Petrological and thermal histories of a lunar breccia 73217 as inferred from pyroxene crystallization sequences, exsolution phenomena, and pyroxene geothermometry

Open Access
Publisher Website
Google Scholar
Abstract
Electron probe microanalyses, X ray diffraction studies, and pyroxene geothermometry of blue‐gray breccia 73217 indicate a record of an early plutonic period and a later thermal event. The breccia contains two domains; one (breccia A) contains coarse orthopyroxene clasts, whereas the other (breccia B) contains coarse pigeonite clasts. Pyroxene trends, deduced from pyroxene clasts from breccias A and B, are analogous to the early and late fractionation stages, respectively, of terrestrial basic layered intrusions. Minor element compositions of both augite and Ca‐poor pyroxenes from breccias A and B vary continuously with FeO/(MgO + FeO) ratio. Optical and X ray observations reveal that pyroxene clasts show a great variety of exsolution textures, but that individual pyroxene grains show relatively simple and less intensive exsolution patterns. There is no overlap in mineral compositions of pyroxenes as well as plagioclase and ilmenite clasts between breccias A and B. These characteristics are consistent with a model that the coarse mineral clasts of both breccias A and B possibly originated from the same relatively small layered intrusion, i.e., breccia A being derived from an early stage (1100°‐1060°C±20°C) and B from a later stage of fractionation (1060°‐1050°C). Both breccias experienced a later common annealing event (1040°‐1070°C).