Mineralogy and 39Ar- 40Ar age of an old pristine basalt: Thermal history of the HED parent body

Abstract

Previous investigations of mineral chemistry and Rb Sr and Sm Nd ages indicated that clast,84 from eucrite Yamato 75011 had preserved the pristine nature of its initial crystallization during an early stage of the HED parent body. Microscale mineralogy and 39Ar- 40Ar ages of this clast, however, revealed local disturbance of micrtextures and partially reset ages. This evidence suggests that, in addition to initial crystallization and rapid cooling, the Y75011,84 clast experienced shock deformation, reheating of short duration at higher temperature, and brecciation. These characteristics suggest two or more impact events. Fe-rich olivine filling fractures in pyroxene may have been introduced during the accompanying shock fracturing. The inferred 39Ar- 40Ar degassing ages for Y75011 matrix and clast,84 are 3.94 ± 0.04 Ga and 3.98 ± 0.03 Ga, respectively. The suggested degassing age for a clast from Y790020, believed to be paired with Y75011, is ∼ 4.03 Ga, but could be younger. We consider it likely that all three samples experienced a common degassing event 3.95 ± 0.05 Ga ago, but we cannot rule out two or more events spaced over a ∼ 0.1 Ga interval. Higher temperature extractions of the two clast samples show significantly older apparent ages up to ∼ 4.5 Ga and suggest that the time/temperature regime of this event was not sufficient to degas Ar totally. Most likely, the K-Ar ages were reset by thermal metamorphism associated with one or more impact events associated with shock fracturing, formation of Fe-rich olivine veins, and/or meteorite brecciation. The pyroxene annealing that commonly occurs in many eucrites is likely to be a much earlier process than the impact-produced textural changes and reset K-Ar ages observed in these meteorites. The existence of mineralogical and chronological evidence for metamorphism in an otherwise pristine eucrite suggests that the HED parent body experienced an extensive degree of early cratering.