Thermal and impact metamorphism on the HED parent asteroid

Abstract

The bulk texture and composition of four monomict eucrites, five polymict eucrites, and one howardite, as well as those of 16 separated clasts and lithological units from these samples were analyzed by optical and scanning electron microscopy and by electron microprobe. Bulk chemical compositions were obtained by INAA. The monomict eucrites Stannern, Millbillillie, Camel Donga, and Juvinas are recrystallized monomict breccias that probably originate from brecciated crater floors or ejecta blocks. The texture of igneous clasts from Juvinas can be explained by interactions of impact and igneous activity that led to disturbance of magma crystallization. Due to the presence of lithic clasts with highly variable chemical compositions and the occurrence of both equilibrated and unequilibrated pyroxenes, the monomict eucrite Pasamonte is redescribed as a polymict eucrite. Three clasts of impact-related lithologies in the polymict eucrite Pasamonte and the howardite EET 87503 contain considerable amounts of chondritic projectile contaminations. The textures of the investigated meteorites reflect a complex post-igneous history dominated by multistage thermal and impact metamorphism. The chronological sequence of thermal and impact events comprises up to six evolutionary phases. Phase I represents crystallization of primary magmas that led to the formation of unequilibrated basalts and other igneous rocks. Phase II represents slow subsolidus cooling or a period of reheating during which pyroxene equilibrated. Phases III and V represent periods of impact brecciation during which the rocks were brecciated in situ or, in the case of polymict HED breccias, mixed with various other rock types. During Phases IV and VI the breccias suffered annealing and recrystallization due to thermal metamorphism. The thermal events that caused recrystallization and equilibration of HED lithologies were active prior, during, and after the formation of impact breccias, indicating that the thermal input by impact might be responsible for thermal overprinting.