Browsing by Author "Mitchell, Jennifer T."
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Item Open Access The fusion crust of the Winchcombe meteorite: a preserved record of atmospheric entry processes(Wiley, 2023-01-02) Genge, Matthew J.; Alesbrook, Luke; Almeida, Natasha V.; Bates, Helena C.; Bland, Phil A.; Boyd, Mark R.; Burchell, Mark J.; Collins, Gareth S.; Cornwell, Luke T.; Daly, Luke; Devillepoix, Hadrien A. R.; van Ginneken, Matthias; Greshake, Ansgar; Hallatt, Daniel; Hamann, Christopher; Hecht, Lutz; Jenkins, Laura E.; Johnson, Diane; Jones, Rosie; King, Ashley J.; Mansour, Haithem; McMullan, Sarah; Mitchell, Jennifer T.; Rollinson, Gavyn; Russell, Sara S.; Schröder, Christian; Stephen, Natasha R.; Suttle, Martin D.; Tandy, Jon D.; Trimby, Patrick; Sansom, Eleanor K.; Spathis, Vassilia; Willcocks, Francesca M.; Wozniakiewicz, Penelope J.Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.Item Open Access The Winchcombe meteorite: a regolith breccia from a rubble pile CM chondrite asteroid(Wiley, 2022-12-21) Suttle, Martin D.; Daly, Luke; Jones, Rhian H.; Jenkins, L.; van Ginneken, Matthias; Mitchell, Jennifer T.; Bridges, J. C.; Hicks, L. J.; Johnson, DianeThe 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.