• Ca,Al-rich inclusions in Rumuruti (R) chondrites

      Rout, S. S.; Bischoff, A. (The Meteoritical Society, 2008-01-01)
      Rumuruti chondrites (R chondrites) constitute a well-characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al-rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al-rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al-rich objects (101 CAIs, 19 Al-rich chondrules, and 6 spinel-rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel-rich inclusions (42), (2) fassaite-rich spherules, (3) complex spinel-rich CAIs (53), (4) complex diopside-rich inclusions, (5) Al-rich chondrules, and (6) Al-rich (spinel-rich) fragments. The simple concentric and complex spinel-rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al-(Cl)-rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R-chondrite group. Some Ca,Al-rich inclusions appear to be primitive (e.g., low FeO-contents in spinel, low abundances of Na,Al-(Cl)-rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al- (Cl)-rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R-chondrite lithologies have abundant secondary Ab-rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.
    • Calcium-aluminum-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites

      Aléon, J.; Krot, A. N.; McKeegan, K. D. (The Meteoritical Society, 2002-01-01)
      Calcium-aluminum-rich refractory inclusions (CAIs) in CR chondrites are rare (<1 vol%), fairly small (<500 micrometers) and irregularly-shaped, and most of them are fragmented. Based on the mineralogy and petrography, they can be divided into grossite +/- hibonite-rich, melilite-rich, and pyroxene-anorthite-rich CAIs. Other types of refractory objects include fine-grained spinel-melilite-pyroxene aggregates and amoeboid olivine aggregates (AOAs). Some of the pyroxene-anorthite-rich CAIs have igneous textures, and most melilite-rich CAIs share similarities to both the fluffy and compact type A CAIs found in CV chondrites. One major difference between these CAIs and those in CV, CM, and CO chondrites is that secondary mineral phases are rare. In situ ion microprobe analyses of oxygen-isotopic compositions of 27 CAIs and AOAs from seven CR chondrites demonstrate that most of the CAIs are 16O-rich (Delta-17O of hibonite, melilite, spinel, pyroxene, and anorthite <-22 ppm) and isotopically homogeneous within 3-4 ppm. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, 16O-rich compositions (-24 ppm < Delta-17O < -20 ppm). In contrast, objects which show petrographic evidence for extensive melting are not as 16O-rich (Delta-17O less than -18 ppm). Secondary alteration minerals replacing 16O-rich melilite in melilite-rich CAIs plot along the terrestrial fractionation line. Most CR CAIs and AOAs are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes, which is reflected in their relatively homogeneous 16O-rich compositions. It is likely that these objects (or their precursors) condensed in an 16O-rich gaseous reservoir in the solar nebula. In contrast, several igneous CAIs are not very enriched in 16O, probably as a result of their having melted in the presence of a relatively 16O-poor nebular gas. If the precursors of these CAIs were as 16O-rich as other CR CAIs, this implies either temporal excursions in the isotopic composition of the gas in the CAI-forming regions and/or radial transport of some CAI precursors into an 16O-poor gas. The absence of oxygen isotope heterogeneity in the primary minerals of melilite-rich CAIs containing alteration products suggests that mineralogical alteration in CR chondrites did not affect oxygen-isotopic compositions of their CAIs.
    • Call for nominations: J. Lawrence Smith Medal

      The Meteoritical Society, 2005-01-01
    • Capture effects in carbonaceous material: A Stardust analogue study

      Fries, M.; Burchell, M.; Kearsley, A.; Steele, A. (The Meteoritical Society, 2009-01-01)
      It is reasonable to expect that cometary samples returned to Earth by the Stardust space probe have been altered to some degree during capture in aerogel at 6.1 km/s. In order to help interpret the measured structure of these particles with respect to their original cometary nature, a series of coal samples of known structure and chemical composition was fired into aerogel at Stardust capture velocity. This portion of the study analyzed the surfaces of aerogel-embedded particles using Raman spectroscopy. Results show that particle surfaces are largely homogenized during capture regardless of metamorphic grade or chemical composition, apparently to include a devolatilization step during capture processing. This provides a possible mechanism for alteration of some aliphatic compoundrich phases through devolatilization of cometary carbonaceous material followed by re-condensation within the particle. Results also show that the possibility of alteration must be considered for any particular Stardust grain, as examples of both graphitization and amorphization are found in the coal samples. It is evident that Raman G band (~1580 cm^(-1)) parameters provide a means of characterizing Stardust carbonaceous material to include identifying those grains which have been subjected to significant capture alteration.
    • Carbon and nitrogen in carbonaceous chondrites: Elemental abundances and stable isotopic compositions

      Pearson, V. K.; Sephton, M. A.; Franchi, I. A.; Gibson, J. M.; Gilmour, I. (The Meteoritical Society, 2006-01-01)
      We have undertaken a comprehensive study of carbon and nitrogen elemental abundances and isotopic compositions of bulk carbonaceous chondrites. A strategy of multiple analyses has enabled the investigation of hitherto unconstrained small-scale heterogeneities. No systematic differences are observed between meteorite falls and finds, suggesting that terrestrial processing has a minimal effect on bulk carbon and nitrogen chemistry. The changes in elemental abundance and isotopic composition over the petrologic range may reflect variations in primary accreted materials, but strong evidence exists of the alteration of components during secondary thermal and aqueous processing. These changes are reflected within the CM2 and CO3 groups and follow the published alteration scales for those groups. The nitrogen isotope system appears to be controlled by an organic host, which loses a 15N-rich component with progressive alteration. This study recommends caution, however, over the use of bulk carbon and nitrogen information for classification purposes; variance in relative abundance of different components in carbonaceous chondrites is significant and reflects intrameteorite heterogeneities.
    • Carbon investigation of two Stardust particles: A TEM, NanoSIMS, and XANES study

      Matrajt, G.; Ito, M.; Wirick, S.; Messenger, S.; Brownlee, D. E.; Joswiak, D.; Flynn, G.; Sandford, S.; Snead, C.; Westphal, A. (The Meteoritical Society, 2008-01-01)
      In this work we present the results of a systematic search for cometary organics in 14 Stardust particles (particles from comet 81P/Wild 2, captured by NASAs Stardust mission) by TEM and multidisciplinary studies (XANES and NanoSIMS) of Febo and Ada, two of the organic-bearing particles identified. The combination of the three analytical techniques has established the presence of organic, cometary degrees C in both particles. Using energy-filtered and high-resolution imaging it was shown that the degrees C is amorphous and rare, given that it is found in grains less than or equal to 200 nm in size that are not abundant throughout the particles. The XANES maps and spectra of the carbonaceous areas identified with the TEM have shown that the carbonaceous material is organic due to the presence of carbonyl (C=O) functional groups and the overlapping of degrees C and N on the same grains. In addition, several different C-XANES spectra were obtained from the same particle, suggesting that there is diversity in the types of carbonaceous phases present in these particles, as well as a heterogeneous distribution of the carbonaceous phases within these particles. The C-XANES spectra obtained are different from C-XANES spectra of carbonaceous chondrites and IDPs. In the particle Febo we found five spots showing a pronounced enrichment in the isotope 15N (delta-15N from 420 to 639 +/- 20 to 70 ppm, 1-sigma) that were clearly associated with the C-rich regions. The carbonaceous material has approximately solar C and D/H isotopic compositions, and the bulk O isotopic composition was found to be delta-17O = -18 +/- 13 ppm and delta-18O = -37 +/- 12 ppm (1-sigma). In the particle Ada we found a C-rich phase with enrichments in the isotope 15N (delta-15N = 550 +/- 70 ppm, 1-sigma) and the isotope D (delta-D = 610 +/- 254 ppm, 1-sigma). The C isotopic composition at this phase is solar (delta-13C = -4 +/-29 ppm, 1 sigma). The bulk O isotopic composition of Ada was found to be delta-17O = 9 +/- 14.6 ppm and delta-18O = -7.3 +/- 8.1 ppm (2-sigma).
    • Carbon isotope fractionation between graphite and diamond during shock experiments

      Maruoka, T.; Koeberl, C.; Matsuda, J.; Syono, Y. (The Meteoritical Society, 2003-01-01)
      Carbon isotopic compositions were measured for shock-produced diamond and shocked graphite formed at peak pressures ranging from 37 to 52 GPa. The delta-13C values of diamonds produced in a sealed container were generally lower than that of the initial graphite. The differences in the carbon isotopic composition between initial graphite and shocked graphite/diamond may reflect kinetic isotopic fractionation during the oxidation of the graphite/diamond and/or analytical artifacts possibly induced by impurities in the samples. The pressure effect on the isotopic fractionations between graphite and diamond can be estimated from the delta-13C values of impurity-free diamonds produced using a vented container from which gases, including oxygen, in pore spaces escaped during or after the diamond formation (e.g., 0.039 +/- 0.085 at a peak pressure of 52 GPa). Any isotopic fractionation induced by shock conversion of graphite to diamond is too small to be detected in natural shock-induced diamond-graphite systems related to terrestrial impact cratering processes.
    • Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite

      Oba, Y.; Naraoka, H. (The Meteoritical Society, 2006-01-01)
      Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the 13C- and D-enriched signature of water-extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270-330 degrees C, monocarboxylic acids with carbon numbers ranging from 2 (C2) to 5 (C5) were detected, acetic acid (CH3COOH; C2) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 degrees C. This result indicates that the Murchison macromolecule has a potential to generate at least ~0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water-extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ~-27 ppm (versus PDB), which is much more depleted in 13C than the water-extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH3-)-C is more enriched in 13C relative to carboxyl (-COOH)-C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270-330 degrees C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.
    • Carbonaceous materials in the acid residue from the Orgueil carbonaceous chondrite meteorite

      Garvie, Laurence A. J.; Buseck, Peter R. (The Meteoritical Society, 2006-01-01)
      Insoluble organic matter (IOM) dominates the HF/HCl residue of the Orgueil (CI) carbonaceous chondrite meteorite. The IOM is composed primarily of two C-rich particle types. The first has a fluffy texture similar to crumpled tissue paper, and the second type occurs as solid or hollow nanospheres. High-resolution transmission electron microscope (HRTEM) images of the fluffy material show it is poorly ordered, with small, irregularly shaped regions having fringes with 0.34-0.38 nm spacings and locally 0.21 nm cross-fringes. Nanodiamonds occur in the fluffy material. The rounded C-rich particles are common in the residue and their HRTEM images show neither fringes nor nanodiamonds. Both types of carbonaceous materials have a high aromatic component, as revealed by electron energy-loss spectroscopy (EELS), with up to 10 at% substitution by S, N, and O. The average compositions of the fluffy material and nanospheres are C100S1.9N3.7O4.9 and C100S2.4N5.0O3.9, respectively. The structural and chemical heterogeneity of the carbonaceous materials may represent material from multiple sources.
    • Carbonates in Vigarano: Terrestrial, preterrestrial, or both?

      Abreu, N. M.; Brearley, A. J. (The Meteoritical Society, 2005-01-01)
      Studies of two separate stones of the CV3 chondrite Vigarano have revealed the presence of previously unreported occurrences of calcite. In the first stone, calcite occurs as thin veins in a type B CAI. In contrast, observations of the second stone, which was recovered one month after its fall, show three calcite occurrences: networks of veins, vesicle fillings in the fusion crust, and pseudomorphic replacement of augite associated with a porphyritic olivine chondrule. The most common occurrence is as veins ranging in thickness from <1 micrometer to 25 micrometers and extending for more than several hundred micrometers. Some veins crosscut the fusion crust and are connected to a carbonate coating on the exterior of the meteorite. Extensive minor element zoning occurs in carbonate masses, indicating variations in the fluid composition and/or redox potential during carbonate growth. Based on the textural evidence and a comparative study with carbonate veins in the CV3 chondrite Leoville, we conclude that the veins are terrestrial in origin. We propose a model for rapid carbonate formation in which calcite precipitation is driven by hydrolysis and oxidation in the meteorite interior that move the fluid composition to alkaline values. In addition, both stones also contain minor occurrences of carbonate that are not readily explained by terrestrial alteration. Minor carbonate in a type B CAI occurs in the first stone and calcite occurs as pseudomorphic replacement of augite in the second stone. Both of these occurrences appear to be preterrestrial, probably asteroidal in origin.
    • Characteristics of cometary dust tracks in Stardust aerogel and laboratory calibrations

      Burchell, M. J.; Fairey, S. A. J.; Wozniakiewicz, P.; Brownlee, D. E.; Hörz, F.; Kearsley, A. T.; See, T. H.; Tsou, P.; Westphal, A.; Green, S. F.; et al. (The Meteoritical Society, 2008-01-01)
      The cometary tray of the NASA Stardust spacecraft’s aerogel collector was examined to study the dust captured during the 2004 flyby of comet 81P/Wild 2. An optical scan of the entire collector surface revealed 256 impact features in the aerogel (width >100 micrometers). Twenty aerogel blocks (out of a total of 132) were removed from the collector tray for a higher resolution optical scan and 186 tracks were observed (track length >50 micrometers and width >8 micrometers). The impact features were classified into three types based on their morphology. Laboratory calibrations were conducted that reproduced all three types. This work suggests that the cometary dust consisted of some cohesive, relatively strong particles as well as particles with a more friable or low cohesion matrix containing smaller strong grains. The calibrations also permitted a particle size distribution to be estimated for the cometary dust. We estimate that approximately 1200 particles bigger than 1 micrometer struck the aerogel. The cumulative size distribution of the captured particles was obtained and compared with observations made by active dust detectors during the encounter. At large sizes (>20 micrometers) all measures of the dust are compatible, but at micrometer scales and smaller discrepancies exist between the various measurement systems that may reflect structure in the dust flux (streams, clusters etc.) along with some possible instrument effects.
    • Characteristics of oceanic impact-induced large water waves—Re-evaluation of the tsunami hazard

      Wünnemann, K.; Weiss, R.; Hofmann, K. (The Meteoritical Society, 2007-01-01)
      The potential hazard of a meteorite impact in the ocean is controversial with respect to the destructive power of generated large ocean waves (tsunamis). We used numerical modeling of hypervelocity impact to investigate the generation mechanism and the characteristics of the resulting waves up to a distance of 100-150 projectile radii. The wave signal is primarily controlled by the ratio between projectile diameter and water depth, and can be roughly classified into deep-water and shallow-water impacts. In the latter, the collapse of the crater rim results in a wave signal similar to solitary waves, which propagate and decay in agreement with shallow-water wave theory. The much more likely scenario for an asteroid impact on Earth is a relatively small body (much smaller than the water depth) striking the deep sea. In this case, the collapse of the transient crater results in a significantly different and much more complex wave signal that is characterized by strong nonlinear behavior. We found that such waves decay much more rapidly than previously assumed and cannot be treated as long waves. For this reason, the shallow-water theory is not applicable for the computation of wave propagation, and more complex models (full solution of the Boussinesq equations) are required.
    • Characterization and significance of shocked quartz from the Woodleigh impact structure, Western Australia

      Hough, R. M.; Lee, M. R.; Bevan, A. W. R. (The Meteoritical Society, 2003-01-01)
      We re-examined the buried Woodleigh structure in Western Australia, which has been inferred to be a multi-ringed, 120 km diameter impact crater, because the proposed size and possible synchronicity with one of the pre-Mesozoic mass extinction events has attracted controversy. We undertook a detailed study of the petrology and mineralogy of a number of samples of core from the Woodleigh-1 borehole that was drilled into the central uplift of the structure. Crystalline Proterozoic basement rocks comprising granites and gneisses in the Woodleigh-1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core. Imaging of laboratory-etched quartz showed that a large number of grains contain shock deformation lamellae. Microstructural and crystallographic analysis of these lamellae by TEM showed that they are planar deformation features (PDFs) that have subsequently undergone annealing and water assisted recrystallization. The available geological, petrographic, and mineralogical evidence suggest that Woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter. Future drilling projects should better constrain the level of erosion, and may reveal any preserved impact lithologies.
    • Characterization of Antarctic micrometeorites by thermoluminescence

      Sedaghatpour, F.; Sears, D. W. G. (The Meteoritical Society, 2009-01-01)
      In order to explore the nature and history of micrometeorites, we have measured the thermoluminescence (TL) properties of four micrometeorites, three cosmic spherules, and one irregular scoriaceous particle, that we found in a survey of 17 micrometeorites. These micrometeorites have TL sensitivities ranging from 0.017 +/- 0.002 to 0.087 +/- 0.009 (on a scale normalized to 4 mg of the H3.9 chondrite Dhajala). The four micrometeorites have very similar TL peak temperatures and TL peak widths, and these distinguish them from CI, most CM, CV, CO, and ordinary chondrites. However, the TL properties of these micrometeorites closely resemble those of the unusual CM chondrite MacAlpine Hills (MAC) 87300 and terrestrial forsterites. Heating experiments on submillimeter chips of a CM chondrite and a H5 chondrite suggest that these TL properties are have not been significantly affected by atmospheric passage. Thus we suggest that there is no simple linkage between these micrometeorites and the established meteorite classes, and that forsterite is an important component of these micrometeorites, as it is in many primitive solar system materials.
    • Characterization of carbon- and nitrogen-rich particle fragments captured from comet 81P/Wild 2

      Gallien, J.-P.; Khodja, H.; Herzog, G. F.; Taylor, S.; Koepsell, E.; Daghlian, C. P.; Flynn, G. J.; Sitnitsky, I.; Lanzirotti, A.; Sutton, S. R.; et al. (The Meteoritical Society, 2008-01-01)
      We studied three Stardust fragments with infrared spectroscopy to characterize organic matter; with synchrotron-induced X-ray fluorescence to determine Fe contents and certain elemental ratios to iron; with scanning electron microscopy (SEM) to image sample morphology and to detect semiquantitatively Mg, Al, Si, Ca, and Fe; and with nuclear reaction analysis (NRA) to measure degrees C, N, O, and Si. A fourth fragment was analyzed by SEM only. Fragment C2054,0,35,21 from track 35 (hereafter C21) is extremely rich in degrees C and contains appreciable concentrations of Mg, Al, and Ca, but little Fe. Fragments C2054,0,35,23 (C23), C2044,0,41 (C41), and C2054,0,35,51,0 (C51), from tracks 35, 41, and 35, respectively, consist largely but not exclusively of aerogel. C23 contains Mg and finely dispersed S, but little Al, Ca or Fe. Pooled CI-normalized elemental ratios for C21, C23, and C41 are as follows: Ti/Fe, 5.0; Cr/Fe, 0.84; Mn/Fe, 0.97; Ni/Fe, 2.4; and Zn/Fe, 31. The enrichments in Ti and Zn may be related to the presence of aerogel. Minimum weight percentages of degrees C and N estimated without correcting for the presence of aerogel are 30 and 0.7 for C21; 2.8 and 0.2 for C23; 1.2 and 0.14 for C41. After corrections for the presence of aerogel containing 1.4 wt% degrees C and 0.02 wt% N, the corresponding results are 37 and 0.85 for C21; and 10 and 1 for C23; and ~1 and ~1, for C41 (The results for C41 have large uncertainties). These weight percentages are larger than or comparable to values for carbonaceous meteorites. degrees C/N atomic ratios without/without aerogel corrections are 51/51 for C21, 17/11 for C23, and 10/~1 for C41. Within the uncertainties these values are within the range for carbonaceous meteorites.
    • Characterization of insoluble organic matter in primitive meteorites by microRaman spectroscopy

      Busemann, H.; Alexander, C. M. O'D.; Nittler, L. R. (The Meteoritical Society, 2007-01-01)
      We have analyzed the chemically and isotopically well-characterized insoluble organic matter (IOM) extracted from 51 unequilibrated chondrites (8 CR, 9 CM, 1 CI, 3 ungrouped degrees C, 9 CO, 9 CV, 10 ordinary, 1 CB and 1 E chondrites) using confocal imaging Raman spectroscopy. The average Raman properties of the IOM, as parameterized by the peak characteristics of the so-called D and G bands, which originate from aromatic degrees C rings, show systematic trends that are correlated with meteorite (sub-) classification and IOM chemical compositions. Processes that affect the Raman and chemical properties of the IOM, such as thermal metamorphism experienced on the parent bodies, terrestrial weathering and amorphization due to irradiation in space, have been identified. We established separate sequences of metamorphism for ordinary, CO, oxidized, and reduced CV chondrites. Several spectra from the most primitive chondrites reveal the presence of organic matter that has been amorphized. This amorphization, usually the result of sputtering processes or UV or particle irradiation, could have occurred during the formation of the organic material in interstellar or protoplanetary ices or, less likely, on the surface of the parent bodies or during the transport of the meteorites to Earth. D band widths and peak metamorphic temperatures are strongly correlated, allowing for a straightforward estimation of these temperatures.
    • Characterization of micron-sized Fe,Ni metal grains in fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: Implications for asteroidal and preaccretionary models for aqueous alteration

      Chizmadia, L. J.; Xu, Y.; Schwappach, C.; Brearley, A. J. (The Meteoritical Society, 2008-01-01)
      The presence of apparently unaltered, micron-sized Fe,Ni metal grains, juxtaposed against hydrated fine-grained rim materials in the CM2 chondrite Yamato (Y-) 791198 has been cited as unequivocal evidence of preaccretionary alteration. We have examined the occurrence, composition, and textural characteristics of 60 Fe,Ni metal grains located in fine-grained rims in Y-791198 using scanning electron microscopy (SEM) and electron microprobe analysis. In addition, three metal grains, prepared by focused ion beam (FIB) sample preparation techniques were studied by transmission electron microscopy (TEM). The metal grains are heterogeneously distributed within the rims. Electron microprobe analyses show that all the metal grains are kamacite with minor element contents (P, Cr, and Co) that lie either within or close to the range for other CM2 metal grains. X-ray maps obtained by electron microprobe show S, P, and/or Ca enrichments on the outermost parts of many of the metal grains. Z-contrast STEM imaging of FIB-prepared Fe,Ni metal grains show the presence of a small amount of a lower Z secondary phase on the surface of the grains and within indentations on the grain surfaces. Energy-filtered TEM (EFTEM) compositional mapping shows that these pits are enriched in oxygen and depleted in Fe relative to the metal. These observations are consistent with pitting corrosion of the metal on the edges of the grains and we suggest may be the result of the formation of Fe(OH)2, a common oxidation product of Fe metal. The presence of such a layer could have inhibited further alteration of the metal grains. These findings are consistent with alteration by an alkaline fluid as suggested by Zolensky et al. (1989), but the location of this alteration remains unconstrained, because Y-791198 was recovered from Antarctica and therefore may have experienced incipient terrestrial alteration. However, we infer that the extremely low degree of oxidation of the metal is inconsistent with weathering in Antarctica and that alteration in an extraterrestrial environment is more probable. Although the presence of unaltered or incipiently altered metal grains in these fine-grained rims could be interpreted as evidence for preaccretionary alteration, we suggest an alternative model in which metal alteration was inhibited by alkaline fluids on the asteroidal parent body.
    • Characterization of the 1.2 micrometer M1 pyroxene band: Extracting cooling history from near-IR spectra of pyroxenes and pyroxene-dominated rocks

      Klima, R. L.; Pieters, C. M.; Dyar, M. D. (The Meteoritical Society, 2008-01-01)
      The 1.2 micrometer band in near-infrared spectra of pyroxenes results from Fe2+ in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near-infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 micrometer band as a function of Fe2+ in the M1 site. Near-infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 micrometer band is shown to be directly related to the amount of Fe2+ in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 micrometer band relative to the combined strengths of the 1.2 and 2 micrometer bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene-dominated rocks.
    • Characterization of the lithological contact in the shergottite EETA79001 A record of igneous differentiation processes on Mars

      Van Niekerk, D.; Goodrich, C. A.; Taylor, G. J.; Keil, K. (The Meteoritical Society, 2007-01-01)
      Elephant Moraine (EET) A79001 is the only Martian meteorite that consists of both an olivine-phyric shergottite (lithology A) and a basaltic shergottite (lithology B). The presence of these lithologies in one rock has previously been ascribed to mixing processes (either magmatic or impactinduced). Here we present data regarding phase changes across the contact between the lithologies. These data show that the contact is gradational and suggest that it is a primary igneous feature consistent with crystallization of a single cooling magma. We present a model to establish a petrogenetic connection between an olivine-phyric and a basaltic shergottite through differentiation. The model involves the shallow or surface emplacement of a magma that contained pre-eruptive solids (phenocrysts and minor xenocrysts). Subsequent differentiation via crystal settling and in situ crystallization (Langmuir 1989) resulted in a layered sequence of lithology A overlain by lithology B, with gradations in modal abundance of maskelynite (increasing from A to B) and pigeonite/ maskelynite (decreasing from A to B), and a gradational change in pattern of pyroxene zonation (zones of magnesian augite separating magnesian and ferroan pigeonite appear and thicken into B) across the contact. A pigeonite phenocryst-bearing zone near the contact in lithology B appears to be intermediate between lithology A and the bulk of lithology B (which resembles basaltic shergottite Queen Alexandra Range [QUE] 94201). Re-examination of Sr isotopic compositions in lithology A and across the contact is required to test and constrain the model.
    • Characterization of the log lithology of cores LB-07A and LB-08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility

      Schell, C.; Schleifer, N.; Elbra, T. (The Meteoritical Society, 2007-01-01)
      Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rockchanging processes, e.g., deformation or sedimentation. To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite. The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), ntermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure.