The number of parent bodies sampled by the CM chondrites, as well as their (former) size and structure, is currently unknown (Cournede et al., 2015 Lee, Cohen, King, et al., 2019 Vernazza et al., 2016). The Mighei-like (CM) carbonaceous chondrites are the largest class of hydrated meteorites, representing collisionally derived fragments of water-rich asteroids (Browning et al., 1996 McSween, 1979 Suttle et al., 2021 Tomeoka & Buseck, 1985). Winchcombe's unweathered state and diversity of lithologies make it an ideal sample for exploring the geological history of the CM chondrite group. The strength of the cataclastic matrix imparts a control on the survival of CM chondrite meteoroids. During atmospheric entry, the Winchcombe meteoroid broke apart with new fractures preferentially cutting through the weaker cataclastic matrix and separating the breccia into its component clasts. Re-accretion formed a poorly lithified body. Its grain size and texture suggest that the disruption of the original parent asteroid responded by intergranular fracture at grain sizes <100 μm, while larger phases, such as whole chondrules, splintered apart. This material is impact-derived fallback debris. In Winchcombe, the fragments of primary accretionary rock are held within a cataclastic matrix (~15 area%). We suggest that this is due to early alteration under localized geochemical microenvironments creating a diversity of compositions and because later reprocessing was incomplete, leaving a record of the parent body's fluid history. Their compositions can vary significantly, even within a single lithology, which can prevent a clear assessment of alteration extent if only TCI composition is considered. Tochilinite–cronstedtite intergrowths (TCIs) are common in several lithologies. Although no lithology dominates, three heavily altered rock types (CM2.1–2.3) represent >70 area%. The degree of aqueous alteration varies between intensely altered CM2.0 and moderately altered CM2.6. The Winchcombe meteorite is a CM chondrite breccia composed of eight distinct lithological units plus a cataclastic matrix.

Chondrite meteorite