stratigraphy, we are examining mineral and rock fragments in a subset of relatively unshocked pristine crustal samples to deduce thermal histories from which we can compute depths of burial. In this study we have applied the orthopyroxene chronometer to calculate the cooling rate from the Fe- Mg ordering states of the orthopyroxene crystals [1, 8, 10]. Sample 76535 is a large, texturally and compositionally pristine, coarse-grained olivineplagioclase cumulate that shows evidence of extensive annealing and recrystallization. The sample contains roughly equal modal volumes of olivine and plagioclase while orthopyroxene makes up ~4% of the rock. McCallum and Schwartz [2] have recently determined that this sample equilibrated at a depth of ~40 kilometers at T~800-900°C. Sample SC-936 (a pristine orthopyroxenite from the Stillwater Complex) was used as a control since it has virtually the same XFe and its thermal history is reasonably well known.