When the most primitive types of meteorite, the chondrites, formed, all of them incorporated small amounts of at least two types of unprocessed pre-solar material: interstellar organic matter and circumstellar grains. The organic matter formed in the protosolar molecular cloud. To date, six types of circumstellar grain have been identified and their isotopic compositions show that they formed around red giant/asymptotic giant branch (RGB/AGB) stars, supernovae and, possibly, novae and Wolf-Rayet stars.

A unique spectroscopic feature has been identified in a Study of twenty-five different samples of meteoritic insoluble organic matter (IOM) spanning multiple chemical classes, groups, and petrologic types, using carbon X-ray Absorption Near Edge Structure (XANES) spectroscopy. The intensity of this feature, a 1s-sigma* exciton, appears to provide a precise measure of parent body metamorphism. The intensity of this exciton is also shown to correlate well with a large negative paramagnetic shift observed through solid state C-13 NMR. Experiments reveal that upon heating primitive IOM is transformed into material that is indistinguishable from that in thermally processed chondrites, including the development of the 1s-sigma* exciton. A thermo-kinetic expression is derived front the experimental data that allows the intensity of the 1s-sigma* exciton to be used to estimated the effective temperature integrated over time. A good correlation is observed between the intensity of the 1s-sigma* exciton and previously published microRaman spectral data. These data provide a self-consistent organic derived temperature scale for the purpose of calibrating Raman based thermometric expressions. (C) 2008 Elsevier B.V. All rights reserved.

We report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba (CV3), and Semarkona (LL3.0). For the unheated CMs, the total ranges of carbonate isotopic compositions are C-13 approximate to 25-75 parts per thousand and O-18 approximate to 15-35 parts per thousand, and bulk carbonate C contents range from 0.03 to 0.60wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0-130 degrees C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (O-18 approximate to 5 parts per thousand)

Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues about their original formation environment. The organic materials in primitive solar system bodies generally have higher D/H ratios and show greater D/H variation when compared to D/H in solar system water. We propose this difference arises at least in part due to (1) the availability of additional chemical fractionation pathways for organics beyond that for water, and (2) the higher volatility of key carbon reservoirs compared to oxygen. We test this hypothesis using detailed disk models, including a sophisticated, new disk ionization treatment with a low cosmic-ray ionization rate, and find that disk chemistry leads to higher deuterium enrichment in organics compared to water, helped especially by fractionation via the precursors CH2D+/CH3+. We also find that the D/H ratio in individual species varies significantly depending on their particular formation pathways. For example, from similar to 20-40 au, CH4 can reach D/H similar to 2 x 10(-3), while D/H in CH3OH remains locally unaltered. Finally, while the global organic D/H in our models can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir, our models are unable to reproduce the most deuterium-enriched organic materials in the solar system, and thus our model requires some inheritance from the cold interstellar medium from which the Sun formed.

Asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. They are also likely the sources of most of the water and other volatiles accreted by Earth. In this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. The asteroidal parent bodies of meteorites formed <= 4 Ma after Solar System formation while there was still a gas disk present. It seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the E-, S-, M- and V-type asteroids formed sunward of Jupiter's orbit, while those associated with C- and, possibly, D-type asteroids formed further out, beyond Jupiter but probably not beyond Saturn's orbit. Comets formed further from the Sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. CI and CM group meteorites are probably related to the most common C- type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the H and N accreted by the terrestrial planets. However, comets may have been major sources

Chondrules are the main components of primitive meteorites and possibly the building blocks of planetary embryos and terrestrial planets. However, their ages and modes of formation are still highly debated. Here, we present high-precision Cr isotope data of nine chondrules from one of the more primitive chondrites, the CO3 chondrite Ornans. These chondrules define an external Mn-53-Cr-53 isochron, with an initial Mn-53/Mn-55 of (7.1 +/- 1.6) x 10(-6), corresponding to an age of 4567.6 +/- 1.3 Ma when anchored to the angrite D'Orbigny (U-corrected). This age is within error of the age of formation of calcium-aluminum-rich inclusions (CAIs). All chondrules show a wide range of epsilon Cr-54 values (+0.20 to +1.22) and a positive correlation between epsilon Cr-53 and epsilon Cr-54 values, suggesting mixing of different isotopic sources in the protoplanetary disk. This could reflect that silicate materials from the CAI-forming region (with complementary compositions to CAIs, i.e., low Mn/Cr and epsilon Cr-54) were transported to the accretion region of the CO chondrite parent body and mixed with CI-like material (high-Mn/Cr and epsilon Cr-54) during chondrule formation. Such mixing must have occurred prior to the formation of chondrule precursors. Furthermore, chondrules from chondrites with more CAIs (CV and CO) exhibit greater variability in epsilon Cr-54 than chondrules from chondrites formed later with fewer CAIs (e.g., CB and CR), suggesting that the accretion regions of the former received more material transported from the inner solar system than the latter. This dichotomy may indicate the CB and CR chondrites accreted at greater orbital distances than other chondrites.

Chondrites are exhumed from their parent bodies by impacts, which at the same time can result in heating and mechanical modification (compaction, deformation, fracturing, etc.). However, whether impacts are responsible for the occurrence of heated C2s remains controversial since radiogenic and solar heating have also been invoked to explain them. Here we report a Raman and infrared study of the composition and structure of Insoluble Organic Matter (IOM) in a series of 39 CM and C2-ungrouped chondrites. These parameters are tracers of the extent and nature of thermal metamorphism a meteorite has experienced and reflect the degree to which the thermally driven and irreversible carbonization of IOM has proceeded. We propose a carbon-based classification of heated C2 chondrites that reveals a high occurrence frequency of thermally processed C2 chondrites (>36%). This classification is in agreement with the mineralogical classification scheme of Nakamura (2005). Strongly heated C2 chondrites (PCA 02012, PCA 91008, Y 96720) display an IOM structural evolution that is dissimilar to that of type 3 chondrites that experienced long duration radiogenic thermal metamorphism. These differences almost certainly reflect kinetic constraints on IOM modification during short duration heating events. QUE 93005 is a weakly heated chondrite that experienced a retrograde aqueous alteration. Its very aliphatic-rich IOM points to a parent body hydrogenation through interactions with water. The closed-system conditions required by this mechanism could be satisfied by a kinetic confinement during a very short duration impact. MET 01072, a heavily compacted and uni-axially deformed chondrite, did not experience post-accretional heating. In this case, the deformation features probably reflect a low-velocity impact. In contrast, the weakly metamorphosed chondrite EET 96029 experienced one or several low pressure impacts that triggered mild heating and partial dehydration without deformation features. The study of a series of lithologies from the Tagish Lake C2-ungrouped chondrite confirms the coexistence of various degrees of post-accretional alteration, the most altered lithologies having experienced a moderate degree of heating. Overall, the high prevalence of heating in C2 chondrites, the evidence of short-duration heating in the most heated C2s and the ability of low velocity collisions to trigger heating favor impacts (against solar heating), as the dominant heating mechanism. Finally, our set of data does not support the action of a low temperature oxidation process that would control the aliphatic abundance in unheated primitive C2s. (C) 2018 Elsevier Ltd. All rights reserved.

To constrain the conditions of aqueous alteration in early planetesimals, we carried out in situ C and O isotope analyses of calcite and dolomite and O isotope analyses of magnetite from the highly altered CM chondrites ALH 83100, ALH 84034, and MET 01070. Petrographic and isotopic analyses of these samples support previous findings of multiple generations of carbonate growth. We observe wide ranges in the C and O isotope compositions of carbonates of up to 8 parts per thousand and 30 parts per thousand, respectively, that span the full range of previously reported bulk carbonate values for CM chondrites. Variations in the Delta O-17 values indicate that fluid evolution varied for each chondrite. ALH 83100 dolomite-magnetite delta O-18 fractionation of 23 parts per thousand +/- 7 parts per thousand (2SD) corresponds to dolomite formation temperature of 125 degrees C +/- 60 degrees C. delta C-13 vs delta O-18 values fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. The positive correlation between delta O-13 and delta O-18 for primary calcite is consistent with the precipitation of calcite in equilibrium with a gas mixture of CO (or CH4 ) and CO2. The isotopic composition of calcite in CM1s and CM2s overlap significantly; however, many CM1 calcite grains are more depleted in delta O-18 compared to CM2s. Altogether, the data indicate that the fluid composition during calcite formation was initially the same for both CM1s and CM2s. CM1s experienced more episodes of carbonate dissolution and reprecipitation where some fraction of the carbonate grains survive each episode resulting in a highly disequilibrium assemblage of carbonates on the thin-section scale. (C) 2019 The Author(s). Published by Elsevier Ltd.

Insoluble organic matter (IOM) is the major organic component of chondritic meteorites and may be akin to organic materials from comets and interplanetary dust particles (IDPs). Reflectance spectra of IOM in the range 0.35-25m are presented as a tool for interpreting organic chemistry from remote measurements of asteroids, comets, IDPs, and other planetary bodies. Absorptions in the IOM spectra were strongly related to elemental H/C (atom) ratio. The aliphatic 3.4m absorption in IOM spectra increased linearly in strength with increasing H/C for H/C>0.4, but was absent at lower H/C values. When meteorite spectra from the Reflectance Experiment Laboratory (RELAB) spectral catalog (n=85) were reanalyzed at 3.4m, this detection limit (H/C>0.4) persisted. Aromatic absorption features seen in IOM spectra were not observed in the meteorite spectra due to overlapping absorptions. However, the 3.4m aliphatic absorption strength for the bulk meteorites was correlated with both H/C of the meteorite's IOM and bulk C (wt%). Gaussian modeling of the 3m region provided an additional estimate of bulk C for the meteorites, along with bulk H (wt%), which is related to phyllosilicate abundance. These relationships lay the foundation for determining organic and phyllosilicate abundances from reflectance spectra. Both the full IOM spectra and the spectral parameters discussed here will aid in the interpretation of data from asteroid missions (e.g., OSIRIS-REx, Hayabusa2), and may be able to place unknown spectral samples within the context of the meteorite collection.