Extended time scales of carbonaceous chondrite aqueous alteration evidenced by a xenolith in LaPaz Icefield 02239 (CM2)

Abstract

LaPaz Icefield (LAP) 02239 is a mildly aqueously altered CM2 carbonaceous chondrite that hosts a xenolith from a primitive chondritic parent body. The xenolith contains chondrules and calcium-and aluminum-rich inclusions (CAIs) in a very finegrained matrix. The chondrules are comparable in mineralogy and oxygen isotopic composition with those in the CMs, and its CAIs are also mineralogically similar to the CM population apart for being unusually small and abundant. The presence of serpentine demonstrates that the xenolith has been aqueously altered, and its phyllosilicate-rich matrix has a comparable oxygen isotopic composition to the matrices of CM meteorites. The xenolith's chondrules lack fine-grained rims, whereas the xenolith itself has a fine-grained rim that is petrographically and chemically comparable with the rims on coarse grained objects in LAP 02239 and other CM meteorites. These properties show that the xenolith's parent body was formed from similar materials to the CM parent body(ies). Following its lithification by aqueous alteration, a piece of the xenolith's parent body was impact-ejected, acquired a fine-grained rim while free-floating in the protoplanetary disc, then was accreted along with rimmed chondrules and other materials to make the LAP 02239 parent body. Subsequent aqueous processing of the LAP 02239 parent body altered the fine-grained rims on the xenolith, chondrules, and CAIs. The xenolith shows that the timespan of geological evolution of carbonaceous chondrite parent bodies was sufficiently long for some of them to have been aqueously altered before others had formed.

Brecciation at the grain scale within the lithologies of the Winchcombe Mighei‐like carbonaceous chondrite

“…Brecciation and reaccretion is an important process that affected many CM chondrites (Bischoff et al., 2017; Lee, Floyd, et al., 2023; Lentfort et al., 2021; Lindgren et al., 2013). Brecciated CMs contain distinct CM lithological clasts juxtaposed with other CM clasts generally with different textures, proportions of chondrules matrix and TCI‐like objects and aqueous alteration histories (Bischoff et al., 2017; Lee, Floyd, et al., 2023; Lentfort et al., 2021; Lindgren et al., 2013). Some clasts can have their own smaller discrete clasts present within them (Lentfort et al., 2021).…”

“…Thus, the primary mineralogy and textures acquired from the accretion of their parent body(ies) have been largely overprinted. Additionally, the majority of CM meteorites are breccias and experienced extensive disaggregation and reaccretion on the regolith of asteroids (Bischoff et al., 2017; Lee, Floyd, et al., 2023; Lentfort et al., 2021; Lindgren et al., 2013; Suttle et al., 2022; Verdier‐Paoletti et al., 2019).…”