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Project Progress

2. Extra-Terrestrial Materials: Origin and Evolution of Organic Matter and Water in the Solar System

1. Organic Matter in Extraterrestrial Samples

The abundant organic compounds in primitive meteorites and interplanetary dust particles (IDPs) are thought to originate largely in the interstellar medium. However, this material may have been modified in the protoplanetary disk and has been modified to varying extents in the asteroidal parent bodies. Thus the organic matter is a record of processes that span the prehistory and early history of the Solar System. Also, it has been suggested that meteorites and IDPs would have been a significant sources of complex organic matter on the early Earth. What compounds would have been either directly delivered to Earth or produced by weathering of extraterrestrial material is of direct relevance to astrobiology. Finally, the organic matter in meteorites is likely to be the best analogue of the refractory organics in cometary dust to be returned in January 2006 by the Stardust mission. The small size of the Stardust dust particles makes analysis of the samples very challenging. We are using the organics to calibrate microanalytical techniques for analysis of the Stardust samples.

1.1 Bulk Elemental and Isotopic Analysis

Alexander and his collaborators completed the preparation of high-purity samples of insoluble organic matter (IOM) from roughly 40 of the most primitive chondritic meteorites as well as the measurement of the bulk elemental and isotopic compositions of the IOM samples. The results have been used to select samples for more detailed studies by nuclear magnetic resonance (NMR) spectroscopy, gas chromatography/mass spectrometry (GCMS), X-ray absorption near edge spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), Raman and infrared (IR) spectroscopy, and transmission electron microscopy (TEM) and ion microscopy.

The matrix-normalized abundances of IOM in the most primitive members of the chondrite groups are all roughly CI-like, consistent with all chondrites having accreted a common IOM in their matrices. However, the IOM exhibits a complex response to asteroidal parent body processes that is also seen in detailed NMR studies of a more limited number of meteorite IOM samples. The general trend in elemental evolution of IOM during parent body processing is a large decrease in H/C (0.75 to 0.1) and more modest change in O/C (0.2 to 0.1). This is very different from terrestrial kerogen evolution in which initially O/C drops much more rapidly than H/C.

The most primitive IOM in this sequence seems to come from the CR chondrites. The IOM from EET92042 (CR2) has an elemental composition (C₁₀₀H₇₅N₄O₁₅S₄) that is very similar to the composition (C₁₀₀H₈₀N₄O₂₀S₂) of comet Halley CHON particles. It is also isotopically very anomalous (δD=3003‰, δ¹⁵N=184‰). These bulk compositions are also comparable to the estimated bulk compositions of IOM in interplanetary dust particles (IDPs). IDPs are often considered to be the most primitive extraterrestrial materials available to us at present.

Alteration in CI (H/C=0.66-0.57) and CM (H/C=0.64-0.57) chondrites has preferentially removed aliphatic material (lowered H/C) and was accompanied by a decrease in D (1000 to 500‰) and ¹⁵N (30 to -10‰) enrichments. These characteristics suggest that the isotopically anomalous H and N are in the aliphatic component. However, the anomalous CM Bells has a H/C=0.57, but δD=2984‰ and δ¹⁵N=415‰. There are very modest differences between the least and most altered CMs, and the CIs, which experienced more extensive alteration at higher temperatures than the CMs, are more isotopically anomalous than all CMs except Bells.

The CV, CO, and OC chondrites experienced varying degrees of both metamorphism and aqueous alteration. Low H/C ratios (0.19-0.16), among other evidence, indicate extensive modification of the IOM in all CVs studied. Oxidized CVs have significantly lower δD values (194-239‰) than reduced CVs 2(500-1360‰). The CO chondrites analyzed exhibit a wider range of H/C ratios than the CVs (0.39-0.09), but their total range in D enrichments is smaller (212-417‰). The OCs studied have a similar range in H/C ratios to the COs, but they show a remarkable inverse correlation between H/C and δD (0.17 to 0.5, and 5560‰ to 2321‰, respectively).

1.2 Raman Spectroscopy

Raman spectroscopy is a sensitive and relatively benign tool for analyzing the gross microstructure of carbonaceous material in meteorites, IDPs, and returned Stardust samples. Single-crystal graphite and disordered finite-sized microcrystallites exhibit the distinctive “G” and “D” Raman bands whose peak height ratio, relative positions, and half widths characterize the crystallinity of carbonaceous material.

Raman spectroscopic analyses have been completed on IOM extracted from >40 chondritic meteorites with a new scanning laser-induced confocal Raman microscope (WiTec α-SNOM). The large, systematic spectral differences observed can be attributed to the distinct histories experienced by the meteorites on their asteroidal parent bodies. Raman features of the IOM from the least thermally metamorphosed CI, CR, and CM chondrites are in general very similar. This finding supports the view that the chondrites all accreted similar IOM. Parent body metamorphism is reflected by strong correlations of D-band position and width or G-band width with metamorphic degree, as shown, for example, for CO3 chondrites. Moreover, the D-band position correlates perfectly with the atomic H/C ratios in the residues. H/C reflects the relative abundances of aromatic and aliphatic organic matter and largely depends on thermal alteration experienced on the parent bodies. The non-destructive Raman analysis allows one to check and identify misclassified meteorites. For instance, Allende is officially classified as CV3.2, but its IOM suggests that it is probably CV3.5/3.6 and, therefore, not as primitive as has been previously thought.

1.3 Ion Microprobe Micro-analysis

In addition to determining bulk properties of organics in primitive meteorites, Nittler, Postdoctoral Fellow Henner Busemann and others have continued microscale investigations of primitive organic matter in both meteorites and IDPs. Using the Carnegie ims-6f ion probe, ~1.5-μm resolution images have been obtained of D/H ratios in matrix fragments of the Tagish Lake (unique CC) and Al Rais (CR2) meteorites and in purified IOM from the EET92042 (CR2), Murchison (CM2), Krymka (LL3) and Allende (CV3) meteorites, and three fragments of cluster IDPs. D/H is highly variable on a 1-2 μm scale in most samples, with D/H ratios in some D-enriched “hotspots” higher than previously observed in meteorites and comparable to the highest ratios found in IDPs.

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Using the NanoSIMS ion probe at the Max Planck Institute for Chemistry in Mainz, the team obtained ~250-nm resolution ¹⁵N/¹⁴N images of many of the same samples, as well as of IOM from the unusual Bells (CM) meteorite. These data also indicate a surprising level of isotopic heterogeneity on very small spatial scales. Although some D hotspots are clearly associated with enhanced ¹⁵N, there is little spatial correlation between the measured H and N isotopic ratios. In fact, most of the highest D and ¹⁵N hotspots are not associated with each other, indicating that, for the most part, presolar D and ¹⁵N anomalies are contained in distinct organic carriers. The Bells data are particularly interesting as they reveal sub-micron hotspots with ¹⁵N/¹⁴N ratios higher than previously observed in most primitive organic matter, even that in IDPs. The highest ratios observed in the Bells organic matter are higher than can be explained by the low-temperature interstellar chemical processes usually invoked to explain ¹⁵N enrichments in primitive organics, perhaps reflecting contributions from stellar nucleosynthesis or UV self-shielding.

A key result of the imaging studies is that even meteorites whose bulk organic matter has been significantly processed on their asteroidal parent bodies preserve on small scales some very primitive D- and ¹⁵N-rich material. Identifying the nature of this material and whether it is associated with specific minerals in the host meteorites is key to understanding its origin and evolution through the solar nebula and on parent bodies. To this end, Stroud and colleagues are using focused ion beam techniques to extract ultra-thin slices of isotopically characterized samples for further characterization by TEM and synchrotron techniques. One section of a Tagish Lake sample traversing two D hotspots has been investigated by TEM so far. In this section, the D-rich material is clearly associated with carbonates; studies of additional fragments in the near future will determine whether this is common to all of the most D-rich material in this meteorite. Electron energy loss spectrometry will also be used to characterize microtomed samples of organic matter from several meteorites.

1.4 NMR and XANES Analysis

NMR analysis of pure meteorite IOM from four different groups, Orgueil (CI1), EET92042 (CR2), Murchison (CM2), and Tagish Lake (C2-ungrouped) reveal considerable variation in bulk organic composition across the different meteorite group’s IOM. The fraction of aromatic carbon increases as CR2 < CI1 < CM2 < Tagish Lake. The increases in aromatic carbon are offset by reductions in aliphatic (sp³) carbon moieties, e.g., “CH_x”, and “CH_x(O,N)”. Oxidized sp² bonded carbon, e.g., carboxyls and ketones grouped as “CO,” are largely conservative across these meteorite groups. The variation in chemistry across these four meteorite groups is consistent with alteration by low-temperature chemical oxidation that occurred in the meteorite parent body. Cody, Alexander, and colleagues expanded these NMR studies to include a suite of IOM derived from CM chondrites that experienced variable degrees of aqueous alteration. Remarkably, only subtle differences in the organic structure of the IOM are found. It would appear that low-temperature aqueous alteration alone has minimal effect on the chemical structure of IOM.

The team also analyzed IOM isolated from thermally metamorphosed CV and CO chondrites by NMR and C-, N-, and O-XANES. The IOM H contents in CVs and COs are lower than the CRs, CIs, and CMs; this IOM contains significant O and N, and is, therefore, still organic. As expected, the chemical structure of IOM from both the CVs and COs differs enormously from that of the CRs, CIs, CMs, and Tagish Lake. The chemical structures of CV and CO IOM do reveal, however, spectroscopic signatures indicative of the extent of thermal metamorphism. These novel spectral features may allow the extent of thermal metamorphism to be independently ranked using IOM as recorder.

Finally, using bulk elemental compositions and NMR spectra, the group developed the first method for quantitatively determining elemental compositions from XANES spectra. Initial heating experiments also show promise for being able to use XANES spectra to determine peak temperatures and heating durations experienced by IOM. This work will have wide applications for meteorite, IDP, and Stardust research.

2. Confocal Raman Imaging Investigation of Inorganic Meteoritic Materials

Confocal Raman imaging is a relatively new technique that has only recently been applied to the study of extraterrestrial materials. Raman imaging can determine chemical composition and structural morphology of silicates, sulfides, sulfates, carbonates, metal oxides, and generally any non-metallic or highly fluorescent material. It is also uniquely sensitive to the presence and structure of carbon compounds and the presence of water both in liquid form and within crystallographic lattices. These capabilities make the confocal Raman technique a powerful new tool for petrographic studies of both terrestrial and extraterrestrial materials. Much work remains in the characterization of Raman spectra of entire families of minerals in terms of their spectral features versus chemical composition, crystallographic orientation, and morphology, but our present understanding is sufficient to provide new insights into the composition and makeup of both meteoritic and terrestrial rock samples.

A broad array of pure samples is currently under examination with the aim of building a spectral database for Raman imaging analysis of meteorites. Postdoctoral Fellow Marc Fries and colleagues have found that it is not sufficient simply to possess spot spectra of minerals as has been done in the past. Raman spectral images of minerals are influenced by minor chemical variations, strain fields, and crystallographic orientation. In order to interpret these data, the team is continuing to analyze laboratory-prepared silicates, oxides, sulfides, and carbon as well as meteorites. Their studies of prepared, thermally annealed carbon samples in particular will allow interpretation of natural carbon in terms of its thermal history. Preparation and examination of additional silicate and sulfide standards is also planned. This new capability will be especially important in the field of meteorite and IDP research.

The aim of the meteorite studies has been to build a spectral database of minerals with different compositions and textures from typical meteorite types. The meteorite types examined thus far include ordinary chondrites (H,L,LL), carbonaceous chondrites (CH, CI, CM, CV, CK), achondrites (lodranite, aubrite, eucrite), rumurutiites, and Martian meteorites. Data collected from this suite of samples have been used to develop methods for analysis of silicates, sulfides, and carbon. However, this study has highlighted the need for carefully synthesized mineral standards of varying chemical composition if Raman imaging is to become truly quantitative. The production of these standards has become an important component of the team’s development of Raman imaging techniques. Even without a complete set of standards, Fries and colleagues have been able to make a comprehensive set of measurements of inclusions within silicate grains in chondrules, which would not be possible with other types of instruments such as SEM or electron microprobe. These inclusions likely contain materials dating to chondrule formation or even earlier that have not been closely investigated. Systematic surveys of chondrule types, inclusions, and matrix materials are currently underway.

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3. Mineralogy, Petrology, and Isotopic Compositions of Sulfides in Carbonaceous Chondrites

Sulfide minerals can reveal much about the conditions of alteration on meteorite parent bodies. They may also act as catalysts for the formation of new organic material during alteration. Boctor and colleagues have studied the mineralogy and S isotopic compositions of sulfides from the Boriskino and Tagish Lake carbonaceous chondrites. Boriskino, a CM chondrite, contains two distinct sulfide mineral assemblages: a primary assemblage consisting of pyrrhotite, Ni-bearing monosulfide, pentlandite, and P-bearing Fe-Ni sulfide; and a secondary assemblage consisting of pyrrhotite veins, magnetite, and Fe-Ni metal with extremely variable Ni contents. The secondary assemblage is associated with aqueous alteration and carbonate precipitation. Pyrrhotite, the only sulfide in the sample investigated from Tagish Lake, occurs either as rims on magnetite framboids or as more disseminated grains.

Primary pyrrhotite in Boriskino is enriched in ³⁴S (δ³⁴S from +1.01 to +5.89 ‰). A single grain of pyrrhotite that exsolved pentlandite had a negative δ³⁴S (-1.28 ‰). Secondary pyrrhotite veins were mostly depleted in ³⁴S (δ³⁴S from —6.35 to +1.96 ‰). The δ³⁴S of pyrrhotite grains in Tagish Lake ranged between +0.99 and +1.97 ‰, whereas the pyrrhotite rims had low negative values (δ³⁴S from —1.31 to —0.05 ‰).

The S isotope data are consistent with a nebular origin for the primary sulfides. The low δ^34%S of secondary pyrrhotites coexisting with magnetite may be attributed to their precipitation under more oxidizing conditions from aqueous fluids that partially dissolved some of the primary sulfides. Alternatively, it may be attributed to kinetic isotope effects due to lack of equilibration between the sulfur-bearing species in the fluid.

4. Fate of Carbon during Planetary Differentiation

During the past year, McCoy and collaborators have documented remarkable carbon isotopic heterogeneity (δ¹³C = -55 to +75‰) in a partially differentiated meteorite, raising fundamental questions about the extent of planetary melting needed to achieve carbon isotopic homogeneity, the origin of carbon isotopic heterogeneity on Earth, and the ultimate fate of carbon incorporated into the Earth prior to differentiation and core formation.

McCoy and colleagues have begun experiments to melt samples of lodranite GRA95209 to investigate these questions, conducting sealed silica-tube experiments with Cr and V buffers to control oxygen fugacity and minimize volatilization. These experiments point to the dissolution of carbon in metal, consistent with the Fe-C phase diagram, which suggests that either austenite or cementite (Fe₃C) should co-exist with a liquid above 1140°C. This result may place upper limits on the temperature reached by GRA95209. Measurement of the carbon isotopic composition of this metal is the next step in our study. These measurements, which require technique development for the Carnegie ion probe, should occur in the next year.

5. The Martian Hydrosphere: Clues from Meteorites

Longstanding questions concerning the surface of Mars are “By what mechanism did globally distributed dust form?” and, importantly, “Was aqueous processing involved?” Vicenzi and collaborators have begun an effort to compare quantitatively the only materials of certain Martian aqueous heritage, preterrestrial secondary minerals in Martian meteorites, with soil from the Mars Exploration Rover (MER) Opportunity and Spirit landing sites. This chemical comparison permits an evaluation of whether the nearly ubiquitous dust covering Mars resulted from the interaction of an ancient ocean and igneous crustal rocks. Because the secondary phases found in veinlets within Martian meteorites are quite small (down to the sub-micrometer scale) and often intergrown, estimating the local bulk chemistry of the alteration assemblage requires a new data processing approach. A poor match between Mars soils and bulk meteorite alteration products for Si/Fe, K/Al, and other element ratios suggests that the dust did not form via low-temperature hydrous process.

Efforts have also been made to evaluate whether the complex phase assemblage often observed in preterrestrial Martian meteorite alteration formed in a single event or represents a more complex history of fluid flow. New trace-element data from clays and hydrous silicates found in contact with carbonates and halites in the Martian nakhlite meteorites indicate that at least two or more pulses of aqueous alteration are needed to explain the observations.

Distinctive bioweathering textures and microchemistry have been documented in terrestrial oceanic volcanic glass. A study aimed at evaluating the nature of olivine and pyroxene alteration from other terrestrial environments, e.g., mantle rocks, as well as Martian meteorites, suggests that similar microscale features can be found upon close examination. However intriguing, the full range of inorganic alteration mechanisms has not yet been explored. Consequently, a definitive connection to a biologically driven weathering mechanism cannot yet be made.