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We have carried out a SEM‐EPMA‐TEM study to determine the textures and compositions of relict primary iron sulfides and their alteration products in a suite of moderately to heavily altered CM1 carbonaceous chondrites. We observed four textural groups of altered primary iron sulfides: (1) pentlandite+phyllosilicate (2P) grains, characterized by pentlandite with submicron lenses of phyllosilicates; (2) pyrrhotite+pentlandite+magnetite (PPM) grains, characterized by pyrrhotite–pentlandite exsolution textures with magnetite veining and secondary pentlandite; (3) pentlandite+serpentine (PS) grains, characterized by relict pentlandite exsolution, serpentine, and secondary pentlandite; and (4) pyrrhotite+pentlandite+magnetite+serpentine (PPMS) grains, characterized by features of both the PPM and PS grains. We have determined that all four groups were initially primary iron sulfides, which formed from crystallization of immiscible sulfide melts within silicate chondrules in the solar nebula. The fact that such different alteration products could result from the same precursor sulfides within even the same meteorite sample further underscores the complexity of the aqueous alteration environment for the CM chondrites. The different alteration reactions for each textural group place constraints on the mechanisms and conditions of alteration with evidence for acidic environments, oxidizing environments, and changing fluid compositions (Ni‐bearing and Si‐Mg‐bearing). [ABSTRACT FROM AUTHOR]

Introduction: As of 2020, the U.S. Antarctic meteorite program has collected >23,500 meteorites. The systematic collection methods employed have provided meteorites of >40 types, many of which are the first of their type ever recognized. One of the early drivers for characterization of the entire U.S. Antarctic collection was to allow statistical comparisons. Early assessments examined mass distributions and the relative frequency of meteorite types as well as comparisons to a defined set of modern falls [1-3]. Using these statistics [4-6] argued that the flux of H chondrites changed over time. [7] used model size distributions to deconstruct the contribution of wind movement, meteorite supply and search losses to the Antarctic collection. Mass-based statistics [8] and size distribution comparisons were examined by [8,9]. [10,11] investigated various statistics, including comparison with modern falls/Saharan finds. [12, 13] discuss geospatial statistics. [14] provides a comprehensive overview of the statistics of the Antarctic collections for the first 35 seasons of U.S. collection by ANSMET. Here we build upon that assessment and that from [15]. Statistics of the U.S. Collection: One of the most important questions surrounding the collection of meteorites in Antarctica is whether the collection procedure is recovering a representative sample of what is actually present at each site. In >40 seasons of searching, we have collected samples from 50 named field sites. Sixteen sites have produced >100 meteorites, and nine have produced >1000 [14]. Field areas with smaller populations (<1000) appear to have an overabundance of unusual meteorite types. However, field sites from which >1000 meteorites have been collected have type populations that converge at approximately 90% ordinary chondrites (OC). Antarctic meteorite populations show interesting trends when comparing certain classes and sizes of falls and hot desert populations. One of these shows large numbers of small samples of OCs, pointing to the possibility of preserved showers that may not have been taken into account in the overall numbers of parent meteorites [16]. These OC populations, combined with the unresolved issue of meteorite pairing, have a significant impact on a comprehensive statistical evaluation of the Antarctic meteorite population. Another visible trend between Antarctic meteorites, worldwide falls and hot desert meteorites is that there is a startling under-abundance of iron meteorites with low masses in the Antarctic collection [17]. Based on a theory that there should be a layer of iron meteorites stranded below the ice, [17] searched for clues to where these meteorites may be. Another obvious absence is CI chondrites. There may be a strength bias, but this is poorly understood as CMs are recovered and irons aren't overepresented. Mass Distribution: An alternative approach is to examine the cumulative mass distribution of a population relative to the number of meteorites represented. [1,14,15,18] point out that the mass of meteorites found in Antarctic field sites peaks at ~10g, while those of modern witnessed falls peak at ~5kg. Saharan meteorite peak at ~300g. As discussed by [18,19], systematic collection of meteorites in Antarctica and elsewhere recovers more small meteorites than do random searches. This remains logical in that small (<2 cm) meteorites are much easier to spot on Antarctic ice than they are in non-Antarctic locations. [18,19] show that the number of meteorites collected in various locations (including falls) has a wide variation when compared with total mass. However, if these meteorites were all thoroughly examined and put into pairing groups, the number of meteorites after pairing would certainly decrease, and if more small modern falls were actually recovered [18] this discrepancy would be minimized. When looking at total mass between Saharan and Antarctic meteorites, it can be seen that while there are ~3x fewer Saharan meteorites (~16000 classified) vs. Antarctics (~47000) [20], the mass of Saharan meteorites (~31.5 metric tons) vs. Antarctics (~6.1 metric tons) is ~5x higher. The major difference between these two populations is that the Antarctic meteorites, in all collections worldwide, are available to science. While we may never completely understand all mechanisms that impact the population of meteorites collected in Antarctica, we can be confident that Antarctic Meteorite Programs have exceeded expectations for providing a broad sampling of Solar System materials and have had a significant impact on meteorite science. [ABSTRACT FROM AUTHOR]

Introduction: The IIAB iron meteorites are one of the largest iron meteorite groups that formed by fractional crystallization [1]. Iron meteorites formed over a range of oxygen fugacities (fO2) [2], most formed relatively reduced at ~ IW-4 to -2.5 (IABs), where IW = iron-wüstite buffer, to relatively oxidized at IW-1 (IVBs) [2,3]. While the IIABs contain reduced mineral phases (daubréelite [1]), the fO2 for IIABs is poorly constrained. The fO2 of iron meteorites may have become more reducing during cooling, with oxidized phases (i.e., chromite) forming at higher temperatures and daubréelite forming at lower temperatures [4]. The O- and Cr-isotope compositions of silicates and chromite in meteorites, including iron meteorites, can determine potential genetic links to other meteorites and constrain if a meteorite is from the non-carbonaceous (NC) or carbonaceous (CC) group [e.g., 5-8]. Iron meteorite groups have been identified as being in the NC or CC group using Mo [9] and Ni [10] isotope compositions of their metallic component. Numerous iron meteorites contain minor amounts of silicates and oxides [e.g., 11], but most have not been analyzed for their Cr or Ti isotope compositions, with some exceptions (e.g., IIIABs [6,7], a IIG, and a IIAB [7]). We analyzed the compositions of silicates and chromite in six IIAB iron meteorites to investigate the relationship between IIABs and known meteorite groups, the fO2 of IIABs, and the origin of chromite in the IIABs. Samples and Analytical Procedures: We identified chromite in six IIABs: Coahuila, Gressk, Kopjes Vlei, Old Woman, Sandia Mountains, and Sikhote-Alin. Electron microprobe analyses (EPMA) were conducted at the Smithsonian Institution, University of Arizona, and Arizona State University (ASU). Chromite grains were mechanically extracted from Coahuila, Sandia Mountains, and Sikhote-Alin. Chromite was analyzed for bulk O-isotopes via laser fluorination at the Open University [e.g., 12] and bulk Cr-isotopes via inductively coupled plasma mass spectrometry at ASU [e.g., 8]. Daubréelite was extracted from North Chile for analysis. Bulk extraction for O-isotope analyses via laser fluorination was not possible for Gressk, Kopjes Vlei, and Old Woman. Instead, in situ O-isotope analyses via secondary ion mass spectrometry of chromite in Kopjes Vlei and Old Woman were determined at the University of Hawaiʻi (UH) to constrain their O-isotope compositions; in situ analyses of chromite in Sandia Mountains and Sikhote-Alin were also obtained for comparison to bulk O-isotope analyses. Chromite in Sikhote-Alin was found in association with olivine and pyroxene, which was also analyzed via EPMA and for in situ O-isotope compositions at UH. The equilibration temperature and fO2 for olivine-spinel in Sikhote-Alin was determined following [3]. Results and Discussion: The bulk chromite O-isotope (mean Δ17O ~ -1.1±0.1‰ [±2σ]) and Cr-isotope (mean ε54Cr ~ -0.97±0.22 [±2σ]) compositions obtained here indicate IIABs are NCs that overlap with the ureilite and acapulcoite/lodranite field. This is consistent with the recent work of [7], and with the assignment of the IIABs to the NCs [9,10]. The e54Cr composition of North Chile daubréelite is similar to that of IIAB chromite. The in situ chromite Δ17O ‰ compositions are indistinguishable from those of bulk chromite, supporting the robustness of Δ17O ‰ data from both techniques. In Sikhote-Alin, the in situ Δ17O ‰ composition of olivine is consistent with chromite, indicating they are cogenetic. Chromite is only surrounded by Fe,Ni metal in Kopjes Vlei, Old Woman, and Sandia Mountains, but is also associated with sulfides in Coahuila, Gressk, and Sikhote-Alin. Chromite in Gressk is associated with troilite and daubréelite, and in Sikhote-Alin it is associated with troilite, schreibersite, silica, kosmochlor, pyroxene, and olivine. Olivine (Fa6) and low-Ca pyroxene (Fs9-14) in Sikhote-Alin are most like acapulcoites (data from [3,13]). The Fe/(Fe+Mg) ratio (range 1-0.62) of chromite in each IIAB is correlated with their bulk Au content [1], a proxy for crystallization sequence [1,4], indicating that crystallization order influenced chromite compositions. Sikhote-Alin was the last IIAB studied here to crystallize, and it has the lowest chromite Fe/(Fe+Mg) ratio. The fO2 of Sikhote-Alin is calculated here to be IW-2.7, similar to IABs [2] and acapulcoites [3], indicating the IIABs are more reduced than the IVBs. The origin and fO2 history of IIABs is complex and will be discussed in detail. [ABSTRACT FROM AUTHOR]