Multi-method Investigation of Several Fragments of the Tagish Lake C2 Carbonaceous Chondrite, a Co-ordinated Mineralogical, Imaging, Chemical and Spectroscopic Reconnaissance
In situ, powdered sample and thin section-based reconnaissance studies of fragments of the Tagish Lake C2 carbonaceous chondrite has identified unique properties of the meteorite requiring further study. Areas and objects of interest within thin sections were investigated at a range of spatial scales using optical petrography, high resolution SEM-BSE, SEM-cathodoluminescence, semi-quantitative SEM-EDX chemical mapping, quantitative chemical analysis (EPMA) and in situ crystal structure analysis using micro X-ray diffraction (μXRD). A touchless reconnaissance study of a Tagish Lake chondrite display-piece was also carried out using μXRD. These reconnaissance studies provided important context for sample correlated bulk mineralogy and reflectance infrared spectroscopy studies of other Tagish Lake samples from seven different collection sites. Rietveld refinement of high-resolution powder X-ray diffraction (XRD) data was used to determine quantitative major mineral abundances. Thermal infrared spectra of the Tagish Lake samples were obtained using biconical (Diffuse) Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) which provides an analogue for remote-sensing emission spectra with higher signal-to-noise and may enable the comparison of Tagish Lake lithologies with asteroid spectra. This integrated study provided several new insights and documented a number of areas for future study: 1) the recognition of a magnetite- and- sulfide- rich lithology, and a carbonate-rich, siderite-dominated lithology in addition to the previously documented carbonate-rich and carbonate-poor Tagish Lake lithologies; 2) grain densities for seven Tagish Lake samples calculated from the refined modal mineral abundances, which are in good agreement with published values for intact fragments; 3) a relict CAI containing spinel, dolomite, phyllosilicates and traces of perovskite; 4) previously unreported relict mesostasis glass in several Tagish Lake chondrules; 5) cathodoluminescence in spinel, calcite, several distinct populations of forsterite; 6) the presence of gypsum of uncertain origin, that would be missed in more conventional petrographic or microbeam studies. This work has also contributed to the development of Rietveld refinement, μXRD and SEM-CL as tools for meteoritics research.
Presolar Organic Globules in Tagish Lake Meteorite and Other Astromaterials
Presolar grains were identified in meteorite residues 20 years ago based on their exotic isotopic compositions . Their study has provide new insights into stellar evolution and the first view of the original building blocks of the solar system. Organic matter in meteorites and IDPs is highly enriched in D/H and 15N/14N at μm scales, possibly due to presolar organic grains [2-4]. These anomalies are ascribed to the partial preservation of presolar cold molecular cloud material. Identifying the carriers of these anomalies and elucidating their physical and chemical properties may give new views of interstellar chemistry and better understanding of the original components of the protosolar disk. However, identifying the carriers has been hampered by their small size and the inability to chemically isolate them. Thanks to immediate careful collection of Tagish Lake meteorite specimen, as well as major advances in nano-scale analytical techniques and advanced sample preparation, we were able to show that in the Tagish Lake meteorite, the principle carriers of these isotopic anomalies are sub-μm, hollow organic globules . The organic globules likely formed by photochemical processing of organic ices in a cold molecular cloud or the outermost regions of the protosolar disk . Organic globules with similar physical, chemical, and isotopic properties are also recently found from Bells CM2 carbonaceous chondrite, in IDPs  and in the comet Wild-2 samples returned by Stardust . These results support the view that microscopic organic grains were widespread constitutents of the protoplanetary disk. Their exotic isotopic compositions trace their origins to the outermost portions of the protosolar disk or a presolar cold molecular cloud.  Zinner E. in Treatise on Geochemistry (2004), pp. 17 - 39.  Messenger S. (2000) Nature 404, 968  Busemann H. et al. (2006), Science 312, 727  Floss C. et al. (2004) Science 303, 1355  Nakamura-Messenger K. et al. (2006) Science 314, 1439  Messenger et al. (2008) LPSC XXXIX, #2391  Matrajt et al. (2007) MAPS 42, 5138
Organic Matter in the Tagish Lake Meteorite: Lithological Differences
The Tagish Lake meteorite is the most carbon-rich of the carbonaceous chondrites, containing nearly 6 wt% C, almost half of which is organic (Grady et al. 2002). The insoluble organic matter (IOM) in the meteorite preserves hydrogen and nitrogen isotopic anomalies consistent with an interstellar or outer solar system origin (Busemann et al. 2006; Nakamura-Messenger et al. 2006). Lithologically, the Tagish Lake meteorite is anything but homogeneous. Besides the carbonate-rich and carbonate-poor lithologies described by Zolensky et al. (2002), there are other lithologies apparent from casual inspection of the exterior of individual samples (Herd and Herd, 2007). Most notable is a dark, dusty lithology that consists almost entirely of very fine-grained matrix material (see Blinova et al., this meeting). By systematically investigating the mineralogical, petrological, geochemical and organic chemical similarities and differences among lithologies within the Tagish Lake meteorite, we hope to gain insights into presolar, nebular and parent-body processes preserved within it. Results of raster ion imaging in the Carnegie NanoSIMS 50L yielded significant nanoscale variations in H, C and N isotopic compositions (Herd et al. 2009, LPSC), and a notable lack of association between 15N and D enrichments and globular structures in contrast to the study of Nakamura-Messenger et al. (2006). Notably, the range of isotopic compositions is greater in the matrix-rich, dark, dusty lithology than in the chondrule-bearing lithologies, suggesting that the dark, dusty lithology may have endured less extensive parent-body alteration, or otherwise has a larger complement of interstellar or outer solar system components. Results of GC-MS analysis of dichloromethane (DCM) and water soluble organics in the Tagish Lake meteorite shows significant differences in the relative proportions of organic molecules among the lithologies (Hilts et al. 2009, LPSC). Compared to chondrule-bearing lithologies, the dark, dusty lithology contains a more reduced suite of (water soluble) monocarboxylic acids, but is depleted in (DCM soluble) alkanes and alkyl- substituted benzenes. One possibility is that this suite represents a primitive set of organic compounds, from which the suite of compounds in other lithologies were derived; for example, the suite of alkanes in chondrule- bearing lithologies may have been derived by hydrothermal decarboxylation of aliphatic monocarboxylic acids. Compound-specific isotopic analysis is underway to test this possibility. In summary, lithological differences within the Tagish Lake meteorite are significant and cannot be ignored. The analysis of organic matter in the context of lithological differences provides novel insights into the processes involved in its synthesis and modification.
Carboxylic Acid Abundances in the Tagish Lake Meteorite: Lithological Differences and Implications for Formic Acid Abundances in Carbonaceous Chondrites
The most abundant soluble organic compounds in carbonaceous chondrites are typically carboxylic acids (Pizzarello et al 2001). Strait-chain monocarboxylic acids up to C-12 have been the focus of considerable attention owing to the exciting possibility that they may have been incorporated into the molecular architecture of prebiotic protomembranes on the ancient Earth (e.g., Silva et al 2004.) The most abundant monocarboxylic acid in interstellar space is formic acid (e.g. Remigen et al. 2003; Snyder 2008). It is generally accepted that the organic material in carbonaceous chondrites such as the Tagish Lake meteorite, which includes carboxylic acids, is derived from interstellar or nebular sources (Cronin et al 1988 and Cronin et al 1993). It is somewhat surprising, therefore, that up until now only what have been described as small or moderate formic acid concentrations have been found in aqueous extracts of carbonaceous chondrites (Huang et al 2005, Naraoka et al 1999, Yuen et al 1973, Shimoyama et al 1986, Yuen et al 1984 and Krishnamurthy et al 1992). Previous reports have ascribed the unexpectedly low formic acid abundances to either compound loss during extraction and subsequent work up, or to depletion caused by evaporation and/or aqueous leaching of the compound from the meteorite upon its exposure to the Earth's hydrosphere (Huang et al 2005 and Naraoka et al 1999). Here we present our analysis of the water-soluble monocarboxylic acids in two different lithologies within the Tagish Lake meteorite using the SPME-GCMS procedures recently developed by (Huang et al 2005) to compare the two lithologies in this respect. Our results conclusively show that formic acid is, by a wide margin, the most abundant monocarboxylic acid in both of the Tagish Lake lithologies investigated thus far. This is in stark contrast to all previous studies of other meteorites in which it was concluded that the formic acid concentration was the lowest or one of the lowest of those monocarboxylic acids present in the extract (Huang et al 2005, Naraoka et al 1999, Yuen et al 1973, Shimoyama et al 1986, Yuen et al 1984 and Krishnamurthy et al 1992). Moreover, our serendipitous discovery that formic acid has a very low response factor when run on either GCMS(quadrupole) or GC-FID (Allen et al 1987) instruments suggests that previous studies on carbonaceous chondrites may have dramatically underestimated the quantities of formic acid present. Also, a close inspection of the relative abundances for the straight-chain monocarboxylic acids in each Tagish Lake lithology has led us to conclude that the overall oxidation levels for the water soluble organics from the two lithologies are different. Lastly, we have found that the monocarboxylic acids within the Tagish Lake meteorite are enriched in deuterium compared to terrestrial organics, with delta D values ranging from + 247 to + 581%o. These results confirm that the acids originate from interstellar space and that terrestrial contamination has been largely avoided.
The Making Of Carbonaceous Chondrites: A Tale of Flash Melting, Metamorphism, Hydrothermal Activity And Organic Chemistry
The carbonaceous chondrites are a diverse class of meteorites, comprising 8 recognized groups and a number of unique members, such as Tagish Lake. They are generally thought to be fragments of Main Belt asteroids, although it has been suggested that some come from the remains of inert comets. Carbonaceous chondrites are classified on the basis of their bulk elemental and O isotopic compositions, and their petrology. They are aggregates of solar nebula material whose major components are high temperature chondrules and Ca-Al-rich inclusions (CAIs) that are set in a fine-grained silicate-dominated matrix. These primary features, which were acquired during accretion of the meteorite parent bodies, are overprinted to varying degrees by thermal metamorphism and/or aqueous alteration that took place in the parent bodies. The metamorphism and alteration were probably driven by internal heating of the asteroids from decay of short- lived radionuclides. CAIs are the oldest known Solar System objects. They are assemblages of high temperature minerals and formed by melting of pre-existing material and condensation from hot gas. Chondrules formed ∼1-3 Ma after CAIs by melting of pre-existing solids in brief nebula heating events that lasted minutes to hours. The heating mechanism(s) for producing CAIs and chondrules remain unidentified, although many have been suggested. The primary mineralogy of the matrix is poorly preserved, but there are indications that it was largely amorphous. It is in the matrix that presolar materials inherited from the protosolar molecular cloud are preserved, such as circumstellar grains that formed around red giant stars and supernovae. The matrix also includes organic material that may have formed in the interstellar medium and/or the Solar System. This organic material includes a macromolecular material with an elemental composition not unlike a mature coal and similar to comet Halley CHON particles, and a number of amino acids and nucleic acids, some with small enantiomeric excesses. Hence the speculation that the organics in carbonaceous chondrites played a role in the origin of life on Earth. Metamorphism resulted in recrystallization of silicates in all components,as well as the destruction of the presolar materials and organics. The aqueous alteration seems to have been a process akin to low temperature serpentinization on Earth. The alteration occurred under a range of conditions that has affected not only the major mineralogy, but also the suite of organics present through processes that remain unclear.