The Winchcombe meteorite—A regolith breccia from a rubble pile CM chondrite asteroid

Abstract

The Winchcombe meteorite is a CM chondrite breccia composed of eight distinct lithological units plus a cataclastic matrix. The degree of aqueous alteration varies between intensely altered CM2.0 and moderately altered CM2.6. Although no lithology dominates, three heavily altered rock types (CM2.1-2.3) represent >70 area%. Tochilinite-cronstedtite intergrowths (TCIs) are common in several lithologies. 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. 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. In Winchcombe, the fragments of primary accretionary rock are held within a cataclastic matrix (~15 area%). This material is impact-derived fallback debris. 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. Reaccretion formed a poorly lithified body. During atmospheric entry, the Winchcombe meteoroid broke apart with new fractures preferentially cutting through the weaker cataclastic

The formation and aqueous alteration of CM2 chondrites and their relationship to CO3 chondrites: A fresh isotopic (O, Cd, Cr, Si, Te, Ti, and Zn) perspective from the Winchcombe CM2 fall

“…However, as a first approximation, Figure 6 can be used as a classification tool to define a subtype for a particular CM sample. As can be seen from Figure 6, there is good agreement between the petrologic subtypes for Winchcombe defined on the basis of mineralogical criteria (Suttle et al, 2022) and the subtype based on D 17 O composition.…”

“…BM.2022, M1-85 (original mass 8.846 g) and BM.2022, M1-86 (original mass 6.931 g) were both recovered from the Wilcock family driveway and represent pieces from the main mass, which had a total recovered mass of ~320 g (King et al, 2022). In contrast, BM.2022, M9-18 ("Field Stone") comprises gram-sized interior chips recovered from the single 152 g fusion crusted stone located in the fields around Rushbury House Farm (Suttle et al, 2022). This stone broke during collection allowing interior material to be sampled.…”

The water content of CM carbonaceous chondrite falls and finds, and their susceptibility to terrestrial contamination

“…However, this suggestion cannot be readily confirmed because none of the published petrologic classifications include all of the falls and finds discussed here. In addition, several of the falls are breccias containing clasts with different degrees of aqueous alteration (i.e., Aguas Zarcas, Kerraouch et al., 2021; Cold Bokkeveld, Lentfort et al., 2021; Winchcombe, King et al., 2022; Suttle et al., 2022). Thus, milligram‐size samples of the same meteorite may differ significantly in phyllosilicate abundance.…”

Insoluble macromolecular organic matter in the Winchcombe meteorite

“…The classification of Winchcombe as a CM meteorite was based on general elemental abundances (King et al, 2022). Detailed scanning electron microscope energy dispersive x-ray analysis of Winchcombe samples (Suttle et al, 2023) has shown that several lithologies are present, with the most common ranging from CM2.0 (most processed) to CM2.5 (least processed). This is consistent with the results found here by the Raman analysis of the carbon bands, which produced results similar to those from the Murray meteorite (type CM2.4/2.5), and with the stepped combustion and pyrolysis-GC-MS data.…”