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The sizes of chondrules are a valuable tool for understanding relationships between meteorite groups and the affinity of ungrouped chondrites, documenting temporal/spatial variability in the solar nebula, and exploring the effects of parent body processing. Many of the recently reported sizes of chondrules within the CM carbonaceous chondrites differ significantly from the established literature average and are more closely comparable to those of chondrules within CO chondrites. Here, we report an updated analysis of chondrule dimensions within the CM group based on data from 1937 chondrules, obtained across a suite of CM lithologies ranging from petrologic subtypes CM2.2 to CM2.7. Our revised average CM chondrule size is 194 μm. Among the samples examined, a relationship was observed between petrologic subtype and chondrule size such that chondrule long‐axis lengths are greater in the more highly aqueously altered lithologies. These findings suggest a greater similarity between the CM and CO chondrites than previously thought and support arguments for a genetic link between the two groups (i.e., the CM‐CO clan). Using the 2‐D and 3‐D data gathered, we also apply numerous stereological corrections to examine their usefulness in correcting 2‐D chondrule measurements within the CM chondrites. Alongside this analysis, we present the details of a standardized methodology for 2‐D chondrule size measurement to facilitate more reliable inter‐study comparisons. [ABSTRACT FROM AUTHOR]

Introduction: Variations in the sizes of chondrules between the classes of chondritic meteotites are well known [1] and are thought to result from sorting processes in the solar nebula between the periods of chondrule formation and chondrite parent body accretion [2]. Similarities in the sizes of chondrules are used as one line of evidence to link chondritic classes, e.g. the proposed CM-CO clan [1]. Unlike for other chondritic groups (e.g. CO, CV, CH, H and L chondrites) chondrule sizes within the CM carbonaceous chondrites have not been the subject of a recent, comprehensive study [3]. CM chondrules are frequently cited as being 270-300 µm in size [1,3,4], with highly variable chondrule abundances. However, several studies have reported average sizes differing significantly from the 270-300 µm value (5-6). Here we present results from a study of just under 2000 chondrules in a suite of CM chondrites with the aim of better quantifying their size ranges, and understanding potential relationships to the CO chondrites. Methods and Samples: The meteorites studied were: Winchcombe (CM2) [7], Cold Bokkeveld (CM2.2) [8], Kolang (CM2.2) [9], Shidian (CM2.2) [10], Aguas Zarcas (CM2.2±0.1) [11], Mighei (petrologic type 1.6) [8], LaPaz Icefield (LAP) 02239 (petrologic type 1.7) [8], Murchison (CM2.5) [8], Lewis Cliff (LEW) 85311 (CM2.7) [8] and Paris (CM2.7) [12]. The apparent sizes of chondrules in 2D were measured from backscattered electron and energy dispersive electron spectroscopy mosaics. Whole chondrules were measured using the CIS method described by [13]. Chondrule sizes in 3D were measured from X-ray computed tomography (XCT) scans of meteorite chips. Chondrules were segmented and measured using the method set out by [14]. XCT scan resolution varied between samples from 12.1 - 2.1 µm/voxel. Results: The results of 2D and 3D chondrule size analysis are in Table 1. 2D analysis used 983 chondrules and reveals an average apparent chondrule size of 2.277 ± 0.896 ϕ-units; 206 µm with individual sample averages consistently < 300 µm. Whole chondrule abundances ranged between 1.23 - 10.38 areal %. 3D analysis of 954 chondrules revealed significant variation in sizes with an average of 1.752 ± 0.832 ϕ-units; 297 µm. In those XCT datasets conducted at higher resolution (LEW 85311 and Winchcombe; <4 µm/voxel) average 3D sizes were comparable with those measured in 2D. Discussion: Results presented here indicate a potential overestimate withinin the literature of the true CM chondrule size, with chondrules significantly closer in size to those observed in the CO chondrites (CO reported average: 2.76 ± 0.923 ϕ-units; 148 !"#$ %&' µm [15]). This finding further supports a genetic link between the CM and CO chondrites (CM-CO clan) and consequently, the use chondrule size differences between the two classes should not be considered evidence of distinct parent asteroid origins with respect to these two chondritic classes [16]. [ABSTRACT FROM AUTHOR]

Introduction: The methodologies used to report the sizes of chondrules and other objects in chondritic meteorites vary between studies [1-5], and in many cases methods are not fully descibed. The variation and the lack of detail in some studies makes comparison of object sizes between samples, both inter- and intra-meteorite class difficult. Here we propose a simple, standardized method for measuring and reporting the sizes of chondrules and other objects (e.g. CAIs) in 2D, in an attempt to facilitate improved comparison of object sizes between chondritic samples. Whilst 2D measurements provide only an apparent size, the current lack of sufficiently high resolution 3D data sets (for example from X-ray computed tomography), and the complexity of 2D-3D true size corrections justifies the use of a uncomplicated, standardised 2D approach for straightforward chondrite descriptions and comparisons. 2D Measurement Method: The methodology set out here, and hereafter referred to as CIS (Chondrule Image Segmentation), can be carried out on image mosaics collected via scanning electron microscopy and conventional transmitted/reflected light microscopy. The steps are outlined in Figure 1 and involve chondrules (or other objects) being identified based on clearly defined criteria. Objects truncated by fractures or the edge of the sample should be excluded as their whole shape/size is unknown. Identified objects should be segmented using an image processing package, our preference is the free software: GNU Image Manipulation Program - GIMP. Segmented chondrules should then be fitted with best-fit ellipses to smooth perimeter irregualrities and produce clearly defineable long and short axes, which can subsequently be measured based off the resolution of the image (μm/pixel). Ellipse fitting and axes measurement can be undertaken in image analysis software, such as ImageJ, under the 'Analyse Particles' function. Analysis and Reporting: Object sizes should be described by long and short axes measurements and as already suggested by [6] reporting of raw size data for all measured objects within chondrites should encouraged to allow greater comparison between samples and allow other object dimensions (e.g. aspect ratio) to be taken into account. Additionally, as the resolution of a given image mosaic impacts on the identification of objects, a statement regarding the image resolution is required. With regards to reporting chondrule size statistics, data should not be considered normally distributed [6], and is typically positively skewed. Reporting statistics (e.g. averages, medians and standard deviations) based on the assumption of a normal distribution can lead to mis-interpretation. The precise size-frequency distribution of chondrules is the subject of significant scientific debate [7]. However, for simplicity the sedimentological function Phi (φ), described by the equation: φ = -log! 𝑑, can be used to approximately describe the data prior to statistics being calculated. Statistics and values reported should be given in both Phi units and a metric equivalent (e.g. 1.723 φ; 303 μm) for ease of comparison and understanding within the community. Phi (φ) has been used previously in studiess of chondrule size by [8-9]. Conclusion: It is hoped that the CIS method described here provides a simple approach to chondrule and chondritic object size measurements which can lead to improved consistency in reporting and better facilitate size comparisons between chondritic samples. [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: Ureilites are primitive achondrites that contain a high concentration of carbon (up to 8.5 wt% [1]). The carbon contained within ureilites is in the form of graphite and diamond [1-3]. The origin of the diamonds is debated with a shock origin [2], high pressure static growth similar to Earth's mantle [3], and chemical vapour deposition (CVD) from the Solar Nebula [4] all being proposed. The shock origin theory suggests that graphite is the original form of carbon, which then gets transformed into diamond through a high pressure phase transformation driven by an impact, and catalysed by the presence of (Fe,Ni,Co)-C liquids, now present as spherule inclusions in ureilite silicate minerals [1-2]. Static high pressure growth involes diamond formation within a large parent body to maintain high pressures for a long enough period of time to allow for large diamonds to grow [3]. CVD is a theory where diamonds directly crystallise out of the Solar Nebula through chemical reactions involving H2CH4 [1, 4]. Diamonds formed in different environments produce distinct peak shapes in Raman spectra. Here we investigated the diamond structure within ureilite meteorite Miller Range 090980 (MIL 090980) using Raman Spectroscopy, to determine their origin. Methods: Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) was used to map MIL 090980 and identify carbon-rich regions. Raman Spectroscopy was conducted on a C-rich region in MIL 090980 using a Renishaw InVia Raman microscope attached to a 45W (max power), 512 nm laser source and a 2400 mm grating. Analysis carried out using a laser power of 5% with 3 second exposure time. All analysis were conducted at the University of Glasgow ISAAC facility. Results: Figure 1 shows a plot of the measured diamonds Raman shift vs the Full Width Half Maximum (FWHM) values. Raman shift is the difference between the ground state of a molecule and the final state post excitation from the laser beam defined by the peak centre [5]. FWHM refers to the width of the Raman peak at half of the maximum intensity. The FWHM and Raman shift of diamonds in MIL 090980 shows wide spread of values (Fig. 1). FWHM range from 1.3 to 53.4 centrered at 6.9 (Fig. 1). The raman shift values range from 1319.9-1335.3 cm-1 centered at 1332.0 cm-1 (Fig. 1). Discussion: The Raman FWHM and Raman shift for diamonds in MIL 090980 were compared with experimental data for laboratory produced shock diamonds, CVD diamonds, high static pressure diamonds and ureilite diamonds previously published [6]. Raman spectra for experimentally shocked diamonds show a trend towards lower Raman shifts (Min: 1315 cm-1 [6]). Experimental CVD diamonds trend towards higher Raman shifts (1340 cm-1 [6]), with a correlation between higher Raman shift and higher FWHM values [6] (Fig. 1). Raman data from high static pressure diamonds plot in a small area of 1332 to 1333 cm-1 with a FWHM value below 5 [6](Fig 1). This new data from MIL 090980 shows a cluster of measurements centered around a Raman shift of 1332 cm-1 with some scattering down towards 1320 cm-1 but no data points plot above 1335.3 cm-1. This indicates that the diamonds within MIL 090980 are more consistent with shock diamonds than CVD diamonds. While some MIL 090980 data do plot in the region consistent with high static pressure growth the large spread of data points around this region indicates that a shock origin for these diamonds is the dominant mechanism for diamond formation in this ureilite. [ABSTRACT FROM AUTHOR]