• An integrated geophysical and geological study of the Monturaqui impact crater, Chile

      Ugalde, H.; Valenzuela, M.; Milkereit, B. (The Meteoritical Society, 2007-01-01)
      The Monturaqui impact crater (350-370 m in diameter and 0.1 Ma old), located in a remote area in northern Chile, was surveyed in December 2003 with detailed geophysics (gravity and magnetics), topography, petrophysics, and geology. The geology of the Monturaqui area is characterized by a basement of Paleozoic granites overlain by Pliocene ignimbrite units. No impact breccia was found in the area. The granites are the main lithology affected by the impact. Although the granite samples analyzed did not show evidence of shock metamorphism, quartz, and to a lesser extent feldspar and biotite grains from impactite samples exhibit different degrees of shock, ranging from planar microdeformation and cleavage to the development of intense planar deformation features (PDFs) and diaplectic glasses in some grains. The differential GPS survey allowed the creation of a detailed digital elevation model of the crater. Its dimensions are 370 m along the eastwest direction, 350 m along the north-south direction, and ~34 m deep. The crater exhibits a circular morphology with a preferred northwest-southeast elongation that coincides with the steepest slopes (~35 degrees) on the southeast edge. The newly acquired gravity data shows a negative anomaly of ~1 mGal at the center and allowed the creation of a 3-D model with a RMS error of <0.1 mGal, which supports the predictions of a fracturing-induced low-density granitic layer on top of the unfractured basement.
    • Mineralogical composition of (25143) Itokawa 1998 SF36 from visible and near-infrared reflectance spectroscopy: Evidence for partial melting

      Abell, P. A.; Vilas, F.; Jarvis, K. S.; Gaffey, M. J.; Kelley, M. S. (The Meteoritical Society, 2007-01-01)
      In March 2001, asteroid (25143) Itokawa, the target of the Japanese Hayabusa spacecraft mission, was in a favorable viewing geometry for ground-based telescopic study. Visible/nearinfrared (VNIR) spectra (~0.48 to 0.9 micrometers) obtained on March 24, 26, and 27 UT, and near-infrared (NIR) spectra (~0.75 to 2.5 micrometers) obtained on March 10, 11, 12, 23, and 24 UT collectively show absorption features centered near 1.0 and 2.0 m, which are indicative of olivine and pyroxene. Analyses of these absorption features indicate an abundance ratio of olivine to pyroxene of approximately 75:25 +/- 5, respectively, with no significant variation in the relative abundance of these minerals across its surface on a regional scale. The band center positions indicate that the mean pyroxene chemistry is ~Wo(14 5)Fs(43 5). There appear to be at least two pyroxene components: primarily a low-Ca orthopyroxene accompanied by a spectrally significant (~15-20%) high Fe-rich pigeonite phase. The mean pyroxene composition is significantly more Fe-rich than the Fs1426 range found in ordinary chondrites. These pyroxene compositions are suggestive of phases crystallized from partial melts. This would indicate that the parent body of (25143) Itokawa reached temperatures sufficient to initiate partial melting (~1050 to 1250 degrees C), but that it did not attain the degree of melting required for significant melt mobilization and efficient segregation of the basaltic melt component from the unmelted residual olivine portion. Itokawas spectral band parameters place it near the S(III)/S(IV) boundary, but within the S(III) taxonomic field. In meteoritic nomenclature, Itokawa would be most analogous to an olivine-rich primitive achondrite. Alternatively, if the high Fs value is not related to partial melting, then Itokawa could also represent a rare atypical LL chondrite, or a previously unsampled oxidized Fe-rich chondritic-like assemblage.
    • Reclassification and thermal history of Trenzano chondrite

      Fioretti, A. M.; Domeneghetti, M. C.; Molin, G.; Cámara, F.; Alvaro, M.; Agostini, L. (The Meteoritical Society, 2007-01-01)
      We present a new single-crystal X-ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low-shocked H6 Trenzano meteorite. The quenched intracrystalline Fe^(2+)-Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature Tc, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene Tc of 522 +/- 13 degrees C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this Tc is about 100 degrees C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two-pyroxene equilibrium temperature of Trenzano (824 +- 24 degrees C), calculated with QUILF95, is similar to the two-pyroxene temperature of 750-840 degrees C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).
    • Mass-dependent fractionation of nickel isotopes in meteoritic metal

      Cook, D. L.; Wadhwa, M.; Clayton, R. N.; Dauphas, N.; Janney, P. E.; Davis, A. M. (The Meteoritical Society, 2007-01-01)
      We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non-magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of approximately 0.4 amu^(-1). The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (approximately 0.0 to approximately 0.3 amu^(-1)) and chondrites (approximately 0.0 to approximately 0.2 amu^(-1)) are similar, whereas the range in pallasite metal (approximately -0.1 to approximately 0.0 amu^(-1)) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (approximately 0.0 to approximately 0.3 amu^(-1)) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by approximately 0.4 amu^(-1). In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe-Ni alloy and the development of the Widmanstätten pattern.
    • Extraterrestrial chromite in latest Maastrichtian and Paleocene pelagic limestone at Gubbio, Italy: The flux of unmelted ordinary chondrites

      Cronholm, A.; Schmitz, B. (The Meteoritical Society, 2007-01-01)
      The distribution of sediment-dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains (63 m) has been studied across the latest Maastrichtian and Paleocene in the Bottaccione Gorge section at Gubbio, Italy. This section is ideal for determining the accumulation rate of EC because of its condensed nature and well-constrained sedimentation rates. In a total of 210 kg of limestone representing eight samples of 14-28 kg distributed across 24 m of the Bottaccione section, only 6 EC grains were found (an average of 0.03 EC grains kg^(-1)). In addition, one probable pallasitic chromite grain was found. No EC grains could be found in two samples at the Cretaceous-Tertiary (K-T) boundary, which is consistent with the K-T boundary impactor being a carbonaceous chondrite or comet low in chromite. The average influx of EC to Earth is calculated to ~0.26 grain m^(-2) kyr^(-1). This corresponds to a total flux of ~200 tons of extraterrestrial matter per year, compared to ~30,000 tons per year, as estimated from Os isotopes in deep-sea sediments. The difference is explained by the EC grains representing only unmelted ordinary chondritic matter, predominantly in the size range from ~0.1 mm to a few centimeters in diameter. Sedimentary EC grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere. The average of 0.03 EC grain kg^(-1) in the Gubbio limestone contrasts with the up to ~3 EC grains kg^(-1) in mid-Ordovician limestone that formed after the disruption of the L chondrite parent body in the asteroid belt at ~470 Ma. The two types of limestone were deposited at about the same rate, and the difference in EC abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup.
    • Book Review: The Surface of Mars, Michael H. Carr

      Byrne, S. (The Meteoritical Society, 2007-01-01)
    • Mössbauer study of Meridiani Planum, the first iron-nickel meteorite found on the surface of Mars by the MER Opportunity

      Van Cromphaut, C.; De Resende, V. G.; De Grave, E.; Vandenberghe, R. E. (The Meteoritical Society, 2007-01-01)
      Meridiani Planum is the first iron meteorite found on Mars. It was discovered in 2005 by the Mars Exploration Rover Opportunity (MER-B). Mössbauer spectra (MS) of the unbrushed and brushed meteorite species were acquired in 10 degrees temperature windows in the range of 210 260 K. Earlier examinations of these MS have led to the conclusion that the meteorite, which contains ~7 wt% Ni, belongs to the IAB meteorite group. Here, making use of a recently developed calibration/ folding procedure for MER MS, we report the results of the MS analyses for the single temperature windows m5 (210-220 K), m6 (220-230 K), m7 (230-240 K), and m89 (240-260 K). All spectra consist of a sextet and a ferric doublet. The hyperfine field of the sextet, extrapolated to room temperature, is ~34.5 T, which is, based on Mössbauer studies of meteorites found on Earth, indeed consistent with the presence of kamacite. The fractional spectral area of the sextet is ~0.96 of the total spectrum. The ferric doublet has an average quadrupole splitting of 0.70 mm/s and is not diagnostic of any specific Fe mineral.
    • The regolith portion of the lunar meteorite Sayh al Uhaymir 169

      Al-Kathiri, A.; Gnos, E.; Hofmann, B. A. (The Meteoritical Society, 2007-01-01)
      Sayh al Uhaymir (SaU) 169 is a composite lunar meteorite from Oman that consists of polymict regolith breccia (8.44 ppm Th), adhering to impact-melt breccia (IMB; 32.7 ppm Th). In this contribution we consider the regolith breccia portion of SaU 169, and demonstrate that it is composed of two generations representing two formation stages, labeled II and III. The regolith breccia also contains the following clasts: Ti-poor to Ti-rich basalts, gabbros to granulites, and incorporated regolith breccias. The average SaU 169 regolith breccia bulk composition lies within the range of Apollo 12 and 14 soil and regolith breccias, with the closest correspondence being with that of Apollo 14, but Sc contents indicate a higher portion of mare basalts. This is supported by relations between Sm-Al2O3, FeO-Cr2O3-TiO2, Sm/Eu and Th-K2O. The composition can best be modeled as a mixture of high-K KREEP, mare basalt and norite/troctolite, consistent with the rareness of anorthositic rocks. The largest KREEP breccia clast in the regolith is identical in its chemical composition and total REE content to the incompatible trace-element (ITE)- rich high-K KREEP rocks of the Apollo 14 landing site, pointing to a similar source. In contrast to Apollo 14 soil, SaU 169 IMB and SaU 169 KREEP breccia clast, the SaU 169 regolith is not depleted in K/Th, indicating a low contribution of high-Th IMB such as the SaU 169 main lithology in the regolith. The data presented here indicate the SaU 169 regolith breccia is from the lunar front side, and has a strong Procellarum KREEP Terrane signature
    • Prebiotic carbon in clays from Orgueil and Ivuna (CI), and Tagish Lake (C2 ungrouped) meteorites

      Garvie, L. A. J.; Buseck, P. R. (The Meteoritical Society, 2007-01-01)
      Transmission electron microscopic (TEM) and electron energy-loss spectroscopic (EELS) study of the Ivuna and Orgueil (CI), and Tagish Lake (C2 ungrouped) carbonaceous chondrite meteorites shows two types of C-clay assemblages. The first is coarser-grained (to 1 micrometer) clay flakes that show an intense O K edge from the silicate together with a prominent degrees C K edge, but without discrete degrees C particles. Nitrogen is common in some clay flakes. Individual Orgueil and Tagish Lake meteorite clay flakes contain up to 6 and 8 at% degrees C, respectively. The degrees C K-edge spectra from the clays show fine structure revealing aromatic, aliphatic, carboxylic, and carbonate degrees C. The EELS data shows that this degrees C is intercalated with the clay flakes. The second C-clay association occurs as poorly crystalline to amorphous material occurring as nanometer aggregates of degrees C, clay, and Fe-O-rich material. Some aggregates are dominated by carbonaceous particles that are structurally and chemically similar to the acid insoluble organic matter. The degrees C K-edge shape from this degrees C resembles that of amorphous degrees C, but lacking the distinct peaks corresponding to aliphatic, carboxylic, and carbonate degrees C groups. Nanodiamonds are locally abundant in some carbonaceous particles. The abundance of degrees C in the clays suggest that molecular speciation in the carbonaceous chondrites is partly determined by the effects of aqueous processing on the meteorite parent bodies, and that clays played an important role. This intricate C-clay association lends credence to the proposal that minerals were important in the prebiotic chemical evolution of the early solar system.
    • Physical properties of Martian meteorites: Porosity and density measurements

      Coulson, I. M.; Beech, M.; Nie, W. (The Meteoritical Society, 2007-01-01)
      Martian meteorites are fragments of the Martian crust. These samples represent igneous rocks, much like basalt. As such, many laboratory techniques designed for the study of Earth materials have been applied to these meteorites. Despite numerous studies of Martian meteorites, little data exists on their basic structural characteristics, such as porosity or density, information that is important in interpreting their origin, shock modification, and cosmic ray exposure history. Analysis of these meteorites provides both insight into the various lithologies present as well as the impact history of the planets surface. We present new data relating to the physical characteristics of twelve Martian meteorites. Porosity was determined via a combination of scanning electron microscope (SEM) imagery/image analysis and helium pycnometry, coupled with a modified Archimedean method for bulk density measurements. Our results show a range in porosity and density values and that porosity tends to increase toward the edge of the sample. Preliminary interpretation of the data demonstrates good agreement between porosity measured at 100 and 300 magnification for the shergottite group, while others exhibit more variability. In comparison with the limited existing data for Martian meteorites we find fairly good agreement, although our porosity values typically lie at the low end of published values. Surprisingly, despite the increased data set, there is little by way of correlation between either porosity or density with parameters such as shock effect or terrestrial residency. Further data collection on additional meteorite samples is required before more definitive statements can be made concerning the validity of these observations.
    • Ultraviolet photolysis of anthracene in H2O interstellar ice analogs: Potential connection to meteoritic organics

      Ashbourn, S. F. M.; Elsila, J. E.; Dworkin, J. P.; Bernstein, M. P.; Sandford, S. A.; Allamandola, L. J. (The Meteoritical Society, 2007-01-01)
      The polycyclic aromatic hydrocarbon (PAH) anthracene was oxidized by exposure to ultraviolet (UV) radiation in H2O ice under simulated astrophysical conditions, forming several anthracene ketones (9-anthrone, 1,4-anthraquinone, and 9,10-anthraquinone) and alcohols (1-anthrol and 2-anthrol). Two of the ketones produced have been detected in the Murchison meteorite but, to our knowledge, there has been no search for the alcohols or other oxidized anthracenes in meteorites. These results seem consistent with the possibility that interstellar ice photochemistry could have influenced the inventory of aromatics in meteorites. Since quinones are also fundamental to biochemistry, their formation in space and delivery to planets is relevant to studies relating to the habitability of planets and the evolution of life.
    • A hydrocode equation of state for SiO2

      Melosh, H. J. (The Meteoritical Society, 2007-01-01)
      The thermodynamic properties of SiO2 are approximated over a range of pressures and temperatures important under the extreme conditions achieved in impacts at typical solar system velocities from 5 to about 70 km/s. The liquid/vapor phase curve and critical point of SiO2 are computed using the equation of state (EOS) program ANEOS. To achieve this goal, two shortcomings of ANEOS are corrected. ANEOS, originally developed at Sandia National Laboratories to describe metals, treats the vapor phase as a monatomic mixture of atoms, rather than molecular clusters. It also assumes a Morse potential for the expanded solid state. Neither of these assumptions is accurate for geologic materials, such as SiO2, that contain molecular clusters in the vapor phase and are better described by a Mie-type potential in the solid state. Using the updates described here, an EOS adequate for numerical hydrocode computations is constructed that agrees well with shock data at pressures up to at least 600 GPa and temperatures up to 50,000 K. This EOS also gives a good representation of the liquid/vapor transition at much lower pressures and temperatures. The estimated critical point parameters for SiO2 are Pc = 0.19 GPa, Tc = 5400K, rho-c = 550 kg/m^3.
    • Indigenous amino acids in primitive CR meteorites

      Martins, Z.; Alexander, C. M. O'D.; Orzechowska, G. E.; Fogel, M. L.; Ehrenfreund, P. (The Meteoritical Society, 2007-01-01)
      CR chondrites are among the most primitive meteorites. In this paper, we report the first measurements of amino acids in Antarctic CR meteorites. Three CRs, Elephant Moraine (EET) 92042, Graves Nunataks (GRA) 95229, and Grosvenor Mountains (GRO) 95577, were analyzed for their amino acid content using high-performance liquid chromatography with UV fluorescence detection (HPLC-FD) and gas chromatographymass spectrometry (GC-MS). Our data show that EET 92042 and GRA 95229 are the most amino acidrich chondrites ever analyzed, with total amino acid concentrations ranging from 180 ppm to 249 ppm. The most abundant amino acids present in the EET 92042 and GRA 95229 meteorites are the alpha-amino acids glycine, isovaline, alpha-aminoisobutyric acid (alpha-AIB), and alanine, with delta-13C values ranging from +31.6 to +50.5. The carbon isotope results together with racemic enantiomeric ratios determined for most amino acids strongly indicate an extraterrestrial origin for these compounds. Compared to Elephant Moraine (EET) 92042 and GRA 95229, the more aqueously altered GRO 95577 is depleted in amino acids. In both CRs and CMs, the absolute amino acid abundances appear to be related to the degree of aqueous alteration in their parent bodies. In addition, the relative abundances of alpha-AIB and Beta-alanine in the Antarctic CRs also appear to depend on the degree of aqueous alteration.
    • Book Review: Volcanism on Io: A Comparison with Earth, Ashley Davies

      Radebaugh, J. (The Meteoritical Society, 2007-01-01)