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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]

Bioavailable Fe is an essential nutrient for phytoplankton that enables the organisms to flourish and draw down atmospheric CO2 thus affecting global climatic conditions. In marine locales, remote from the continents, extraterrestrial dust provides an important source of Fe and thus moderates primary productivity. Here, we provide constraints on partitioning of extraterrestrial Fe between seawater and sediments from the observations of dissolution and the alteration of cosmic spherules recovered from deep‐sea sediments and Antarctica. Of the ∼3000–14,000 t a−1 extraterrestrial dust that reaches Earth's surface, ∼2–5% material falling in the oceans survives in marine sediments while the remainder is liberated into seawater. Both processes contribute ∼(3–10) × 10−8 mol Fe m−2 yr−1. The Fe contribution of surviving particles due to etching is estimated to be ∼10% of Fe contribution of meteoric smoke. Changes in extraterrestrial dust flux over geological time scales not only vary Fe delivery to the oceans by up to three orders of magnitude but also the partitioning of Fe between surface and abyssal waters depending on entry velocity and evaporation. [ABSTRACT FROM AUTHOR]

Fractionation effects related to evaporation and condensation had a major impact on the current elemental and isotopic composition of the Solar System. Although isotopic fractionation of moderately volatile elements has been observed in tektites due to impact heating, the exact nature of the processes taking place during hypervelocity impacts remains poorly understood. By studying Fe in microtektites, here we show that impact events do not simply lead to melting, melt expulsion and evaporation, but involve a convoluted sequence of processes including condensation, variable degrees of mixing between isotopically distinct reservoirs and ablative evaporation during atmospheric re-entry. Hypervelocity impacts can as such not only generate isotopically heavy, but also isotopically light ejecta, with δ56/54Fe spanning over nearly 5‰ and likely even larger variations for more volatile elements. The mechanisms demonstrated here for terrestrial impact ejecta modify our understanding of the effects of impact processing on the isotopic evolution of planetary crusts. Fe isotopic composition of the distal ejecta of a terrestrial impact crater records both evaporation and condensation, refining the nature of the isotopic fractionation taking place during hypervelocity impacts in the Solar System. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The early stages of atmospheric entry are investigated in four large (250–950 μm) unmelted micrometeorites (three fine‐grained and one composite), derived from the Transantarctic Mountain micrometeorite collection. These particles have abundant, interconnected, secondary pore spaces which form branching channels and show evidence of enhanced heating along their channel walls. Additionally, a micrometeorite with a double‐walled igneous rim is described, suggesting that some particles undergo volume expansion during entry. This study provides new textural data which links together entry heating processes known to operate inside micrometeoroids, thereby generating a more comprehensive model of their petrographic evolution. Initially, flash heated micrometeorites develop a melt layer on their exterior; this igneous rim migrates inwards. Meanwhile, the particle core is heated by the decomposition of low‐temperature phases and by volatile gas release. Where the igneous rim acts as a seal, gas pressures rise, resulting in the formation of interconnected voids and higher particle porosities. Eventually, the igneous rim is breached and gas exchange with the atmosphere occurs. This mechanism replaces inefficient conductive rim‐to‐core thermal gradients with more efficient particle‐wide heating, driven by convective gas flow. Interconnected voids also increase the likelihood of particle fragmentation during entry and, may therefore explain the rarity of large fine‐grained micrometeorites among collections. [ABSTRACT FROM AUTHOR]

The orientations of dehydration cracks and fracture networks in fine-grained, unmelted micrometeorites were analyzed using rose diagrams and entropy calculations. As cracks exploit pre-existing anisotropies, analysis of their orientation provides a mechanism with which to study the subtle petrofabrics preserved within fine-grained and amorphous materials. Both uniaxial and biaxial fabrics are discovered, often with a relatively wide spread in orientations (40°-60°). Brittle deformation cataclasis and rotated olivine grains are reported from a single micrometeorite. This paper provides the first evidence for impact-induced shock deformation in fine-grained micrometeorites. The presence of pervasive, low-grade shock features in CM chondrites and CM-like dust, anomalously low-density measurements for C-type asteroids, and impact experiments which suggest CM chondrites are highly prone to disruption all imply that CM parent bodies are unlikely to have remained intact and instead exist as a collection of loosely aggregated rubble-pile asteroids, composed of primitive shocked clasts. [ABSTRACT FROM AUTHOR]

Cosmic spherules are unique igneous objects that form by melting due to gas drag heating during atmospheric entry heating. Vesicles are an important component of many cosmic spherules since they suggest their precursors had finite volatile contents. Vesicle abundances in spherules decrease through the series porphyritic, glassy, barred, to cryptocrystalline spherules. Anomalous hollow spherules, with large off-center vesicles occur in both porphyritic and glassy spheres. Numerical simulation of the dynamic behavior of vesicles during atmospheric flight is presented that indicates vesicles rapidly migrate due to deceleration and separate from nonporphyritic particles. Modest rotation rates of tens of radians s−1 are, however, sufficient to impede loss of vesicles and may explain the presence of small solitary vesicles in barred, cryptocrystalline and glassy spherules. Rapid rotation at spin rates of several thousand radians s−1 are required to concentrate vesicles at the rotational axis and leads to rapid growth by coalescence and either separation or retention depending on the orientation of the rotational axis. Complex rapid rotations that concentrate vesicles in the core of particles are proposed as a mechanism for the formation of hollow spherules. High vesicle contents in porphyritic spherules suggest volatile-rich precursors; however, calculation of volatile retention indicates these have lost >99.9% of volatiles to degassing prior to melting. The formation of hollow spherules, by rapid spin, necessarily implies preatmospheric rotations of several thousand radians s−1. These particles are suggested to represent immature dust, recently released from parent bodies, in which rotations have not been slowed by magnetic damping. [ABSTRACT FROM AUTHOR]

Introduction: Micrometeorites (MMs) are millimeter to sub-millimeter-sized dust particles that are liberated as debris tails from asteroids and comets and are constantly captured by Earth. They are decelerated at high altitudes, often heated and partially melted, yet survive atmospheric entry to be recovered from Earth's surface environments. They have been successfully separated from Antarctic ice and investigated in detail using numerous analytical techniques, which has improved our understanding of the source population of small bodies, their compositions and aqueous processing of materials [1-4]. Additionally, the study of MMs provides a complementary perspective on meteorite studies as well as regolith samples obtained from sample return missions like Hayabusa2 and OSIRIS-REx. Identifying the parent bodies of MMs and understanding their relationship to known meteorite groups improves our knowledge of solar system geological diversity during the age of early planetesimals. Such a classification requires a detailed investigation of the MM mineralogy, chemical characteristics, and oxygen isotope compositions. Here, we report on a quantitative and semi-quantitative analysis of the mineralogy of several unmelted MMs from Antarctica. We focus on the textural setting of P-bearing minerals with surrounding phases, which has not received particular attention. Furthermore, selected MMs will be investigated for O-isotopes. Samples and Methods: We investigated a subset of small (<150 µm) MMs from the Cap Prud'homme MM collection [4]. These are blue-ice derived particles, hand-picked from filter papers in 1994. Mineralogy of the particles was ascertained using energy dispersive spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS) on the JEOL JXA-8530F electron microprobe (EPMA) at Arizona State University. Analyses were conducted with a beam current of 15 nA, an acceleration voltage of 15 keV, and a beam size of 1 to 5 µm. High-resolution elemental maps of K, P, Si, Al, Na, Mg, Cl, S, Ti, Fe, Mn, Ca were collected. Results: CP94-050 epoxy mount has 80 MMs; data on sixty-seven MMs, often with CI and CM-like characteristics have been previously published [4-7]. The remaining MMs range in size from ~80 µm to ~135 µm in diameter and are primarily irregular in shape. Of these, four are fine-grained MMs (FgMMs), two are scoriaceous fine-grained MMs, and seven are coarse-grained MMs. Of the FgMMs, three are type C2 (compact and chemically heterogenous), and one is type C3 (highly porous) based on the classification scheme of [4]. We conducted sixty-nine WDS spot analyses of about 5×5 µm2 regions in the MM suite. Thirty-one of these target regions are from MMs previously unstudied in this capacity. These regions are either fine-grained matrix or larger coarse grains. Individual grains vary from spherical to irregular to elongated. WDS analyses outside the range of 97 to 101 wt. % were rejected, unless the spot was taken from a porous, fine-grained matrix, in which case a threshold of >94 wt. % was used (e.g., [4]). Twenty-four grain analyses met the former criteria. Eight of these spots are olivine (Fo85.1-99.3), 12 are pyroxene (En88.5-97.4Fs1.5-7.8Wo0.6-4.2), one is Ca-rich augite (En55.5Fs0.8Wo43. 7), and two are SiO2 grains. The remaining grain is non-stoichiometric with a Si/O ratio is 1/3 and with Mg and Al enrichments. Sixty-nine spots were analyzed for P with WDS. Considering 97 to 101 wt. % totals, we identified two grains that are olivine while 2 are pyroxene; the remaining grains show non-stoichiometric compositions. The P content of these silicate grains vary from >0.01 to 0.035 wt.%. We also found five fine-grained, porous regions with lower totals (>94 wt. % criteria) that contain P (0.012-0.0127 wt.%). Several of these contain Mg and Ca, suggestive of thermally metamorphosed apatite minerals. Elemental maps show that P is evenly distributed in most MMs, although a few MMs show hotspots. For example, one MM displays a 5×5 µm2 hotspot, contains elevated levels of Fe (18.3 wt. %) and S (1.07 wt. %), and exhibits non-stoichiometric elemental composition. Several P hotspots contain Fe and are either Fe-phosphide inclusions or Fe-Ni metal altered to ferrihydrite. Discussion: The study of the phases containing S and P can provide valuable insights into (1) element reservoirs and (2) redistribution during metasomatic fluid processing on asteroid parent bodies. Phosphorus is present in Fe-Ni metal in the most primitive carbonaceous chondrites [8]. Secondary processes oxidize P to form phosphates and phosphides [9]. We identify P in olivine and pyroxene minerals, and in grains that contain P in association with S, Mg, and Ca in non-stoichiometric proportions. We argue that fluid-mediated processing occurred in these MM parent bodies and P is not completely lost from the MMs during re-entry through Earth's atmosphere. [ABSTRACT FROM AUTHOR]

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: 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]

Introduction: Within the geologic record, sub-millimetre microspherules may be found in trace amounts, most likely formed by high-temperature processes. Lightning strikes discharge an average of 4 x 108 J per strike [1], creating temperatures in excess of 2,500 K [2]. This process may form molten droplets from the target rock that are rapidly quenched, resulting in microspherules. Given that ~44 lightning flashes occur per second globally [3], with the majority occurring over landmasses and over 25% reaching the ground [2], the abundance of lightningogenic spherules throughout the geologic record is likely to be non-trivial. Other microspherules on the Earth's surface may be formed by the atmospheric entry of cosmic dust, impact events, volcanic activity and anthropogenic activities. They are likely to have different mineralogical, textural and compositional features allowing them to be distinguished. Characterising the nature of lightningogenic microspherules would allow them to be identified in the rock record, which may have implications for palaeoenvironment and palaeoclimate models. Methods: Fine-grained sediment was collected from pan holes on a 30 m high exposure of granite gneiss in the Pilbara region of Western Australia. Spherical particles were picked from the sediment under a binocular microscope, mounted in an epoxy stub and analysed using a Hitachi TM4000Plus scanning electron microscope (SEM) at Imperial College London. Textural features were identified using backscattered electron imaging and compositional data were obtained using energy-dispersive spectroscopy (EDS) via point analyses and maps. Results: The particles have a size range of 20-60 µm, with near-perfect spherical morphologies. Textures are diverse and include glassy, microcrystalline, vesicular and coarse-grained. Where present, vesicles are spherical, offcentre and typically < 10 µm in diameter. Two over-all categories of particle were identified: (1) silicate dominated particles, primarily composed of K, Ca, Al-silicate glass and containing irregular, relict crystals of quartz and orthoclase, and (2) oxide dominated particles, usually with dendritic oxide crystals. The spherules show significant compositional variation. Some are Ti-rich (< 25 wt%), resembling mineralogies such as rutile, anatase or ilmenite. Others have high Fe, Al and O abundances, indicating spinel or magnetite, where Al content is low. Silicate particles contain high Al (< 21 wt%) and appreciable K (< 8 wt%) or Ca (< 6 wt%), which may be derived from orthoclase or plagioclase feldspars, respectively. While Mg abundances are low (< 6 wt%), Fe is found in most spherules (< 45 wt%). Discussion: A lightningogenic origin of the Pilbara spherules is suggested by their mineralogies, textures and compositions. The silicate spherules are non-chondritic and oxides contain Ti indicating they are not cosmic spherules [4, 5]. The compositional variability contrasts with distal impact microspherules [4], and no local impact craters are present to suggest they are proximal ejecta. A volcanic origin can be discounted owing to the large abundance of spherules and absence of recent volcanism in the Pilbara. The remote location and absence of industry suggest they are not anthropogenic in origin. Finally, the compositions of spherules and their inventory of relict minerals corresponds closely to the underlying granite gneiss, suggesting generation by localised energetic events capable of melting or vaporising the substrate bedrock. Lightning strikes are thus the most likely mechanism. The compositional variability of the spherules suggests direct melting of individual minerals rather than gas phase condensation owing to the minimal mixing observed. The presence of Ti-rich spherules, formed by melting of ilmenite and rutile, suggest a minimum temperature of 2,130 K [6]. Vesicles imply an indigenous volatile content, since exogenic diffusion of volatiles is unlikely during the short time these small spherules spent above the liquidus. Localised weathering of the exposure surface is likely to be a source of water and iron in the particles. The unexpected high abundance of spherules may relate to the elevation of the exposure compared to the local topography making it more susceptible to lightning strikes. Implications: The absence of micrometeorites in the Pilbara collection may give clues as to the frequency of cloud-to-ground lightning strikes, given a steady background flux of cosmic dust [5]. The textural and compositional features described here - particularly variable, mineral-derived compositions - may form the basis for identifying lightningogenic microspherules in the geologic record. [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]

The discovery of magnetic spherules in acid-insoluble residues from conodont samples encouraged a systematic search for Ordovician micrometeorites from northwestern Argentina. Some 220 melted micrometeorites were recovered from the magnetic fraction of six samples (total rock weight: 23 kg) from the Cordillera Oriental (Santa Rosita Formation) and 17 from five samples (total rock weight: 8.9 kg) from the Argentine Precordillera (Las Aguaditas, Gualcamayo and Las Vacas formations). The specimens resemble I-type cosmic spherules, in their chemistry and distinct dendritic and polygonal crystalline structures. They represent a flux of micrometeorites several orders of magnitude greater than present. The wide differences in spherule abundance between the Precordillera and the Cordillera Oriental samples could reflect uncertainties in the sedimentary rates or temporal variations in the flux of extraterrestrial matter to Earth. The micrometeorite-bearing formations span the late Tremadocian to the late Darriliwian (~480-460 Ma), which is consistent with a period of elevated flux of extraterrestrial material, as recorded several thousand kilometres away from coeval horizons in Scotland, Sweden and central China. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]