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Project Progress

1. Organic Matter in Extraterrestrial Samples: An integrated multi-technique study

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 source 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 returned by the Stardust mission. The small size of the Stardust dust particles makes analysis of the samples very challenging. We are using the organics from meteorites to develop and calibrate microanalytical techniques for analysis of the Stardust samples.

Co-Investigator Conel Alexander and colleagues have continued the preparation of high-purity samples of insoluble organic matter (IOM) of the most primitive chondritic meteorites, now numbering 69, 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 most primitive IOM found to date comes from the CR chondrites and the CM chondrite Bells. 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. Indeed the results of an ion imaging study by Postdoctoral Fellow Henner Busemann and colleagues showed that the IOM in the most primitive meteorites contain the same isotopic hotspots as IDPs. These hotspots are greatly enriched in D and/or ¹⁵N and are thought to be the best evidence that the IOM formed in the interstellar medium (ISM) and survived the turbulent formation of the solar system. These hotspots seem to be robust particles that also survive the chemical and physical treatments used to purify the IOM from meteorites. Compared with IDPs, meteorites are many orders of magnitude larger, which means that large enough quantities of primitive IOM can be prepared from meteorites for a broad range of standard analytical techniques to be applied to it. Preliminary TEM, XANES, and FTIR studies of sections prepared by focused-ion-beam techniques of hotspots found by ion imaging in residues and in situ illustrate the types of promising studies by Busemann, Postdoctoral Fellow Thomas Zega, and others, made possible with the much larger samples available from meteorites. As do the degradative chemical studies being conducted by Postdoctoral Fellow Hikaru Yabuta. Also, the presence of thermally sensitive primitive organic matter in meteorites from the asteroid belt and IDPs that may come from comets has important implications for the thermal structure of and transport of material within the solar protoplanetary disk.

As reported last year, large, systematic spectral differences observed in Raman spectra of meteoritic IOM can be attributed to the distinct histories experienced by the meteorites on their asteroidal parent bodies. Carbon Raman features (the so-called D “disordered” and G “graphite” bands) 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 position and width with metamorphic degree, 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.

According to Co-Investigator Larry Nittler and colleagues, carbon Raman spectra of primitive IDPs are very similar to those of the most primitive meteorites, again confirming the relationship between IOM in meteorites and IDPs. To better understand the mechanisms responsible for the modification of IOM during parent body metamorphism we have carried out a number of heating experiments on meteoritic IOM and various terrestrial analogues. After heating, the samples were analyzed by XANES and Raman. Heating of the IOM at various temperatures and for a range of times has enabled the team to begin to understand the kinetics of the process. Extrapolation of the experiments to the longer times believed to be appropriate for meteorite metamorphism indicate that processing temperatures are significantly higher than mineralogical indicators. Cody and colleagues suspect that shear stresses associated with compaction, shock , and recrystallization in the parent bodies may enhance the rate of transformation of the IOM relative to the laboratory heating experiments. However, to confirm this, the underlying process(es) that mediates the transformation of the IOM must be understood.

Observations of organic material in situ may also help to understand the transformation of IOM during metamorphism. Studies of chondrules in CV, CM, L, and LL meteorites by Postdoctoral Fellow Marc Fries have revealed wide ranges in the character and context of carbonaceous matter both within mineral grains and in intergranular mesostasis, reflecting the wide variety of processing histories found in different chondrule types. A new type of carbon-bearing primary melt inclusion has been identified in a porphyritic chondrule from the CV3 QUE 93466 that shows evidence of cooling from a metamorphic temperature near the olivine liquidus. While heavily processed, this type of material condensed within its parent chondrule without communication with the environment, preserving in its composition the chemical conditions within the chondrule during formation.

Analysis of carbonaceous matter in the ureilite LAP 03587 by Fries has revealed the presence of sub-micron diamond aggregates within a highly metamorphosed graphitic phase. Similar diamond clusters have also been found in extracts of the Murchison meteorite and serendipitously within the matrix of the CV3 NWA 3118 meteorite. The diamonds within carbonaceous chondrites differ from those in ureilites in their very strong latent strain, indicating formation through explosive processing as opposed to high temperature, high-pressure genesis. These samples lie in contrast to each other, as ureilite diamonds were formed by parent body processing while those found in carbonaceous chondrites are derived from pre-accretion sources, perhaps from a long-gone supernovae or some severe shock suffered by the early Sun’s accretion disk.

All the microanalytical techniques Alexander, Nittler, and colleagues have developed for studying the IOM in meteorites and IDPs have or will be applied to the samples of comet Wild-2 returned by the Stardust mission, as part of the Preliminary Examination Team (PET). Unfortunately, at this stage all results are embargoed until publication of the results in Science in November.

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

Sulfides in meteorites are indicators of the conditions during parent body alteration and potential catalysts for organic synthesis in meteorites. As a continuation of Collaborator Nabil Boctor and colleagues’ study of sulfides in CM carbonaceous chondrites, the team presented new data on the petrography and metal sulfide mineralogy in the two meteorites Dom 03182 and Dom 03183. Dom 03182 and Dom 03183 are composed of fragments of olivine and pyroxene chondrules, single crystals of olivine, and zoned olivine crystal aggregates (core Fo 98, rim Fo 69). Interstitial clinopyroxene (En₆₈Fs₁Wo₃₁) in the olivine aggregates is rare. Some olivine aggregates are mantled by a rim of enstatite (En₉₇Fs₃). Partial alteration of phyllosilicates affected the matrix and to a lesser degree the chondritic fragments, crystals, and olivine aggregates. Fine-grained rims on some of the olivine aggregates believed to be rims of accretionary dust by some investigators were found to have the same composition as the phyllosilicates. The metal is kamacite (5 to 8 wt.% Ni), which occurs as irregular grains in the matrix or as spheres in olivine. Troilite spheres show eutectic-like intergrowths with metal. The metal-troilite assemblage shows partial oxidation to magnetite and pyrrhotite. Examination of the sulfides with confocal Raman spectroscopy showed no carbon species; however, C was detected in the matrix. Boctor, Postdoctoral Fellow Marc Fries, Alexander, and others interpret the metal-sulfide assemblage as a primary nebular assemblage and the pyrrhotite-magnetite as a secondary assemblage formed in association with the phyllosilicate alteration, probably in a planetary environment.

3. Fate of Carbon during Planetary Differentiation

Co-Investigator Tim McCoy, Nittler, and Co-Investigator Rhonda Stroud along with collaborators Bill Carlson (UT-Austin) and Don Bogard and Dan Garrison (NASA Johnson Space Center) 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 completed experiments on melting of GRA 95209 to investigate these questions, conducting sealed silica tube experiments at 1000ºC and 1300ºC with Cr and V buffers to control oxygen fugacity and minimize volatilization. These experiments point to the dissolution of carbon in metal. In the 1300ºC experiment, we observe a carbide phase (either austenite or cementite (Fe₃C)) co-existing with blebs of molten metal armored by shells of molten sulfide. This assemblage is predicted by the Fe-S-C phase diagram, where a carbide should co-exist with liquid metal above 1140°C. The presence of this assemblage in the 1300ºC experiment suggests that metal-suflide veins in GRA 95209 never reached these temperatures, consistent with textural and mineralogical evidence for limited silicate partial melting. Measurement of the carbon isotopic composition of this metal is the next step in our study.

4. The Martian Hydrosphere: Clues from Meteorites

Information regarding the ancient Martian hydrosphere, and potentially the Martian atmosphere, can be derived from the study of preterrestrial secondary minerals in the Martian meteorite collection. A review of the mineralogical and textural relationships in alteration assemblages in Martian meteorites, and the constraints on the information yielded by stable isotope geochemistry of secondary mineralization, suggests that mixing occurred between atmospheric, magmatic, and crustal reservoirs. According to studies by Co-Investigator Vicenzi and Collaborator Laurie Leshin, this fluid alteration does not appear to be extensive and is probably limited in space and time according to the information currently in-hand.

Vicenzi and colleagues added the new Martian meteorite MIL 03346 to the suite of nakhlites the team has analyzed for aqueous alteration products. The phases in these products, frequently including poorly crystalline clays, give important clues to the aqueous history of the nakhlites’ source region on Mars — all nakhlites are thought to originate from one single impact on Mars ~11 Ma ago. First results found less preterrestrial alteration in MIL 03346 than in other nakhlites, e.g., Nakhla or Lafayette. This might be related to other findings suggesting that its host layer was in the upper stratum of the nakhlitic lava flows. Initially, Postdoctoral Fellow Detlef Rost and Vicenzi hoped to identify the first stages of mesostasis alteration by use of cathodoluminescence spectroscopy. However, little alteration was found in the team’s MIL 03346 section. Instead, Rost and colleagues found patches of a Li-rich mineral in the mesostasis. This phase has a composition close to fayalite, but a distinct Raman spectrum. Further investigations by Rost and colleagues strongly suggest that this mineral is laihunite, Fe²⁺Fe³⁺₂(SiO₄)₂, a rare oxidized form of fayalite that forms at ~700° C under oxidizing conditions. The discovery of Li-rich grains in MIL 03346 mesostasis is important for understanding the preterrestrial aqueous alteration processes in nakhlites, since a readily dissolvable Li host phase is a likely source for the Li-enrichment observed in secondary clay minerals found in Nakhla and Lafayette — by way of percolating fluids.

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Bioweathering textures have been documented in oceanic volcanic glass. A study aimed at evaluating the nature of olivine and pyroxene alteration from a variety of terrestrial environments, as well as Martian meteorites, suggest similar microscale features are present in other tectonic settings. However intriguing, the full nature of inorganic alterations mechanisms has not yet been fully explored, and as such, a connection to a biologically-driven mechanism has not been conclusively made. The observations from this study by Collaborator Martin Fisk, Vicenzi, and others, will undoubtedly provide motivation for further bioweathering examinations to determine if the morphological features are genetically linked to either biological or abiological alteration.