Extraterrestrial Organic Chemistry in Meteorites
Unraveling the Organic Chemistry of the Early Solar System by Solid State NMR Spectroscopy, X-ray Absorption Near Edge Spectroscopy (XANES), and Pyrolysis Gas Chromatography-Mass Spectrometry (Pyr-GCMS)
Figure 1: This amazing image of the trail of the Tagish Lake meteorite I found on a Candian web site documenting the fall and collection of the Tagish Lake meteorite in Western Canada, British Columbia. Please check out Google@ or other search engine to find the owner of this image. The image serves to document the fact that the solar system “kindly” provides samples of its earliest history to us via meteorite impact.
One of the truly remarkable aspects of early solar system materials (that available for study on Earth) is that certain meteorite groups, in particular carbonaceous chondrites, contain a rich inventory of organic carbon that pre-dates the formation of the solar system. This organic matter records a complex succession of chemical histories that started with reactions that occurred in the interstellar medium, followed by reactions that accompanied the formation and evolution of the early solar nebula, and, ultimately, ended with reactions driven by hydrothermal alteration that occurred in the meteorite parent body.
Figure 2: This is an image of piece of the Tagish Lake meteorite, recovered from the ice in British Columbia, that was kindly passed to meteorite scientist, Mike Zolensky (NASA, JSC) [this image was obtained courtesy of Google “Tagish Lake” from Mike Zolensky, NASA, JSX]. Please note that the block on the left is a centimeter a side. The chondrules and CAI’s are obvious as white bodies embedded in a dark black matrix. The insoluble organic matter exists in the dark matrix. This meteorite has, so far, defied classification beyond identifying it as a generic carbonaceous chondrite exhibiting type 2 alteration (i.e., an ungrouped C2 meteorite).
One of the challenges in understanding the early evolution of the solar system involves establishing whether one can identify chemical signatures of these reactions encoded in the chemical structure of the meteoritic organic fractions and whether there exists any relationship between the molecular structure(s) of organic matter with the classic designations of meteorite group and type, e.g. the degree of alteration. One of the problems with addressing such questions is that between 70-99% of the organic carbon contained with carbonaceous chondrites is insoluble in any solvent. Thus, the classic molecular methods available for the study of complex organic assemblages (e.g., Gas Chromatography-Mass Spectrometry-GCMS) are not available.
Cody in collaboration with DTM colleague Alexander have set out to utilize solid state Nuclear Magnetic Resonance (NMR) Spectroscopy as a primary method of deriving self-consistent and quantitative analyses of the insoluble organic matter (IOM) fractions obtained from carbonaceous chondrites. In order to this end we utilize a custom designed demineralization procedure derived by our colleague Fuoad Tera (DTM) that employs high molarity CsF aqueous solutions with the pH adjusted with HF to achieve nearly perfect demineralization. With these isolates we are able to apply solid-state nuclear magnetic resonance spectroscopy to thoroughly characterize the meteorite IOM. We have developed a protocol that employs 8 independent NMR experiments to yield an internally consistent analysis of the essential molecular characteristics. These experiments are time consuming, however, and it typically takes about one month to completely analyze a given IOM fraction. We use these data as a means of comparing the IOM fractions from different meteorites in order to assess the role that parent body and other processes have modified IOM from its pre-solar or otherwise pristine state.
Figure 1: This 13C variable amplitude-cross polarization magic angle spinning (VA-CPMAS) NMR spectrum of the IOM from Murchison (a CM2 Chondrite) reveals the enormous chemical complexity of extraterrestrial organic matter. Analysis is limited to designating specific spectral regions a likely reflecting the contribution from various functional groups, e.g. alcohols and/or ethers (CHxO).
A Comparison of Organic Matter From Four Different Meteorite Groups: We have recently completed a series of solid-state 1H and 13C Nuclear Magnetic Resonance (NMR) Spectroscopic experiments on isolated meteoritic Insoluble Organic Matter (IOM) obtained from four different carbonaceous chondrite meteorites; a CR2 (EET92042), a CI1 (Orgueil), a CM2 (Murchison), and an undesignated rank 2 meteorite, Tagish Lake. We find that the solid state NMR experiments reveal considerable variation in bulk organic composition across the meteoritic IOM fractions. For example, the fraction of aromatic carbon increases as CR2 < CI1 < CM2 < Tagish Lake (C2). These increases in aromatic carbon are largely offset by reductions in aliphatic (sp3) carbon moieties, e.g, CH3, CH2, CH grouped as “CHx”, and CH2O, CHO, CO grouped as “CHxO”. Oxidized sp2 bonded carbon, e.g. carboxyls and ketones grouped as “CO”, are largely conservative across these meteorite groups. Single pulse (SP) 13C magic angle spinning (MAS) NMR experiments reveal the presence of nanodiamonds in each IOM fraction with an apparent concentration ranking of CR2 < CI1 < CM2 < Tagish Lake. A pair of independent NMR experiments reveals that the aromatic moieties in all four meteoritic IOM fractions are structurally similar with high degrees of substitution.
We conclude from these data that both the aromatic moieties and nanodiamonds may have been unaffected by low temperature parent body processes and constitute inert markers of organic reaction progress. Fast spinning SP 1H MAS NMR spectra provide information on the fractions of aromatic and aliphatic hydrogen that when combined with other NMR experimental data reveal that the average hydrogen content of sp3 bonded carbon functional groups is universally low indicating a high degree of branching in each IOM fraction. Inspection of Figure 4 (below) reveals the enormous differences in the organic structure of IOM obtained from these four meteorites. The reduction in intensity at ~ 15-70 ppm corresponds to a loss in saturated organic carbon, CHx and CHxO. The apparent gains at ~ 129 ppm reflect a relative increase in aromatic carbon. As a product of our analysis, we believe that the aromatic carbon is inert under these parent body conditions.
Figure 4: These NMR spectra reflect ENORMOUS differences in the carbon chemistry of meteoritic IOM material spanning meteorite group. We believe that these differences reflect differences in parent body processing (see Cody and Alexander, GCA 2004 for the gory details). Note that this is just the “tip of the ice berg” of what solid state NMR is uniquely providing us. (Please see Cody et al. GCA to be published late 2004-earlier 2005)
These data lead us to conclude that the dominant chemical reaction that yields these chemical differences involve low temperature chemical oxidation during the earliest stages of parent body alteration. The Tagish lake IOM, evidently, suffered considerably more chemical oxidation during parent body processing than did the CR2 (EET92042) IOM. As to the nature of the oxidant, the chemical modification of the IOM requires a fairly strong oxidant. Our current best guess is that the oxidant was hydrogen peroxide, a likely constituent of the ices from which the aqueous fluids were derived. Hydrogen peroxide may have reacted with any Fe2+ in the system to form Fe(OH)2+ and hydroxy radicals •OH. Hydroxy radicals would have severely degraded the IOM fraction yielding chemical changes identical to those recorded in Figure 1.
The Effects of Thermal Metamorphism Recorded in IOM: Our interest in thermal metamorphism’s effects on IOM were initiated by an early NMR paper on IOM by Cronin, Pizzarello, and Frye (GCA, 1987); where they analyzed a partially purified IOM fraction of Allende. Remarkably, their sample appears to have substantial aliphatic carbon (CHx), in spite of the numerous reports of Allende being subjected to moderate heating (e.g. up to ~ 530 °C). The problem is that Allende has, relatively speaking, only a small amount of organic carbon (~ 0.2 wt %). It took a considerable time to isolate Allende’s IOM fraction (approximately 3 months of repeated treatments and washes) with a yield of ~ 35 mg of nearly pure IOM. We were fortunate to also receive a sample of Y86720, a well studied thermally metamorphosed CM chondrite. According to some in the literature, the mineralogy of Y86720 may record peak temperatures of up to 850 °C.
Whereas it was an enormous challenge to obtain organic matter from Allende, it turned out that acquiring NMR data on this sample also provided a significant challenge. After numerous attempts we could obtain no signal when using acquisition parameters that were optimized for all previous IOM NMR studies. We came to understand that both Y86720 and Allende IOM are distinctly different from other meteorite IOM fractions and have now optimized the NMR acquisition parameters to analyze these. Once we obtained these spectra it became immediately obvious what the problem had been (please stay tuned!).
Compositional Variation Accompanying Hydrothermal Alteration: We are about half way through our analysis of a suite of CM chondrite IOM fractions with varying degrees of hydrothermal alteration. To date we have completely analyzed Murchison, Cold Bokkefeld, and ALH83100. We working on Murray, Mighei, Bells, and eagerly anticipate a CM1.
Micro XANES analysis of Meteoritic IOM: With the return of the Stardust sample collector anticipated in early 2006 there is considerable interest in how to maximize the information on organic matter that may have survived sample capture. Obviously, these samples will be much too small to use NMR for analysis. We are, therefore, applying micro C-, N-, and O-X-ray Absorption Near Edge Spectroscopy (XANES) to our collection of well-characterized meteorites to provide a definitive data base by which Stardust samples (and IDP’s) may be compared. In August 2003 we presented our approach to this issue at the Stardust sample return workshop at Crystal Mountain.
Figure 1: Carbon Near Edge X-ray Absorption Spectroscopy of Insoluble Organic Matter derived from four different carbonaceous chondrites. Note the systematic differences in the intensity of various 1s to bound-state transitions that correlate with the chemical differences observed via solid state NMR (see above).
Elemental Analysis and Stable isotopic Abundances: In collaboration with Dr. Marilyn Fogel at the Geophysical Laboratory we are obtaining H/C, O/C, and N/C data was well as dD, d15N, d13C, and d18O on each of the IOM fractions.
Figure (above): Meteoritic Insoluble Organic Carbon differs radically in compositional trends of O/C vs. H/C relative to terrestrial type III kerogens.
Figure (above): Pyrolysis-GCMS selected ion chromatograms highlighting the relative distribution of alkyl aromatics, phenol, and naphthalene/alkylnaphthalene. Qualitative trends in molecular structure are revealed in these pyrolysate data.
Pyrolysis Gas chromatography and Mass Spectrometry (Pyr-GC-MS): In addition to the analyses described above we have been analyzing our IOM samples with pyr-GC-MS. Pyr-GC-MS can provide a molecular fingerprint of meteoritic IOM requiring less than 1 mg. of sample. The data derived from pyr-GC-MS is complicated by the fact that one detects the stable products derived from rapid heating in a helium stream. The molecular distribution so derived cannot readily be re-integrated to derive a quantitative picture of the extraterrestrial macromolecule that is IOM. We have been developing an approach wherein we can use pyr-GC-MS data to define a compositional space where in trends in IOM evolution with parent body processing can be revealed.
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