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Introduction: Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred in the last few seconds of their deceleration in the atmosphere [1]. Although fusion crusts are ubiquitous they are rarely characterised and studied because they obscure the primary features of meteorites. Here we report the results of a study of the fusion crust of the Winchcombe CM2 chondrite. The Winchcombe meteorite fell at 21:54 hours on 28 February 2021 in Gloucestershire in the UK and was recovered over the next week. The fall was observed on UKFAll network cameras and recorded by CCTV. The meteoroid had a low entry velocity compared to other observed falls of 13.5 km/s. Study of the fusion crust reveals unique textural features that testify to previously unknown processes related to vigorous degassing of this intensely altered CM2 chondrite. Methods: Six polished blocks of Winchcombe were studied using backscattered electron imaging, elemental mapping, energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD) and micro-X-ray fluorescence (XRF). Apparent size distributions and abundances were obtained by threshold analysis using ImageJ. Results: The fusion crust consists of an inner thermally altered substrate and outer melted crust. The altered substrate exhibits unusually abundant dehydration cracks extending up to 5 mm into the meteorite. The crack network encompasses fragments up to 70 μm in diameter (dense rock equivalent) with increasing abundance with decreasing size. Loss of sheet-like habits for phyllosilicates and tochilinite testifies to progressive dehydration towards the exterior. The outer melted crust has a vesicular porphyritic texture with olivine phenocrysts and magnetite in a glassy mesostasis. Grain-size and magnetite abundance increase outwards similar to other CI/CM2 fusion crusts [2]. High Ni (<80 wt%) sulphide-metal droplets occur - often as menisci on vesicles. A magnetite rim occurs on the exterior surface and some vesicles, and include some tabular, rim-parallel magnetite crystals. Unique features in the fusion crust are oscillatory zoned olivine crystals, monolayers of magnetite and silicate warts. Monolayers form chains of magnetite crystals within the mesostasis that have tabular crystals similar to magnetite rims. EBSD data reveals [111] is parallel to the length of tabular crystals and is layer parallel in rims and monolayers. Oscillatory zoned crystals are equant with up to 4 Mg-rich zones. Silicate warts form lenticular features on the surface of the fusion crust and contain dendritic olivine - their compositions are, however, similar to the rest of the crust. Magnetite monolayers lie between warts and the underlying crust. Discussion: The unusualy high abundance of dehydration cracks suggests the tochlinite-rich matrix of the Winchcombe meteorite is particularly sensitive to dehydration, owing to the low decomposition temperature of this mineral (250oC [3]). Mechanical failure of the substrate, in part driven by gas pressure, is likely to inject large abundances of particulates into the meteoroid gas stream. Observations of episodic pulsed plasma in the trail of the fireball may be a phenomena associated with calving of the dehydrated substrate and generate thermal pulses explaining the presence of oscillatory zoning. Other features also are consistent with vigorous degassing. Magnetite monolayers appear to have formed as surface magnetite rims - owing to their similar alignment of tabular crystals. Trapping of surface magneite rims through collapse of melt protrusions is likely to explain how these layers become buried within the crust and is probably driven by perturbation of surface melt by rapid vesicle loss. Finally, silicate warts are likely to be droplets attached to the crust surface. Their dendritic textures suggest higher peak temperatures and strongly suggest they represent droplets removed from other stones in the shower. Warts represent the first discovery of intershower transport of ablation materials, possibly owing to enhanced ablation as a result of vigorous degassing. Implications: The fusion crust of the Winchcombe meteorite illustrates the complexity of processes affecting meteorites during atmospheric flight. Features such as magnetite monolayers and silicate warts have not previously been described, and may be unique to tochlinite-rich CM2 chondrites, which experience vigorous degassing. They may also allow ablation debris to be related to particular types of meteorite, thus providing a distributed record of the meteorite flux. Winchcombe underlines the utility of fusion crust, which should be routinely characterised in addition to meteorite interiors. [ABSTRACT FROM AUTHOR]

Introduction: The Mighei-like (CM) carbonaceous chondrites are the largest class of hydrated meteorites, representing collisionally derived fragments of water-rich asteroids [1,2]. Most (>95%) are breccias, whose clasts sample a range of aqueous alteration extents [3]. They can therefore act as "snapshots" recording the progression of fluidrock interaction on the CM parent body. Conversely, analysis of the material between clasts (termed cataclastic matrix) provides an opportunity to study the post-hydration history of the CM parent body, specifically its fragmentation and re-accretion. Here, we investigate both aspects of the CM chondrites' geological history through study of the newly recovered fall: Winchcombe [4, 5]. Methods: Sixteen polished sections with a total area of 190 mm2 were generated for this work. They were studied under scanning electron microscopy (SEM) using backscattered electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and electron microprobe analysis (EMPA). These sections sample the two largest masses (the main mass [320 g] and the agricultural field stone [152 g]) recovered from the Winchcombe strewnfield [4]. Results: Winchcombe is a breccia, composed of lithological clasts held within a cataclastic matrix. We identified eight distinct lithologies. Their aqueous alteration extents vary between intensely altered CM2.0 and moderately altered CM2.6 [6]. Although no lithology dominates, three rock types represent >70% of the studied area. Several lithologies contain abundant tochilinite-cronstedtite intergrowths (TCIs). Type-II forms with zoned textures are most common, typically they have Fe-rich rims ("FeO"/SiO2 wt.%: 1-5) and Mg-rich cores ("FeO"/SiO2 wt.%: < 1), however, forms with hollow cores or cores containing a mix of phyllosilicate and calcite or phyllosilciates and anhydrous silicate are also found. The cataclastic matrix represents ~15% of the studied area. It has a coarse, heterogenous texture and includes abundant subangular fragments. Fragments include the full range of CM chondrite components (e.g. Fe-sulphides, whole chondrules with or without fine-grained rims, olivine and pyroxene grains, serpentine, carbonate grains, TCI clusters, as well as coherent blocks of fine-grained matrix). The cataclastic matrix is, therefore, a complex mix of components, with both heavily altered and mildly altered phases found in close association. Another striking feature is the apparent low abundance (< 3 area%) of identifiable whole chondrules. Discussion and conclusions: Our data suggest that both anhydrous silicates and carbonates (T1a calcites) act as precursor phases for type-II TCI formation. Cross-cutting relationships allow the sequence of mineralization to be reconstructed. Initially, inward dissolution by Fe-rich and S-rich fluids forms rims composed of intermixed tochilinite and cronstedtite. In the intermediate stages of type-II TCI formation, further dissolution continues without concurrent precipitation, resulting in the formation of hollow structures. These voids were later infilled, most often by Mg-rich phyllosilicates. As alteration advanced, early-formed secondary phases became unstable and were either dissolved (e.g. T1a calcites) or chemically altered (e.g. TCI rims). The presence of numerous lithological clasts with variable aqueous alteration extents and abrupt boundaries found in close juxtaposition indicates that the cataclastic matrix formed by the deposition of fines, alongside larger fragments (the clasts), on or near the surface of the parent asteroid. Furthermore, the composition of the cataclastic matrix is consistent with formation by fragmentation and mixing of debris derived from the entire clast population. The cataclastic matrix is, therefore, interpreted as an impact-derived fallback breccia. Analysis of grain size and texture suggests that disruption of the original parent asteroid responded by intergranular fracture at grain sizes <100 µm, while larger phases, such as whole chondrules, splintered apart. Re-accretion formed a poorly lithified rubble-pile body. During atmospheric entry, the meteoroid broke apart with new fractures preferentially cutting through the weaker cataclastic matrix and thereby separating the Winchcombe meteoroid into its component-lithological clasts. Thus, the strength of the cataclastic matrix imparts a significant control on the survival of CM chondrite meteoroids. [ABSTRACT FROM AUTHOR]