Pyroxene as a recorder of cumulate formational processes in asteroids, Moon, Mars, Earth: Reading the record with the ion microprobe

摘要

Igneous cumulate rocks provide an important record of planetary magmatism, but there are pitfalls in their interpretation. Cumulus minerals may react with the trapped melt and other cumulus phases during subsolidus reactions, thus losing a direct record of their igneous history. One of the best approaches for estimating the melt compositions parental to the cumulates is to analyze the cores of cumulus phases for elements with slow diffusion rates (e.g., REE in pyroxene) because these most reliably retain a record of the mineral-melt partitioning. We have conducted SIMS studies of pyroxenes from a variety of planetary cumulates, including lunar norites, martian orthopyroxenites, asteroidal orthopyroxenites (diogenites), asteroidal pyroxene-plagioclase cumulates (cumulate eucrites), and terrestrial orthopyroxenites and norites (Stillwater Complex, Montana). We emphasized the REE (La, Ce, Nd, Sm, Eu, Dy, Er, Yb) along with Sr, Y, and Zr in these investigations. Our studies yielded the several conclusions. (1) Lunar Mg-suite norites crystallized from highly evolved (KREEPy) melts that were emplaced into the lunar anorthositic crust. One viable model suggests remobilization of KREEP lithologies that formed late in the crystallization of the lunar magma ocean. (2) Orthopyroxenites from asteroid 4 Vesta (diogenites) likely formed as cumulates from melts derived from depleted mantle, which had previously experienced eucritic basalt removal. The calculated parental melts show a limited range of major element compositions but an exceedingly large range of trace element variation that is difficult to explain by any simple crystallization or melting models. (3) The diogenites of 4 Vesta are cumulates from melts derived from many depleted mantle reservoirs that either formed numerous orthopyroxene plutons, later mixed by impact brecciation, or commingled by magma mixing to form a limited number of differentiated plutons. We base this conclusion on a comparative investigation of a well-studied terrestrial orthopy-roxenite sequence (Bronzitite zone of the Stillwater Complex). An important lesson to be learned from these studies in comparative planetology is that if simple petrogenetic models do not work for well-studied terrestrial occurrences there is little reason to believe that they will work for planetary environments where we have little geologic control on sampling.