A primitive asteroid that lived fast and died young sampled by a xenolith in the Cold Bokkeveld CM2 carbonaceous chondrite

Abstract

Xenoliths in carbonaceous chondrites include lithologies that are unrepresented in the meteorite record and so are a rich source of information on asteroid diversity. Cold Bokkeveld is a CM2 regolith breccia that contains both hydrous and anhydrous lithic clasts. Here, we describe a hydrous clast with a fine-grained rim. This rim shows that the clast is a xenolith that interacted with dust in the protoplanetary disk between liberation from its protolith and incorporation into Cold Bokkeveld's parent body. Prior to its fragmentation, the xenolith's protolith had undergone brittle deformation, with the fractures produced being cemented by carbonates to make veins. After being incorporated into Cold Bokkeveld's parent body, the veined xenolith experienced a second phase of aqueous alteration leading to hydration of its fine-grained rim, replacement of carbonate by tochilinite–cronstedtite intergrowths, and formation of magnetite within its fine-grained matrix. The veined xenolith's protolith underwent its entire geological evolution (accretion–aqueous alteration–fracturing–fragmentation) before Cold Bokkeveld's parent body had accreted. Such a short lifespan may be explained by explosive breakup of the protolith due to overpressure from gases produced internally during water–rock interaction. Early fragmentation effectively acted as a thermostat to limit runaway heating that may have otherwise resulted from the body's high concentrations of ²⁶Al. Many other hydrous lithic clasts in CM carbonaceous chondrite meteorites could be the remains of such ephemeral early asteroids, but they are hard to identify without evidence that they were accreted as hydrous lithologies and contemporaneously with chondrules.