Search

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]

The joint NASA‐ESA Mars sample return campaign aims to return up to 31 sample tubes containing drilled sedimentary and igneous cores and regolith. The titanium alloy tubes will initially still be sealed when they are retrieved. Several types of measurement will be carried out on sealed samples in the pre‐basic characterization phase of scientific investigation. We show that powder x‐ray diffraction (XRD) analysis can be successfully carried out on sealed samples using an x‐ray source at the I12 beamline of Diamond Light Source synchrotron. Our experiment used an analog sample tube and a Martian regolith analog (Icelandic basaltic sand). The titanium walls of the tube analog give strong but few diffraction peaks, making identification of the major constituent mineral phases feasible. A more significant constraint on quantification of mineral phase abundances by this XRD technique is likely to be the grain size of the sample. This technique opens up the possibility of initial mineralogical analysis of samples returned from Jezero crater without opening the sample tubes and the potential changes to the sample that entails. [ABSTRACT FROM AUTHOR]

Introduction: The nakhlite secondary alteration assemblage consists mainly of Fe-rich carbonate partially corroded and replaced by ferric saponite and an odinite-like serpentine phase [1,2]. This mineral assemblage and its alteration stages can illustrate key aspects of the Amazonian carbon cycle. For instance, the origin of methane in the Martian atmosphere is one of the key unresolved questions in Mars' atmospheric evolution and current near-surface processes [3,4]. A possible route for methane production was described in [5] which demonstrated that CH4 is expected to dominate over carbon oxides under the redox conditions associated with martian hydrothermal fluids and the nakhlite secondary minerals. Here we use mineralogical studies of the Amazonian [6] nakhlite secondary assemblage to explore links between the carbonate-bearing crust and routes for organic formation. Methods and Samples: Mineralogical data on the nakhlite secondary minerals have been gathered with FE-SEM, EDX, WDS, FIB-(S)TEM at University of Leicester's Advanced Microscopy Facility and University of Nottingham, and Fe-K XANES at Diamond Light Source e.g. [1,2]. The 12 nakhlites carry varying abundances and parts of the overall secondary assemblage depending on depth of the parent rock within the nakhlite pile and extent of exposure to the secondary fluid [1,2,7]. We have studied many of the nakhlites as part of this project, but the near pristine nakhlite Lafayette has particular importance due to the abundance of its secondary assemblage and the textural evidence for an evolving fluid. We have anlysed Lafayette sections USNM 7849 and a section originally from the CFM. Results: Carbonate is up to 4% of one Lafayette thin section, and Mg0.0-2.0Cc13.2-38.6Sd17.7-81.9Rh3.1-42.9 [1]. A key feature of the nakhlite phylloslicate-carbonate assemblage in the nakhlites is that carbonate is variably replaced by ferric saponite in olivine fractures and odinite-like serpentine in mesostasis textural sites (Fig. 1). For instance, another thin section of the same Lafayette meteorite contained only minute traces of unaltered carbonate, most having been replaced. The odinite-like phase has d001-spacings of 0.7 nm and (Fetot+Mg)/Al at. of 0.8-0.9 [1]. Fe-K XANES show Fe3+/SFe ratios from 0.86-1.0 within the odinite and 0.59-0.96 within the saponite. Small traces of ferrihydrite were identified near the margins of corroded carbonate and saponite. Discussion: The nakhlite secondary fluid was initially a HCO3-charged brine associated with the precipitation of carbonate within olivine and mesostasis sites. Once the fluid's HCO3 was exhausted, the evolving, neutral-mildly alkaline, =50 oC brine corroded the carbonate and precipitated the phyllosilicate as a replacement of carbonate and directly within fractures. As the fluid was no longer buffered by Fe2+-carbonate, the iron oxidation state increased, as recorded in the Fe3+-rich phyllosilicates and precipitation of small traces of ferrihydrite near the margins of corroded carbonate and saponite. Extensive dissolution of such carbonate across the upper martian crust, producing bicarbonate and carbon dioxide, and the coupled formation of ferric phyllosilicates, would lead to the formation of CH4 in substantial amounts via a Fischer-Tropsch type reaction [8-10]. Such reactions could be substantially inhibited by kinetic factors but the likely relatively high abundance of carbonate and clay-bearing assemblages in much of the Mars upper crust [11] suggest that substantial quantities of CH4 and PAHs would still have been created. Seasonal release of some trapped CH4 reservoirs formed from this process are, for instance, of sufficient likely magnitude to explain localised atmospheric CH4 enrichments detected by Mars Science Laboratory [3]. As more of the crust is being studied via meteorites, MSL [12,13], Mars2020-MSR and orbiters, the role and significance of carbonate-phyllosilicate abundances within the ancient carbon cycle and organic reservoirs of Mars will continue to be revealed [14]. [ABSTRACT FROM AUTHOR]

Introduction: The Askival sample is a geochemically and texturally unique sample investigated by the Curiosity rover in Gale crater. Located at the Bressay site and analyzed between mission sols 2015 to 2021, Askival has a feldspathic cumulate texture, but presents geochemical evidence of aqueous alteration beyond initial crystallization and accumulation. An earlier target, Bindi, provides an important comparative sample as it also appears to be a feldspathic cumulate but lacks the alteration present in Askival. Methods: Geochemical analysis of Askival and Bindi was performed using the Chemistry and Camera (Chem-Cam) instrument on the MSL Curiosity rover [1]. ChemCam is a laser induced breakdown spectrometer (LIBS) instrument that performs geochemical remote sensing on targets up to 7 m away from the rover. LIBS analyses occur by creating an energized plasma at a micron-scale point on the target rock, allowing for sampling of different phases within coarse-grained targets such as Askival. Supporting imagery from the ChemCam remote micro-imager (RMI) allows these chemical analyses to be correlated with visual identification of different phases. Further geochemical analysis of Askival using the Alpha-Particle X-Ray Spectrometer (APXS) [2] and images from the Mars Hand Lens Imager (MAHLI) [3] provide complementary datasets. Results & Analysis: Texturally, Askival has strong resemblance to feldspar cumulate samples, with roughly 65% of the sample consisting of light-toned, subhedral crystals with some examples of elongation. This phase is separated and, in some cases, enclosed by a fine-grained darker matrix, which comprises approximately 30% of the sample. The remaining texture is comprised of a fibrous grey/brown phase that occurs only in a couple of visible areas of the sample, as well as a secondary light-toned phase observed as small veins occupying interstitial areas. Geochemically, the primary light-toned phase has a range of compositions which form a linear trend away from stoichiometric feldspar composition. The most prominent chemical divergence is an enrichment in silica, with some of the sampled LIBS points containing >80% SiO2. This enrichment correlates with a detected increase in H, and a decrease in alkali elements and enrichment in Mg. This is consistent with the production of a leached layer and precipitation of secondary Mg-phyllosilicates under low temperature acidic conditions [4]. Other rocks located at Bressay do not show similar alteration, and are not of the same cumulate origin, indicating either a diversity of sources within a single locality or that Bressay has a collection of samples that have undergone different transportation pathways. The secondary phase presents a diverse mafic composition, indicating multiple mineral endmembers contributing to the phase that are not visibly distinguished in the RMI imagery. Normative calculations support a composition made of Fe/Mg silicates including pyroxenes and amphibole, supporting a model of formation under an upper crustal setting. [ABSTRACT FROM AUTHOR]

Introduction: C-type asteroid Ryugu grains returned by the JAXA Hayabusa2 mission show a variety of space weathering (sw) effects overprinting a CI chondrite like mineralogy [1]. Notably, the Fine Grained Mineralogy team demonstrated that sw on the asteroid's surface included amorphization and partial melting of phyllosilicate [2]. In previous work on sw samples from the Itokawa S class asteroid we used Fe K XANES, STEM, TEM to demonstrate changing Fe3+/SFe, vesicular blistering, and npFe0 growth associated with that style of sw. Here, and in [2], we use the same techniques to provide accurate Fe3+/SFe and mineral-textural identifications on sw Ryugu grains to help characterise a different set of SW processes, likely associated with micrometeorite bombardment [2]. Samples and Methods: We have analysed FIB-TEM wafer samples A0058-C2001-08, A0104-00200502, and A0104-01700602 prepared from Hayabusa2 near surface samples (Chamber A) by Noguchi at Kyushu University [2]. As part of a Hayabusa2 consortium of complementary analytical work, we conducted Fe-Ka XAS measurements using the I-14 X-ray Nanoprobe Beamline at Diamond Light Source. To achieve XAS mapping, a series of maps were obtained over regions of interest, each map measured from 7050 to 7350 eV with a higher energy resolution over the XANES features (~7100-7150 eV). Similar to Itokawa asteroid samples [3], mineralogical identity can be constrained, including oxidation states in Fe-oxides and Fe-silicates, by observing increased energy shifts in the 1s?3d pre-edge peak centroid positions, and comparing to reference minerals of known Fe3+/SFe. EELS was performed using the JEOL ARM200CF instrument at ePSIC in the Diamond facility, using an accelerating voltage of 200 keV, current 15 µA, and measuring 0.25 eV/ch. Additional TEM imaging, calibrated STEM-EDX have been performed using JEOL 2100 FEG and LaB6 TEM's at the University of Nottingham, UK. Results: The interior of these samples are a fibrous phyllosilicate and magnetite rich (Fig. 1) with lesser amounts of phosphate and sulfides. The phyllosilicate has two layered lattice spacings of 0.75-0.78 nm and 100*(Si+Al)/(Mg+Fetot) atomic of 48-60. Its Fe3+/SFe varies considerably on the 3 wafers we have analysed from 20-50%. However, the melt zone, as shown by vesicularity and less crystalline nature, has Fe3+/SFe of 15%. Discussion and Future Work: The reduction of the phyllosilicate-rich Ryugu grains associated with micron scale surface melt zones is in contrast to the >Fe3+/SFe associated with sw nm-thick zones in Itokawa samples returned by Hayabusa [3,4]. The latter is thought to be a result of the implanted solar wind H+ ions reacting with the segregated ferrous Fe in the surface material [3]. The Ryugu results are demonstrating the diversity of processes associated with sw that act to alter the spectral characteristics of airless bodies. This report forms part of our study into the origin of the phyllosilicate in Ryugu Chamber A grains and the conditions of space weathering that have altered them. [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]