End of magmatism in the upper crust of asteroid 4 Vesta

Abstract

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 40 Ar/ 39 Ar 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 µm 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 39 Ar) or mini-plateau (50-70% of 39 Ar) 40 Ar/ 39 Ar ages with a weighted mean of 4531 AE 6 Ma (P = 0.67). Thermodynamic cooling and 40 Ar 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~35 Ma after accretion. 620F. Jourdan et al.

Petrology and mineralogy of mesosiderite Northwest Africa 12949: Implications for geological history on its parent body

“…Mesosiderites have experienced complicated thermal histories. The well-accepted formation model of mesosiderites (e.g., Rubin, 1997;Rubin & Mittlefehldt, 1993;Scott et al, 2001) can be divided into the following stages: (1) accretion and initial crystallization of mesosiderite silicates at 4.56-4.47 Ga (Haba et al, 2019;Stewart et al, 1994;Wadhwa et al, 2003); (2) collisional disruption and intense brecciation (Haba et al, 2017;Jourdan et al, 2021);…”

“…Mesosiderites have experienced complicated thermal histories. The well‐accepted formation model of mesosiderites (e.g., Rubin, 1997; Rubin & Mittlefehldt, 1993; Scott et al., 2001) can be divided into the following stages: (1) accretion and initial crystallization of mesosiderite silicates at 4.56–4.47 Ga (Haba et al., 2019; Stewart et al., 1994; Wadhwa et al., 2003); (2) collisional disruption and intense brecciation (Haba et al., 2017; Jourdan et al., 2021); (3) mixing of metal and silicates and rapid cooling (Delaney et al., 1981; Ganguly et al., 1994; Ruzicka et al., 1994; Tamaki et al., 2006); (4) deep burial and slow cooling (Bogard & Garrison, 1998; Haack et al., 1996; Hopfe & Goldstein, 2001; Yang et al., 1997); and (5) impact excavation and ejection at <4 Ga (Bogard, 2011; Bogard et al., 1990; Kring & Cohen, 2002). However, several key processes of mesosiderite formation have remained unclear, including the number of impact and reheating events, the time of metal and crustal silicate mixing, the degree and the peak temperatures of thermal metamorphism, and the emplacement depths in each thermal event (e.g., Caves, 2019; Pittarello et al., 2019; Sugiura et al., 2022).…”