• Noble gases in ten Nullarbor chondrites: Exposure ages, terrestrial ages, and weathering effects

      Schultz, L.; Franke, L.; Bevan, A. W. R. (The Meteoritical Society, 2005-01-01)
      We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of 84Ke, 129Xe, and 132Xe. From the measured cosmogenic 14C concentrations (Jull et al. 1995), shielding-corrected production rates of 14C are deduced using cosmogenic 22Ne/21Ne ratios. For shielding conditions characterized by 22Ne/21Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic 3He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.
    • Peak metamorphic temperatures in type 6 ordinary chondrites: An evaluation of pyroxene and plagioclase geothermometry

      Slater-Reynolds, V.; McSween, H. Y. (The Meteoritical Society, 2005-01-01)
      Quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies. Various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade (type 6), but results are inconsistent and span hundreds of degrees. Because different geothermometers and calibration models were used with different meteorites, it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations. We addressed this problem by performing twopyroxene geothermometry, using QUILF95, on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer (Nakamuta and Motomura 1999). We also calculated temperatures for published pyroxene analyses from other type 6 H, L, and LL chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites. Pyroxenes record a narrow, overlapping range of temperatures in H6 (865-926 degrees C), L6 (812-934 degrees C), and LL6 (874-945 degrees C) chondrites. Plagioclase temperature estimates are 96-179 degrees C lower than pyroxenes in the same type 6 meteorites. Plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak. The average temperature for H, L, and LL chondrites (~900 degrees C), which agrees with previously published oxygen isotope geothermometry, is at least 50 degrees C lower than the peak temperatures used in current asteroid thermal evolution models. This difference may require minor adjustments to thermal model calculations.
    • Recipes for making synthetic CAIs, refractory residues, and minerals for rim-forming experiments

      Wark, David (The Meteoritical Society, 2005-01-01)
      Recipes are presented for synthesizing various type A and type B Ca-Al-rich inclusions (CAIs), refractory volatilization residues, and the minerals forsterite and melilite that are required for experiments. These experiments (described in other works) aim to make two determinations: 1) the conditions under which the surfaces of CAIs were either "flash-heated" or "volatilized subsolidus" to form a temporary ultra-refractory residue, and 2) the conditions under which the residue was then metasomatized to form the mineral layers making up Wark-Lovering (WL) rims on CAIs.
    • Solid-state 13C NMR characterization of insoluble organic matter from Antarctic CM2 chondrites: Evaluation of the meteoritic alteration level

      Yabuta, H.; Naraoka, H.; Sakanishi, K.; Kawashima, H. (The Meteoritical Society, 2005-01-01)
      Chemical structures of the insoluble organic matter (IOM) from the Antarctic CM2 chondrites (Yamato [Y-] 791198, 793321; Belgica [B-] 7904; Asuka [A-] 881280, 881334) and the Murchison meteorite were analyzed by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Different types of carbons were characterized, such as aliphatic carbon (Ali-C), aliphatic carbon linked to hetero atom (Hetero-Ali-C), aromatic carbon (Aro-C), carboxyls (COOR), and carbonyls (C=O). The spectra of the IOM from Murchison and Y-791198 showed two major peaks: Ali-C and Aro-C, while the spectra from the other meteorites showed only one major peak of Aro-C. Carbon distribution was determined both by manual integration and deconvolution. For most IOM, the Aro-C was the most abundant (49.8-67.8%) of all carbon types. When the ratios of Ali-C to Aro-C (Ali/Aro) were plotted with the atomic hydrogen to carbon ratio (H/C), a correlation was observed. If we use the H/C as a parameter for the thermal alteration event on the meteorite parent body, this result shows a different extent of thermal alteration. In addition, IOM with a lower Ali/Aro showed a lower ratio of Ali-C to COOR plus C=O (Ali / (COOR + C=O)). This result suggests that the ratio of CO moieties to aliphatic carbon in IOM might reflect chemical oxidation that was involved in hydrothermal alteration.