Browsing by Author "Johnson, Diane"
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Item Open Access Brecciation at the grain scale within the lithologies of the Winchcombe Mighei-like carbonaceous chondrite(Wiley, 2024-04-15) Daly, Luke; Suttle, Martin D.; Lee, Martin R.; Bridges, John; Hicks, Leon; Martin, Pierre-Etienne M. C.; Floyd, Cameron J.; Jenkins, Laura E.; Salge, Tobias; King, Ashley J.; Almeida, Natasha V.; Johnson, DianeThe Mighei-like carbonaceous (CM) chondrites have been altered to various extents by water–rock reactions on their parent asteroid(s). This aqueous processing has destroyed much of the primary mineralogy of these meteorites, and the degree of alteration is highly heterogeneous at both the macroscale and nanoscale. Many CM meteorites are also heavily brecciated juxtaposing clasts with different alteration histories. Here we present results from the fine-grained team consortium study of the Winchcombe meteorite, a recent CM chondrite fall that is a breccia and contains eight discrete lithologies that span a range of petrologic subtypes (CM2.0–2.6) that are suspended in a cataclastic matrix. Coordinated multitechnique, multiscale analyses of this breccia reveal substantial heterogeneity in the extent of alteration, even in highly aqueously processed lithologies. Some lithologies exhibit the full range and can comprise nearly unaltered coarse-grained primary components that are found directly alongside other coarse-grained components that have experienced complete pseudomorphic replacement by secondary minerals. The preservation of the complete alteration sequence and pseudomorph textures showing tochilinite–cronstedtite intergrowths are replacing carbonates suggest that CMs may be initially more carbonate rich than previously thought. This heterogeneity in aqueous alteration extent is likely due to a combination of microscale variability in permeability and water/rock ratio generating local microenvironments as has been established previously. Nevertheless, some of the disequilibrium mineral assemblages observed, such as hydrous minerals juxtaposed with surviving phases that are typically more fluid susceptible, can only be reconciled by multiple generations of alteration, disruption, and reaccretion of the CM parent body at the grain scale.Item Open Access 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(Wiley, 2023-05-15) Greenwood, R. C.; Findlay, R.; Martins, R.; Johnson, DianeAs part of an integrated consortium study, we have undertaken O, Cd, Cr, Si, Te, Ti, and Zn whole rock isotopic measurements of the Winchcombe CM2 meteorite. δ66Zn values determined for two Winchcombe aliquots are +0.29 ± 0.05‰ (2SD) and +0.45 ± 0.05‰ (2SD). The difference between these analyses likely reflects sample heterogeneity. Zn isotope compositions for Winchcombe show excellent agreement with published CM2 data. δ114Cd for a single Winchcombe aliquot is +0.29 ± 0.04‰ (2SD), which is close to a previous result for Murchison. δ130Te values for three aliquots gave indistinguishable results, with a mean value of +0.62 ± 0.01‰ (2SD) and are essentially identical to published values for CM2s. ε53Cr and ε54Cr for Winchcombe are 0.319 ± 0.029 (2SE) and 0.775 ± 0.067 (2SE), respectively. Based on its Cr isotopic composition, Winchcombe plots close to other CM2 chondrites. ε50Ti and ε46Ti values for Winchcombe are 3.21 ± 0.09 (2SE) and 0.46 ± 0.08 (2SE), respectively, and are in line with recently published data for CM2s. The δ30Si composition of Winchcombe is −0.50 ± 0.06‰ (2SD, n = 11) and is essentially indistinguishable from measurements obtained on other CM2 chondrites. In conformity with petrographic observations, oxygen isotope analyses of both bulk and micromilled fractions from Winchcombe clearly demonstrate that its parent body experienced extensive aqueous alteration. The style of alteration exhibited by Winchcombe is consistent with relatively closed system processes. Analysis of different fractions within Winchcombe broadly support the view that, while different lithologies within an individual CM2 meteorite can be highly variable, each meteorite is characterized by a predominant alteration type. Mixing of different lithologies within a regolith environment to form cataclastic matrix is supported by oxygen isotope analysis of micromilled fractions from Winchcombe. Previously unpublished bulk oxygen isotope data for 12 CM2 chondrites, when combined with published data, define a well-constrained regression line with a slope of 0.77. Winchcombe analyses define a more limited linear trend at the isotopically heavy, more aqueously altered, end of the slope 0.77 CM2 array. The CM2 slope 0.77 array intersects the oxygen isotope field of CO3 falls, indicating that the unaltered precursor material to the CMs was essentially identical in oxygen isotope composition to the CO3 falls. Our data are consistent with earlier suggestions that the main differences between the CO3s and CM2s reflect differing amounts of water ice that co-accreted into their respective parent bodies, being high in the case of CM2s and low in the case of CO3s. The small difference in Si isotope compositions between the CM and CO meteorites can be explained by different proportions of matrix versus refractory silicates. CMs and COs may also be indistinguishable with respect to Ti and Cr isotopes; however, further analysis is required to test this possibility. The close relationship between CO3 and CM2 chondrites revealed by our data supports the emerging view that the snow line within protoplanetary disks marks an important zone of planetesimal accretion.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 QEMSCAN® automated mineralogical analysis of PM2.5 and PM4: a preliminary study of underground coal mine dust from Poland and Slovenia(Frontiers, 2022-09-21) Johnson, Diane; Rollinson, Gavyn. K.; Arif, Ali Talib; Moreno, Teresa; Trechera Ruiz, Pedro; Lah, Robert; Lubosik, Zbigniew; Pindel, Thomas; Gminsk, Richard; Williamson, Ben J.Determining the physical and chemical properties of airborne dusts in occupational settings is essential for assessing their potential toxicity as well as the effectiveness of respiratory protective equipment and dust mitigation measures. Here, we report the first successful QEMSCAN® automated mineralogical analysis of potentially toxic PM4 and PM2.5 dust from deep coal mines in Poland and Slovenia. QEMSCAN® was setup to automatically delimit 100,000 ‘particles’ per sample, based on average atomic number contrast, subject these to X-ray elemental analysis at points in a grid pattern (0.5 µm spacing), assign a mineral name to each point and then output the results as particle size, shape, mineralogy and mineral associations data and as mineral maps. The dusts were prepared as dispersions on a polyethylene sheet so that coal particles, with a slightly higher BSE signal, could be recognized from their substrate. Samples were analyzed repeatedly and in different orientations to determine the effects of sample geometry and topography. QEMSCAN® mineral identifications were manually checked using standard SEM X-ray elemental analysis. From a pilot study of Polish and Slovenian coal dust samples, PM4 and PM2.5 contain varying proportions of coal, quartz and other silicates, sulphides, sulphates, carbonates, oxides and other minerals, and notable concentrations of fly-ash particles. That some of these components may be toxic when inhaled, particularly the quartz and fly-ash, highlights the need for larger scale and wider ranging studies. The further potential of the newly developed QEMSCAN® methodology is discussed.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.