Impact histories of angrites, eucrites, and their parent bodies

Abstract

Abstract-Eucrites, which are probably from 4 Vesta, and angrites are the two largest groups of basaltic meteorites from the asteroid belt. The parent body of the angrites is not known but it may have been comparable in size to Vesta as it retained basalts and had a core dynamo. Both bodies were melted early by 26 Al and formed basalts a few Myr after they accreted. Despite these similarities, the impact histories of the angrites and eucrites are very different: angrites are very largely unshocked and none are breccias, whereas most eucrites are breccias and many are shocked. We attribute the lack of shocked and unbrecciated angrites to an impact, possibly at 4558 Myr ago-the radiometric age of the younger angrites-that extracted the angrites from their original parent body into smaller bodies. These bodies, which may have had a diameter of approximately 10 km, suffered much less impact damage than Vesta during the late heavy bombardment because small bodies retain shocked rocks less efficiently than large ones and because large bodies suffer near-catastrophic impacts that deposit vastly more impact energy per kg of target. Our proposed history for the angrites is comparable to that proposed by Bogard and Garrison (2003) for the unbrecciated eucrites with Ar-Ar ages of 4.48 Gyr and that for unbrecciated eucrites with anomalous oxygen isotopic compositions that did not come from Vesta. We infer that the original parent bodies of the angrites and the anomalous eucrites were lost from the belt when the giant planets migrated and the total mass of asteroids was severely depleted. Alternatively, their parent bodies may have formed in the terrestrial planet region and fragments of these bodies were scattered out to the primordial Main Belt as a consequence of terrestrial planet formation.

Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures

“…This is consistent with the observation that angrites are less metamorphosed than eucrites (Mittlefehldt et al. ; Scott and Bottke ).…”

“…Oxygen isotope ratios of the angrites are distinct from those of any other meteorite group, indicating that the angrites are an independent meteorite group (Greenwood et al 2017). Unlike other achondrites, such as howardite, eucrite, and diogenite (HED) meteorites, the angrites show little evidence of shock (Mittlefehldt et al 2002;Scott and Bottke 2011). The angrites are subdivided into two major groups based on their petrology: finer-grained "quenched" angrites and coarser-grained "plutonic" angrites (Mittlefehldt 2003;Keil 2012).…”

Iron isotope fractionation in planetary crusts

“…All angrites (except Angra dos Reis and NWA2999) are unshocked and do not show particular impact features (Scott and Bottke, 2011). In addition, angrites are depleted in moderately volatile elements, but they are not notably more depleted in moderately volatile elements (e.g., Zn, Cd) than eucrites (Weisberg et al, 2006).…”

The early impact histories of meteorite parent bodies

“…; Krot et al. ), and the angrite and eucrite parent bodies (Scott and Bottke ). Recently, the effects of individual collisions between porous bodies have been quantified (Davison et al.…”