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Author's Abstract, COPYRIGHT 1999, Mineralogical Society of America Micro-Raman spectra of carbonates, silica and amorphous plagioclase, and both micro-Raman and IR reflectance spectra of phosphates in ALH84001 are reported. Data from these vibrational techniques combined with electron microprobe analyses show that (1) the carbonates exhibit complex compositional heterogeneity on a sub-micrometer scale, (2) the phosphates, chlorapatite and merrillite, are largely anhydrous, (3) amorphous silica and plagioclase experienced peak shock pressures >32 GPa and >50 GPA, respectively, and (4) vitreous plagioclase was quenched from a shock-induced melt after relaxation of the peak shock pressure. The observed general Raman band broadening of lattice and internal modes of carbonates in ALH84001 indicates complex sub-microscopic compositionalheterogeneity and possibly structural disorder. Any search for biogenic markers in ALH84001 must recognize the complex shock and thermal history of these minerals.

Abstract: The metal-rich carbonaceous chondrites (CB and CH) have the highest whole-rock 15N-enrichments (δ15N up to 1500‰) among planetary materials. They are also characterized by the absence of interchondrule fine-grained matrix. The only fine-grained material is present as lithic clasts, which experienced extensive aqueous alteration in contrast to the surrounding high-temperature components (chondrules, refractory inclusions, metal grains). Hence, the clasts are foreign objects that were incorporated at a late stage into the final parent body of Isheyevo. Their origin is poorly constrained. Based on mineralogy, petrography, and thermal processing of the aromatic carbonaceous component, different types of clasts have been previously identified in the CB/CH-like chondrite Isheyevo. Here, we focus on the rare lithic clasts characterized by the presence of anhydrous silicates (chondrules, chondrule fragments, and CAIs). Their mineralogy and oxygen isotopic compositions reveal them to be micro-chondrules, fragments of chondrules, and refractory inclusions related to those in the Isheyevo host, suggesting accretion in the same region. In contrast to previously studied IDPs or primitive chondritic matrices, the fine-grained material in the clasts we studied is highly and rather uniformly enriched in heavy nitrogen, with bulk δ15N values ranging between 1000‰ and 1300‰. It is also characterized by the presence of numerous 15N hotspots (δ15N ranging from 1400‰ to 4000‰). No bulk (δD <–240‰) or localized deuterium enrichments were observed. These clasts have the highest bulk enrichment in heavy nitrogen measured to date in a fine-grained material. They represent a unique material, of asteroidal or cometary origin, in our collection of cosmomaterials. We show that they were 15N-enriched before their incorporation in the final parent body of Isheyevo. They experienced an extensive aqueous alteration that most likely played a role in redistributing 15N over the whole fine-grained material and may have significantly modified its initial hydrogen isotopic composition. Based on a review of isotopic fractionation models, we conclude that the nitrogen isotopic fractionation process, its timing, and its location are still poorly constrained. The 15N-rich clasts may represent the surviving original carrier of the 15N anomaly in Isheyevo whole-rock. [Copyright &y& Elsevier]

Abstract: The 26Al–26Mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) – one ungrouped (Acfer 094), six CR, five CV, and one CO – reveal large variations in the initial 26Al/27Al ratio, (26Al/27Al)0. Well-resolved 26Mg excesses (δ26Mg) from the in situ decay of the short-lived nuclide 26Al (t 1/2 ∼0.72Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with (26Al/27Al)0 values ranging from ∼3×10−6 to ∼1.5×10−5. Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of 26Al but do not yield well defined values for (26Al/27Al)0, while the remaining chondrules do not contain excess radiogenic 26Mg and yield corresponding upper limits of (11–2)×10−6 for (26Al/27Al)0. The observed range of (26Al/27Al)0 in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0–3.4). However, unlike the UOC chondrules, there is no clear trend between the (26Al/27Al)0 values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of (26Al/27Al)0 are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower (26Al/27Al)0 values in PRCs from CCs, relative to most CAIs, are consistent with a model in which 26Al was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of (26Al/27Al)0 values in PRCs from CCs is most plausibly explained in terms of an extended duration of ∼2–3Ma for the formation of CC chondrules. This interval is in sharp contrast to most CAIs from CCs, whose formation appears to be restricted to a narrow time interval of less than 105 years. The active solar nebula appears to have persisted for a period approaching 4Ma, encompassing the formation of both CAIs and chondrules present in CCs, and raising important issues related to the storage, assimilation and mixing of chondrules and CAIs in the early solar system. [Copyright &y& Elsevier]

It is inferred that magnesian non-porphyritic chondrules in the CB (Bencubbin-type) carbonaceous chondrites formed in an impact generated plume of gas and melt at 4562.49 ± 0.21 Ma (Bollard et al., 2015) and could be suitable for the absolute age normalization of relative chronometers. Here xenon isotopic compositions of neutron irradiated chondrules from the CB chondrites Gujba and Hammadah al Hamra (HH) 237 have been analyzed in an attempt to determine closure time of their I–Xe isotope systematics. One of the HH 237 chondrules, #1, yielded a well-defined I–Xe isochron that corresponds to a closure time of 0.29 ± 0.16 Ma after the Shallowater aubrite standard. Release profiles and diffusion properties of radiogenic 129* Xe and 128* Xe, extracted from this chondrule by step-wise pyrolysis, indicate presence of two iodine host phases with distinct activation energies of 73 and 120 kcal/mol. In spite of the activation energy differences, the I–Xe isotope systematics of these two phases closed simultaneously, suggesting rapid heating and cooling (possibly quenching) of the CB chondrules. The release profiles of U-fission Xe and I-derived Xe correlate in the high temperature host phase supporting simultaneous closure of 129 I– 129 Xe and 207 Pb– 206 Pb systematics. The absolute I–Xe age of Shallowater standard is derived from the observed correlation between I–Xe and Pb–Pb ages in a number of samples. It is re-evaluated here using Pb–Pb ages adjusted for an updated 238 U/ 235 U ratio of 137.794 and meteorite specific U-isotope ratios. With the addition of the new data for HH 237 chondrule #1, the re-evaluated absolute I–Xe age of Shallowater is 4562.4 ± 0.2 Ma. The absolute I–Xe age of the HH 237 chondrule #1 is 4562.1 ± 0.3 Ma, in good agreement with U-corrected Pb–Pb ages of the Gujba chondrules (Bollard et al., 2015) and HH 237 silicates (Krot et al., 2005). All I–Xe data used here, and in previous estimates of the absolute age of Shallowater, are calculated using 15.7 ± 0.6 Ma value for 129 I half-life. The slopes of I–Xe – Pb–Pb correlation lines plotted for different sets of samples for Shallowater normalization are always ⩽1. Assuming uranium half-life values are correct; this restricts the half-life of 129 I to ⩽15.7 Ma. [ABSTRACT FROM AUTHOR]