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Recent Submissions
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An evaporite sequence from ancient brine recorded in Bennu samplesAbstract: Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth<jats:sup>1</jats:sup> and detected by remote sensing on icy bodies in the outer Solar System<jats:sup>2,3</jats:sup>. The mineralogical evolution of these brines is well understood in regard to terrestrial environments<jats:sup>4</jats:sup>, but poorly constrained for extraterrestrial systems owing to a lack of direct sampling. Here we report the occurrence of salt minerals in samples of the asteroid (101955) Bennu returned by the OSIRIS-REx mission<jats:sup>5</jats:sup>. These include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine that existed early in the history of Bennu’s parent body. Discovery of diverse salts would not be possible without mission sample return and careful curation and storage, because these decompose with prolonged exposure to Earth’s atmosphere. Similar brines probably still occur in the interior of icy bodies Ceres and Enceladus, as indicated by spectra or measurement of sodium carbonate on the surface or in plumes<jats:sup>2,3</jats:sup>.
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An overview of the petrography and petrology of particles from aggregate sample from asteroid BennuAbstract: The OSIRIS‐REx mission returned a sample of regolith from the carbonaceous asteroid Bennu in September 2023. We present preliminary in situ investigations of the petrology and petrography of selected particles ranging in size from 0.5 to 3 mm. Using a combination of optical and electron beam techniques, we investigate whole specimens and polished sections belonging to morphologically and visually distinct categories of particles. We find that morphological differences in the particles are reflective of petrographic and petrologic differences, leading to the conclusion that we have at least two distinct major lithologies in the bulk sample. Our findings support predictions from remote sensing, suggesting that the morphological differences observed in the boulder population of Bennu correspond to petrologic differences. Our data provide insight into the geologic activity on Bennu's parent body and the petrographic framework needed to contextualize the detailed analyses of this pristine asteroidal material.
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Origin of the Jadar Volcano-Sedimentary Li-B Deposit, SerbiaAbstract: The Jadar deposit (Serbia) is a unique end member of the volcano-sedimentary Li deposit class, where the main ore mineral is jadarite (LiNaSiB3O7(OH)), to date only recorded at the Jadar locality. We provide the first account of the features of the Jadar deposit based on the study of drill hole material, complemented by petrographic analysis, whole-rock, and isotopic geochemistry. The Li-B mineralization is hosted by sediments interlayered with tuffs that were deposited in a fault-bounded lacustrine basin. Mineralization processes initially involved alteration of volcanic glass through near-neutral and heated meteoric fluids, which resulted in the formation of hectorite. Lithium clays were later overprinted by higher-pH brines, which underwent significant evaporation, combined with conditions of Na+ and SiO2(aq) saturation, resulting in formation of siliceous gels that were the precursors to jadarite formation and extensive zeolitization of the intrabasinal sediments. Formation of jadarite occurred in two stages: (1) early diagenesis accompanied by zeolitization and precipitation of primary dolomite, which resulted in the sediment-hosted Li mineralization, (2) late remobilization of Li(B-Na)–saturated fluids, leading to formation of epigenetic jadarite-albite infills of fractured rocks. Isotopic geochemistry combined with mineral chemistry proxies indicate that early mineralizing processes were largely dominated by diagenetic fluids that experienced a thermal influence from the coeval cooling of the Miocene S-type granite member of the nearby Cer Mountain Complex. Late thermal maturation of organic matter then resulted in acidic fluids that dissolved the early ore assemblage and led to formation of late diagenetic lithiophosphate and low-Na borates.
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Facilitating high throughput collections-based genomics: a comparison of DNA extraction and library building methodsWhile DNA barcoding methods are an increasingly important tool in biological conservation, the resource requirements of constructing reference libraries frequently reduce their efficacy. One efficient way of sourcing taxonomically validated DNA for reference libraries is to use museum collections. However, DNA degradation intrinsic to historical museum specimens can, if not addressed in the wet lab, lead to low quality data generation and severely limit scientific output. Several DNA extraction and library build methods that are designed to work with degraded DNA have been developed, although the ability to implement these methods at scale and at low cost has yet to be formally addressed. Here, the performance of widely used DNA extraction and library build methods are compared using museum specimens. We find that while our selected DNA extraction methods do not significantly differ in DNA yield, the Santa Cruz Reaction (SCR) library build method is not only the most effective at retrieving degraded DNA from museum specimens but also easily implemented at high throughput for low cost. Results highlight the importance of lab protocol on data yield. An optimised “sample to sequencing” high-throughput protocol which incorporates SCR is included to allow for easy uptake by the wider scientific community.
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Petrography, geochemistry and mineralogy of the Stonehenge Sarsens: Digital data collectionThis collection includes a suite of digital materials that, in combination, characterise the petrography, mineralogy and geochemistry of a sarsen upright (Stone 58) from the central trilithon horseshoe at Stonehenge. The collection arises from work undertaken during the British Academy/Leverhulme Trust project “Geochemical fingerprinting the sarsen stones at Stonehenge” (Small Research Grant SG-170610), led by the University of Brighton. As part of the project, permission was granted by English Heritage to sample a section from one of three cores drilled through the full thickness of Stone 58 during conservation work in 1958. This core had been returned to the UK from Florida in 2018 by Mr Robert Phillips, an employee of Van Moppes (Diamond Tools) Ltd, Basingstoke, who had been at Stonehenge during the drilling work. Mr Phillips was granted permission by the Ministry of Works to retain the core on behalf of the company and was gifted it by Van Moppes on his retirement to the USA. This core – referred to as the Phillips’ Core – is now held in the English Heritage Collections Store at Temple Cloud (Bath, UK). The Phillips’ Core is 108cm long, has a 2.5cm diameter and is broken into six pieces ranging in length from 7 to 29cm. The digital materials within this collection result from the analysis of section 2-3 of the core, from 29 to 36cm along its length. Full details of sampling, analytical approaches and interpretation are provided in Nash et al. (2021) (see Metadata).
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Crustal structure of the Western Azuero Peninsula, Panama: Insights into the structure of accretionary complexes and forearc ophiolitesDetailed geologic mapping (639 field stations in ~700 km2) and a ~50 km-long gravity survey (142 stations) in the western Azuero Peninsula revealed two faulted and folded slivers of oceanic crust attached to the trailing edge of the Caribbean Large Igneous Plateau (CLIP). Our new data, along with published geochronology, allowed us to reconstruct the Cretaceous forearc configuration of the trailing edge of the CLIP prior to seamount collision, ophiolite accretion, and whole-margin deformation. The ophiolite in western Azuero is composed of two tectonic slivers arranged in south-verging, imbricated thrust faults that stack a ~73 Ma pillow, flow, and picritic basalt and black chert, together with a ~ 89–93 Ma and older basalt flows and capping red chert sequences. Accretion of these slivers to form a supra-subduction zone ophiolite resulted from the middle Eocene collision and accretion of Galapagos seamounts against the trailing edge of the CLIP. Accreted seamounts are arranged in a north-verging antiformal stack duplex, and below the thrust sheets. Change in kinematics after fission of the Cocos-Nazca Plate during early Miocene times prompted the propagation of the Azuero-Sona fault zone flower structure, favouring the preservation of these slivers of oceanic crust.
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Spatial controls on the generation of low-δ18O basalts on São Miguel, AzoresVolcanism in the Azores is generally accepted to result from the decompression melting in the upper mantle associated with the Azorean plume. Basalts on the island of Sao ˜ Miguel have been previously recognised as having δ18O values lower than MORB with different authors ascribing this low-δ18O signature to either a component within the plume itself or as the result of assimilation of crustal materials at shallow level. Here we assess these contrasting models by using a suite of samples that are both geographically and temporally spread. We find that while low-δ18O mafic magmas (reaching 4.98 ‰) do occur on S˜ ao Miguel, they are spatially restricted to the Picos fissure system, between the central volcanoes of Sete Cidades and Fogo. Basalts of similar age from outside this restricted region return δ18O values that are similar to MORB. This spatial constraint argues that the source of the observed low-δ18O signature is within the crust rather than a component of the plume. Previous studies on mafic to intermediate rocks of the Picos fissure system have identified the assimilation of broadly syenitic lithologies, which were found as co-erupted lithic clasts at the Fogo volcano and may have δ18O values as low as 2.6 ‰. However, trace element geochemistry indicates that these lithologies have not been assimilated within our low-δ18O samples, a more likely assimilant is altered oceanic crust or mafic lavas within the island edifice.
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Obsidian clasts as sintered remnants of agglutination processes in volcanic conduits, evidence from the Pepom tephras (Sete Cidades), São Miguel, AzoresThe youngest explosive eruptions of the Sete Cidades volcano, São Miguel, Azores, are recorded by a series of relatively small-volume (<0.03 to 0.13 km3 DRE) trachytic pyroclastic deposits termed the Pepom tephra deposits. While dominated by crystal-poor to crystal-moderate (<10%) pumice clasts, these deposits also contain a suite of dense glassy clasts of broadly similar crystallinity. The obsidian clasts from a single deposit vary in texture from entirely dense to those that are moderately vesicular and typically single clasts will contain multiple textural domains. The majority (∼71%) of these dense clasts have compositions both from bulk rock and in-situ glass measurements that are identical to those of the pumice clasts within the same deposit. We interpret these dense clasts to reflect sintering of previously fragmented magma at shallow levels in the conduit prior to being re-entrained and erupted with the vesicular magma, in agreement with recent studies focussing on textural observations. Notably, across the exposed volcanic stratigraphy of São Miguel obsidian domes, flows/coulees are not preserved, arguing against the idea that the dense glass clasts within the Pepom tephras are sourced from existing surficial rocks. In contrast, the neighbouring island of Terceira exhibits domes and coulees with large obsidian bands that cut through the crystalline groundmass. Most silicic rocks of Santa Bárbara and Pico Alto volcanoes on Terceira are peralkaline, comenditic to pantelleritic in composition, and at similar conditions (e.g., temperature and water content) have lower viscosities than the trachytic Pepom obsidian clasts. However, the Santa Bárbara silicic lavas on Terceira (the less peralkaline suite) have more obsidian than the more peralkaline Pico Alto domes and coulees indicating that while peralkalinity, developed during magmatic evolution in the crust, may play a role, sintering occurring at shallow levels within the conduit likely is more important in producing obsidian.
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Lithium systematics in the Krafla volcanic system: comparison between surface rhyolites and felsic cuttings from the Iceland deep drilling project -1 (IDDP-1)Abstract - The unexpected discovery of felsic magma by the Iceland Deep Drilling Project-1 (IDDP-1) in the Krafla volcanic system (KVS) presents a unique opportunity to investigate pre-eruptive lithium (Li) dynamics and establish a more direct connection between magma reservoirs and volcanic deposits. Our study provides new insights into Li abundances and isotope compositions in bulk-rock, minerals, and groundmass glass from rhyolitic lavas at KVS, encompassing various stages of groundmass crystallisation. Additionally, we examined felsic cuttings retrieved from the IDDP-1 well, comprising crystal-poor obsidian and crystal-bearing to -rich ‘felsite’ particles. Groundmass glasses from surface lavas show limited variability in K/Na, indicating limited secondary hydration of the glasses and that their Li contents seem to not be affected by this post-eruptive process. Lithium inventories in groundmass glasses and minerals within lavas exhibit variations consistent with the cooling history of the deposit, resembling patterns seen in Snake River Plain ignimbrites. Lithium contents of glassy rhyolitic lavas, whether bulk-rock (avg. 27.2 ± 3.1 μg/g) or groundmass glass (average 28.4 ± 4.7 μg/g), and their bulk isotopic compositions (avg. δ<jats:sup>7</jats:sup>Li =+ 4.4 ± 0.2‰) overlap with those observed in IDDP-1 obsidian cuts (avg. 24.9 μg/g Li in bulk, 28.6 ± 1.5 μg/g in groundmass glass, and δ<jats:sup>7</jats:sup>Li = 4.5 ± 0.2‰). Glassy lavas lacking spherulites may potentially preserve pristine magmatic Li element and isotope compositions, while areas with extensive groundmass crystallisation reveal Li enrichments in phenocrysts. Plagioclases in slowly cooled parts of the deposit record a two-fold increase in Li contents compared to plagioclase found in glassy counterparts, along with evidence of open-system degassing marked by heavier bulk Li isotope compositions and lower bulk Li contents of the crystallised lava portions (avg. δ<jats:sup>7</jats:sup>Li = +7.2 ± 0.1‰ and 7 ± 0.8 μg/g Li) relative to bulk glassy lithologies (avg. δ<jats:sup>7</jats:sup>Li = +4.1 ± 0.1‰ and 28 ± 2 μg/g Li). Partition coefficients derived from IDDP-1 cuts successfully predict Li inventories in vitrophyres of rhyolites on the surface of the KVS. Lithium isotope compositions of the crystal-rich IDDP-1 cuts are significantly heavier (avg. δ<jats:sup>7</jats:sup>Li = +7.2 ± 0.2‰) than lavas and IDDP-1 obsidian cuts, casting doubt on the notion that the IDDP-1 rhyolitic magma could result from the melting of felsite lenses in the KVS. Lithium contents in groundmass glasses within IDDP-1 crystal-rich cuts show higher Li contents (avg. 55.1–60.7 μg/g), correlating with the higher crystal content and an increase in other incompatible elements (avg. 250 μg/g Rb) relative to obsidian cuttings (avg. 75 μg/g Rb).
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The explosive-effusive transition within the Miocene Fataga suite, Gran CanariaMany volcanoes show transitions between explosive and effusive eruptive styles both through the history of the volcano as a whole and occasionally within the course of a single eruption. These differing eruptive styles have vastly different implications for hazard assessments in surrounding regions and so understanding such changes is important. Here, we investigate the intercalated lavas and ignimbrites of the Miocene Fataga Group on the island of Gran Canaria, Spain. Ignimbrites reflect the products of explosive events from the Tejeda caldera, while lavas found within the pyroclastic succession were erupted from extra-caldera sources some 5–10 km from the caldera margin. The ignimbrites exhibit textural complexity containing both crystal-poor juvenile pyroclasts and late-erupted crystal-rich juvenile clasts interpreted to reflect interaction between cumulates and recharge magmas. The lavas meanwhile are almost phenocryst-free, with the exception of few large (cm-scale), unzoned sanidine crystals. Despite their textural differences, the lavas are geochemically similar (in terms of bulk rock and feldspar compositions) to the crystal-poor juveniles in the ignimbrites. Oxygen and lead isotopic compositions of the lavas and surrounding ignimbrites reveal that the magmas shared a deeper source and that petrographic variability is imprinted upon the magmas at shallow levels. We interpret the lavas as originating from peripheral magmatic pockets, on the edges of the main caldera-feeding reservoir. These peripheral magma chambers felt the effects of recharge only as slight thermal fluctuations that fostered the production of the large sanidine crystals. Our findings highlight the potential for storage of magmas aside from the main magmatic system that may represent an underappreciated hazard at volcanoes worldwide.
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Geochemistry of the Pepom tephra deposits: The most recent intracaldera volcanism of Sete Cidades volcano, São Miguel, AzoresMany volcanoes have the ability to impact human infrastructure with explosive activity and understanding the processes of magma generation and conditions of storage in these systems remains a priority. The Sete Cidades volcano on the island of São Miguel, Azores archipelago is exactly such a volcano lying only 12 km from the island's capital city of Ponta Delgada and has been repeatedly active during the Holocene. Sete Cidades is the westernmost central volcano of São Miguel and its most recent explosive volcanism produced the Pepom series of 17 trachytic pumice deposits that were erupted from vents within the caldera. These variably magmatic and phreatomagmatic eruptions involved trachytic magmas that represent the end point of a magmatic evolution dominated by crystal fractionation processes. The magmas contain a mineral assemblage dominated by feldspar, with biotite, clinopyroxene, Fesingle bondTi oxides, amphibole, and trace apatite. Mineral thermometry and hygrometry reveal that the Pepom magmas were stored at temperatures of 789 ± 23 °C to 894 ± 20 °C under volatile-rich conditions (mostly >5 wt% water). Intercalated within the trachytic pyroclastic succession are mafic products that were erupted from vents on the flanks of the volcano suggesting that the evolved, shallow magmatic system may be acting as a barrier to ascending mafic magmas. The Pepom tephra deposits represent trachytic magmas that show limited evidence for the involvement of feldspar-dominated cumulates as has been observed in other settings whereas the preceding, caldera-forming, Santa Bárbara eruption suggests such involvement. Despite the predominant role of fractionation in petrogenesis, the Pepom tephras formed from mildly low-δ18O magmas (4.1 to 5.6‰ for trachyte) and as such require the addition of hydrothermally altered material. The cause of the low-δ18O values remains unknown, but low-δ18O, hydrothermally altered syenitic clasts found within deposits from the nearby Fogo volcano may be potentially analogues.
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Linking monitoring data and timescales of mafic recharge during the 2013–17 eruption at Volcán de Colima, MexicoTimescales of mafic magma recharge beneath Volcán de Colima have been calculated from diffusion modelling of reversely zoned pyroxene crystals erupted in the 2013–17 interplinian eruptive period. The results suggest that injections of low volume mafic magma are periodic and ephemeral, residing in the plumbing system for only weeks to months before eruption. At least three separate periods of magma recharge and mixing occurred within c. 3 years in October 2013–April 2014, September 2014–June 2015 and November 2015–September 2016. These timescales have been compared with the continuous seismic monitoring of the volcano and quasi-continuous gas monitoring data. Each eruptive phase shows different patterns between petrological and monitoring data, suggesting a complex mixing-eruption relationship, which may be related to the frequency and volume of recharge events, as well as the thermal state of the magma reservoir. Low frequency or low volumes of magma injection may result in a lack of correlation with any change in the monitoring data. High frequency of magma injections priming the magma reservoir results in a strong correlation between recharge events and monitoring record, which may be useful for interpreting future monitoring data.
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A Snakemake Toolkit for the Batch Assembly, Annotation and Phylogenetic Analysis of Mitochondrial Genomes and Ribosomal Genes From Genome Skims of Museum CollectionsABSTRACT - Low coverage ‘genome‐skims’ are often used to assemble organelle genomes and ribosomal gene sequences for cost‐effective phylogenetic and barcoding studies. Natural history collections hold invaluable biological information, yet poor preservation resulting in degraded DNA often hinders polymerase chain reaction‐based analyses. However, it is possible to generate libraries and sequence the short fragments typical of degraded DNA to generate genome‐skims from museum collections. Here we introduce a snakemake toolkit comprised of three pipelines <jats:italic>skim2mito</jats:italic>, <jats:italic>skim2rrna</jats:italic> and <jats:italic>gene2phylo</jats:italic>, designed to unlock the genomic potential of historical museum specimens using genome skimming. Specifically, <jats:italic>skim2mito</jats:italic> and <jats:italic>skim2rrna</jats:italic> perform the batch assembly, annotation and phylogenetic analysis of mitochondrial genomes and nuclear ribosomal genes, respectively, from low‐coverage genome skims. The third pipeline <jats:italic>gene2phylo</jats:italic> takes a set of gene alignments and performs phylogenetic analysis of individual genes, partitioned analysis of concatenated alignments and a phylogenetic analysis based on gene trees. We benchmark our pipelines with simulated data, followed by testing with a novel genome skimming dataset from both recent and historical solariellid gastropod samples. We show that the toolkit can recover mitochondrial and ribosomal genes from poorly preserved museum specimens of the gastropod family Solariellidae, and the phylogenetic analysis is consistent with our current understanding of taxonomic relationships. The generation of bioinformatic pipelines that facilitate processing large quantities of sequence data from the vast repository of specimens held in natural history museum collections will greatly aid species discovery and exploration of biodiversity over time, ultimately aiding conservation efforts in the face of a changing planet.
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A Tube-Dwelling Early Cambrian LobopodianFacivermis yunnanicus [1, 2] is an enigmatic worm-like animal from the early Cambrian Chengjiang Biota of Yunnan Province, China. It is a small (<10 cm) bilaterian with five pairs of spiny anterior arms, an elongated body, and a swollen posterior end. The unusual morphology of Facivermis has prompted a history of diverse taxonomic interpretations, including among annelids [1, 3], lophophorates [4], and pentastomids [5]. However, in other studies, Facivermis is considered to be more similar to lobopodians [2, 6-8]-the fossil grade from which modern panarthropods (arthropods, onychophorans, and tardigrades) are derived. In these studies, Facivermis is thought to be intermediate between cycloneuralian worms and lobopodians. Facivermis has therefore been suggested to represent an early endobenthic-epibenthic panarthropod transition [6] and to provide crucial insights into the origin of paired appendages [2]. However, the systematic affinity of Facivermis was poorly supported in a previous phylogeny [6], partially due to incomplete understanding of its morphology. Therefore, the evolutionary significance of Facivermis remains unresolved. In this study, we re-examine Facivermis from new material and the holotype, leading to the discovery of several new morphological features, such as paired eyes on the head and a dwelling tube. Comprehensive phylogenetic analyses using parsimony, Bayesian inference, and maximum likelihood all support Facivermis as a luolishaniid in a derived position within the onychophoran stem group rather than as a basal panarthropod. In contrast to previous studies, we therefore conclude that Facivermis provides a rare early Cambrian example of secondary loss to accommodate a highly specialized tube-dwelling lifestyle.
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Evidence for shock‐induced anhydrite recrystallization and decomposition at the UNAM‐7 drill core from the Chicxulub impact structureAbstract: Drill core UNAM‐7, obtained 126 km from the center of the Chicxulub impact structure, outside the crater rim, contains a sequence of 126.2 m suevitic, silicate melt‐rich breccia on top of a silicate melt‐poor breccia with anhydrite megablocks. Total reflection X‐ray fluorescence analysis of altered silicate melt particles of the suevitic breccia shows high concentrations of Br, Sr, Cl, and Cu, which may indicate hydrothermal reaction with sea water. Scanning electron microscopy and energy‐dispersive spectrometry reveal recrystallization of silicate components during annealing by superheated impact melt. At anhydrite clasts, recrystallization is represented by a sequence of comparatively large columnar, euhedral to subhedral anhydrite grains and smaller, polygonal to interlobate grains that progressively annealed deformation features. The presence of voids in anhydrite grains indicates SOx gas release during anhydrite decomposition. The silicate melt‐poor breccia contains carbonate and sulfate particles cemented in a microcrystalline matrix. The matrix is dominated by anhydrite, dolomite, and calcite, with minor celestine and feldspars. Calcite‐dominated inclusions in silicate melt with flow textures between recrystallized anhydrite and silicate melt suggest a former liquid state of these components. Vesicular and spherulitic calcite particles may indicate quenching of carbonate melts in the atmosphere at high cooling rates, and partial decomposition during decompression at postshock conditions. Dolomite particles with a recrystallization sequence of interlobate, polygonal, subhedral to euhedral microstructures may have been formed at a low cooling rate. We conclude that UNAM‐7 provides evidence for solid‐state recrystallization or melting and dissociation of sulfates during the Chicxulub impact event. The lack of anhydrite in the K‐Pg ejecta deposits and rare presence of anhydrite in crater suevites may indicate that sulfates were completely dissociated at high temperature (T> 1465 °C)—whereas ejecta deposited near the outer crater rim experienced postshock conditions that were less effective at dissociation.
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Impact‐Induced Porosity and Microfracturing at the Chicxulub Impact StructureAbstract: Porosity and its distribution in impact craters has an important effect on the petrophysical properties of impactites: seismic wave speeds and reflectivity, rock permeability, strength, and density. These properties are important for the identification of potential craters and the understanding of the process and consequences of cratering. The Chicxulub impact structure, recently drilled by the joint International Ocean Discovery Program and International Continental scientific Drilling Program Expedition 364, provides a unique opportunity to compare direct observations of impactites with geophysical observations and models. Here, we combine small‐scale petrographic and petrophysical measurements with larger‐scale geophysical measurements and numerical simulations of the Chicxulub impact structure. Our aim is to assess the cause of unusually high porosities within the Chicxulub peak ring and the capability of numerical impact simulations to predict the gravity signature and the distribution and texture of porosity within craters. We show that high porosities within the Chicxulub peak ring are primarily caused by shock‐induced microfracturing. These fractures have preferred orientations, which can be predicted by considering the orientations of principal stresses during shock, and subsequent deformation during peak ring formation. Our results demonstrate that numerical impact simulations, implementing the Dynamic Collapse Model of peak ring formation, can accurately predict the distribution and orientation of impact‐induced microfractures in large craters, which plays an important role in the geophysical signature of impact structures.
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Brecciation at the grain scale within the lithologies of the Winchcombe Mighei‐like carbonaceous chondriteAbstract: The 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.
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A microchondrule‐bearing micrometeorite and comparison with microchondrules in <scp>CM</scp> chondritesAbstract: We report the discovery of a partially altered microchondrule within a fine‐grained micrometeorite. This object is circular, <10 μm in diameter, and has a cryptocrystalline texture, internal zonation, and a thin S‐bearing rim. These features imply a period of post‐accretion parent body aqueous alteration, in which the former glassy igneous texture was subject to hydration and phyllosilicate formation as well as leaching of fluid‐mobile elements. We compare this microchondrule to three microchondrules found in two CM chondrites: Elephant Moraine (EET) 96029 and Murchison. In all instances, their formation appears closely linked to the late stages of chondrule formation, chondrule recycling, and fine‐grained rim accretion. Likewise, they share cryptocrystalline textures and evidence of mild aqueous alteration and thus similar histories. We also investigate the host micrometeorite's petrology, which includes an unusually Cr‐rich mineralogy, containing both Mn‐chromite spinel and low‐Fe‐Cr‐rich (LICE) anhydrous silicates. Because these two refractory phases cannot form together in a single geochemical reservoir under equilibrium condensation, this micrometeorite's accretionary history requires a complex timeline with formation via nonequilibrium batch crystallization or accumulation of materials from large radial distances. In contrast, the bulk composition of this micrometeorite and its internal textures are consistent with a hydrated carbonaceous chondrite source. This micrometeorite is interpreted as a fragment of fine‐grained rim material that once surrounded a larger parent chondrule and was derived from a primitive carbonaceous parent body; either a CM chondrite or Jupiter family comet.
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Post-cratering melting of target rocks at the impact melt contact: Observations from the Vredefort impact structure, South AfricaImpact melt is generated following hypervelocity impact events. Emplacement of impact melt dikes, such as the Vredefort Granophyre Dikes, allow for this high temperature melt to come into contact with deeply-buried target rocks after the cratering process is completed. Our study analyzes the effects of this interaction by examining the direct contact between the Vredefort Granophyre and the granitic host at the Kopjeskraal and Lesutoskraal Granophyre Dikes using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), electron backscatter diffraction (EBSD), and X-ray micro-computed tomography (μCT). A several-mm-thick transition zone between the host rock and the impact melt is enriched in SiO2 and indicates preferential melting of feldspar and mica in the host rock by interaction with the impact melt. Immiscible droplets of newly-formed silicate melt migrated from the transition zone into the impact melt. We observe inundations of the impact melt along narrow fractures into the host rocks, which, in some cases, surround and incorporate fragments of the host rock into the melt body. We suggest three possible mechanisms by which components of the host rock can enter the impact melt: 1) fragmentation of the host rock prior to melt emplacement and subsequent entrainment into the melt; 2) inundations of melt around fragments of host rock at the contact, followed by incorporation of the host rock into the melt; 3) melting of the host rock and immiscible migration of melt fragments within the impact melt. The lack of observed assimilation of the granitic fragments into the impact melt, either because of silica saturation or viscosity contrast between the melts, suggests that the bulk composition of the Granophyre Dike matrix approximately represents the composition of the impact melt sheet.
