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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. Re‐accretion formed a poorly lithified body. 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. The strength of the cataclastic matrix imparts a control on the survival of CM chondrite meteoroids. Winchcombe's unweathered state and diversity of lithologies make it an ideal sample for exploring the geological history of the CM chondrite group. [ABSTRACT FROM AUTHOR]

Introduction: All of the CM carbonaceous chondrites contain carbonates, which formed by parent body aqueous alteration at ~4,563.4 Ma [1]. They show a considerable range in mineralogy and chemical composition, which tracks the degree of alteration of the host meteorite [2]. Calcite is the most abundant mineral and occurs in all CMs. Aragonite has been recorded only from the mildly altered meteorites, and dolomite in the more highly altered ones [3]. Ankerite and breunnerite have been described just from Queen Elizabeth Range (QUE) 93005 [4]. The carbon and oxygen stable isotopic compositions of carbonates provide a 'snapshot' of the properties of precipitating fluids, e.g., their provenance, extent of interaction with the host rock, and temperature. However, in order to construct a narrative of parent body aqueous alteration these snapshots need to be placed in the correct sequence. The relative ages of the different minerals have been difficult to determine because carbonates are scarce such that intergrowths or cross-cutting relationships are rare. Here we report petrographic and microstructural observations from Winchcombe, a recent CM fall, which provide good evidence for the sequence of carbonate mineralization. Materials and methods: Winchcombe fell on 28th February 2021 in Gloucestershire, England. It is a breccia of CM lithologies that range from completely hydrated (CM2.0) to moderately altered (CM2.6) [5]. We studied carbonates in C-coated thin sections P30540, P30552 and P30555 loaned by the Natural History Museum, London. P30540 and P30552 were studied at the University of Glasgow using a Zeiss Sigma field-emission SEM, and P30555 at Oxford Instruments (OI) using a Hitachi SU70 SEM equipped with an OI Symmetry S2/UltimMax 65 EBSD/EDS detector. EBSD data were acquired with OI Aztec v6.0 software and processed using OI AZtecCrystal v2.3. Results: Thin section P30555 contains both aragonite and calcite, with the latter being more abundant. In several areas the two minerals are juxtaposed. The calcite occurs as a cluster composed of small, undeformed grains with faceted shapes. By contrast the aragonite displays prominent deformation microstructures - twins (some curved), intragranular misorientation, and subgrains. Low angle boundaries show rotation about <100>, and weighted Burgers vectors are preferentially parallel to <001> and <010>. P30552 contains mainly calcite, and many grains show cathodoluminescence zoning indicative of crystal growth into fluid-filled pores. Most grains are rimmed by fibers of coherently interstratified tochilinite/serpentine, but some have been completely replaced by tochilinite/serpentine fibers intergrown with finely crystalline Mg-serpentine. P30540 contains both calcite and dolomite, and one composite grain has a core of calcite that is rimmed by phyllosilicate and overgrown by euhedral dolomite crystals. Discussion: Our observations indicate that the Winchcombe carbonates formed in at least three discrete episodes. Aragonite was first to crystallize then was deformed, probably by an impact, and its microstructures are consistent with dislocation creep. As adjacent calcite is undeformed, it must have grown after aragonite and the impact event. Calcite was then partially or completely replaced by tochilinite and serpentine. Subsequently, fluids became supersaturated with respect to dolomite, which in some cases precipitated as an overgrowth on calcite-phyllosilicate substrates. The findings from Winchcombe are consistent with oxygen isotopic data from the CM Lonewolf Nunataks 94101 showing that aragonite grew before calcite [6]. They also agree with interpretations of Δ17O and clumped isotope temperature measurements of carbonates in the CM Allan Hills 83100 indicating that dolomite precipitated after calcite and from fluids that were hotter and had interacted more with anhydrous silicates [7]. A caveat is that multiple generations of same carbonate mineral may have formed in Winchcombe, in common with other CMs [8, 9], and more observations are needed in order to test this model of carbonate mineralization. Nonetheless, Winchcombe shows how petrographic and microstructural studies can reveal the mineralization history of the CMs during aqueous alteration. Acknowledgements: Thanks to the UK meteorite community for the successful recovery of the Winchcombe meteorite, and the Natural History Museum London for loan of the samples. This work was funded by the UK STFC through grant ST/T002328/1. [ABSTRACT FROM AUTHOR]

Introduction: Winchcombe is a CM2 (Mighei-like) carbonaceous breccia composed of eight main lithologies within a cataclastic matrix. These lithologies display varying degrees of aqueous alteration, spanning from 2.0-2.6 [1], according to Rubin's scale [2, 3]. CM chondrites are amongst the most common carbonaceous meteorite samples, yet the abundance of Calcium-Aluminium-rich Inclusions (CAI) within them remains poorly constrained. This study is focused on describing and studying the CAI populations found within the Winchcombe lithologies and how they inform our understanding of the formation and evolution of the CM parent bodies. Methods: CAIs were sought in 18 polished blocks of Winchcombe using sample-wide Backscattered Electron images combined with Ca-Al-Mg Energy-Dispersive X-ray Spectroscopy (EDS) maps. The samples were initially mapped by Scanning Electron Microscopy (SEM) at the Natural History Museum (NHM), London (20 kV/3 nA, live frame time 270s). Three sections (P30540, P30548, and P30552) were further studied to confirm the mineralogy of the CAIs using a Zeiss Sigma Variable Pressure Analytical SEM (20 kV) at the University of Glasgow (UoG), and a Cameca SX100 Electron-Probe Micro-Analyser (EPMA; 15 kV) at the NHM. Another section (P30542) was also further analysed using a Hitachi S3400 SEM (20 kV/20nA) and an Oxford Instrument X-Max SSD Energy Dispersive X-Ray (EDX) spectrometer at the University of Kent (UoK). Results: The Winchcombe lithologies have a diverse population of CAIs. They display a wide range of alteration states from near-completely altered (heavily calcitised; cf. Fig. 1) to well preserved with a quasi-spherical intact pyroxene rim. The polished blocks contain 18 confirmed CAIs, with 22 potential CAIs requiring further investigation. Most of the CAIs fall into two categories: (1) spinel-rich with a relatively-well preserved diopside rim (Spinel-Pyroxene, [2, 3, 4]); (2) a forsterite core with ~2 µm rounded grains of perovskite (Pyroxene-Olivine, [2, 3, 4]). Calcitised CAIs seem to be outliers among the observed CAI populations. They are heavily altered and are composed of small Ti-Ca-rich perovskite grains (<10 µm) and long globular Mg-Al-rich spinel (~ 10 µm) within a predominantly calcite matrix, that are, in a few cases, surrounded by a discontinuous diopside rim. Discussion: Calcitised CAIs occur in CM lithologies of very different degrees of alteration [5] and therefore cannot provide context as to the extent of aqueous alteration processes the parent-body or the meteorite itself underwent. The calcite-forming process would have had to occur early as these specimens are visible even in lithologies with lower degrees of aqueous alteration. However, these objects do remain relevant as calcite might originate from the alteration of melilite or anorthite, which could grant us the necessary clues to determining the origin of the many lithological constituents of the meteorite. Future work will focus on the link between CAI abundance relative to the degree of aqueous alteration of the CM lithologies, and will include additional sample-wide EDS mapping, Electron Backscatter Diffraction (EBSD) maps of CAIs, as well as Transmission Electron Microscopy (TEM) analyses of sections of the grossmanite grains within the calcitised CAI. [ABSTRACT FROM AUTHOR]

Introduction: CM carbonaceous chondrites are among the most chemically primitive meteorites, yet are also some of the most aqueously altered [1,2]. This pervasive parent body processing destroys much of the primary mineralogy and texture [2]. Nano-scale investigations indicate that the permeability of aqueously altered carbonaceous chondrites is low, permitting fluid flow only over distances <100 µm [3], and that aqueous alteration is locally heterogeneous [4]. In contrast, recent observations of large veins of carbonate on the B-type asteroid Bennu [5], which has spectroscopic similarities to CMs [6], suggest macro-scale movement of fluids. On the 28th February 2021 a bright fireball was observed by the UK Fireball Alliance (UKFall) [7]. This resulted in the recovery of the Winchcombe meteorite 12 hours later [7]. It is a CM chondrite breccia with eight distinct lithologies, with variable alteration histories ranging from CM2.0-CM 2.6 plus a cataclastic matrix [8]. The limited terrestrial alteration of Winchcombe, combined with the orbital parameters determined by UKFAll [7], make it an ideal sample to explore macro-nanoscale constraints on aqueous alteration of CM chondrites. Here, we present the key results of the investigations undertaken by the fine-grained sub-team of the Winchcombe consortium study. Methods: Several rock chips and petrographic sections of the Winchcombe meteorite were analysed using X-ray computed tomography (XCT), electron probe microanalysis (EPMA), scanning electron microscopy (SEM) techniques (including secondary electron (SE) and backscatter electron (BSE) imaging, energy dispersive X-ray spectrometry (EDS), QEMSCAN, electron backscatter diffraction (EBSD)), focused ion beam (FIB) microscopy (including FIB based time of flight secondary ion mass spectrometry (TOF-SIMS) transmission electron microscopy (TEM) techniques (including, HAADF, EELS, STEM, EDS, XANES), atom probe tomography (APT). Permeability at the macroscale were established via numerical simulations. Key results: XCT data indicate that some Winchcombe lithologies have a preferred alignment of relic ellipsoidal chondrules defining a foliation fabric. These foliations are in separate orientations in different lithologies from the same rock chip. Some lithologies also exhibit a 'fracture cleavage' defined by sub-parallel fracture sets. Numerical simulations indicate that Winchcombe has an anisotropic permeability with a much lower permeability along one axis. SEM imaging reveals a range from complete to incomplete replacement of carbonate and silicate phases in some tochilinite cronstedtite intergrowths (TCIs) with some carbonate-TCI grains separated by only a few 100 µm. EPMA data show a moderate spread of compositions within the matrix, fine-grained rims (FGR), and TCIs within each lithology. TEM investigations of an FGR around a chondrule within a subtype 2.2 lithology reveal the presence of inclusions that resemble glass with embedded metal and sulphide (GEMS). Coordinated TOF-SIMS and APT data indicate that Na is concentrated at the boundary between Mg-rich and Fe-rich serpentine intergrowths in TCIs. Discussion and conclusions: At the macro-scale, XCT results and numerical simulations show that fluids flowed more readily along a 2D plane and less-readily along the pole to that plane. Such an anisotropic permeability network may be generated via compaction or through precipitation of minerals during aqueous alteration, and would serve to limit fluid transfer, resulting in the segregation of fluid compositions which may enhance the preservation of pockets of unaltered material. At the nanoscale, the survival of both GEMS-like phases that are readily altered by minor degrees of aqueous alteration, and the heterogeneity in the extent of carbonate alteration to TCIs over short distances, suggest that regions within otherwise severely aqueously altered regions of the Winchcombe meteorite experienced little fluid interaction. This was likely caused by local variations in permeability on the Winchcombe parent. The cause of this lower permeability that limited fluid migration to reactive surfaces could be a primary texture, a compaction texture, an impact texture, or generated by progressive alteration. Therefore, even in pervasively aqueously altered meteorites it is likely that some nano-macro scale volumes can preserve their primary mineralogy and texture. [ABSTRACT FROM AUTHOR]

The remarkable physical properties of dental enamel can be largely attributed to the structure of the hydroxyapatite (HAp) crystallites on the sub-micrometre scale. Characterising the HAp microstructure is challenging, due to the nanoscale of individual crystallites and practical challenges associated with HAp examination using electron microscopy techniques. Conventional methods for enamel characterisation include imaging using transmission electron microscopy (TEM) or specialised beamline techniques, such as polarisation-dependent imaging contrast (PIC). These provide useful information at the necessary spatial resolution but are not able to measure the full crystallographic orientation of the HAp crystallites. Here we demonstrate the effectiveness of enamel analyses using transmission Kikuchi diffraction (TKD) in the scanning electron microscope, coupled with newly-developed pattern matching methods. The pattern matching approach, using dynamic template matching coupled with subsequent orientation refinement, enables robust indexing of even poor-quality TKD patterns, resulting in significantly improved data quality compared to conventional diffraction pattern indexing methods. The potential of this method for the analysis of nanocrystalline enamel structures is demonstrated by the characterisation of a human enamel TEM sample and the subsequent comparison of the results to high resolution TEM imaging. The TKD - pattern matching approach measures the full HAp crystallographic orientation enabling a quantitative measurement of not just the c-axis orientations, but also the extent of any rotation of the crystal lattice about the c-axis, between and within grains. Results presented here show how this additional information highlights potentially significant aspects of the HAp crystallite structure, including intra-crystallite distortion and the presence of multiple high angle boundaries between adjacent crystallites with rotations about the c-axis. These and other observations enable a more rigorous understanding of the relationship between HAp structures and the physical properties of dental enamel., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Patrick Trimby reports a relationship with Oxford Instruments NanoAnalysis that includes: employment., (Copyright © 2024 Elsevier B.V. All rights reserved.)