End Of Magmatism In The Upper Crust Of Asteroid 4 Vesta

Asteroid 4 Vesta is the only largely preserved differentiated asteroid and thus is an excellent proxy to study early magmatism occurring on planets and moons. In this study, we focus on eucrite Pecora Escarpment (PCA) 82502, a medium- to fine-grained eucrite which chemical analyses suggest belongs to the main howardite-eucrite-diogenite clan, albeit with some peculiarities. We carried out backscattered electron and electron backscattered diffraction microscopy analyses of the meteorite along with step-heating Ar-40/Ar-39 dating analyses of various types of groundmass aliquots. We show that Pecora Escarpment 82502 is composed of medium-grained igneous crystalline clasts and smaller fractured satellite clasts surrounded by approximately 50 mu m wide impact melt veins of the same composition. Our results show that the large crystalline clasts and fine-grained veins both display little evidence of shock processes. Six groundmass aliquots from large crystalline clasts returned concordant plateau (>70% of Ar-39) or mini-plateau (50-70% of Ar-39) Ar-40/Ar-39 ages with a weighted mean of 4531 +/- 6 Ma (P = 0.67). Thermodynamic cooling and Ar-40 diffusion models suggest that the K/Ar system recorded and preserved the igneous age despite subsequent infiltration of hot and quickly quenched melt veins. Our new igneous age, combined with evidence for four other young volcanic and plutonic eucrites of similar age, shows that Vesta was still magmatically active around 4531 Ma. The lack of younger ages suggests that this age might well represent the end of the magmatic activity in the upper crust of Vesta. When combined with existing paleomagnetic constraints, our data suggest that 4 Vesta had an active dynamo that was still active similar to 35 Ma after accretion.