• Book Review: Mars—A Warmer, Wetter Planet, Jeffrey S. Kargel

      Chapman, C. R. (The Meteoritical Society, 2005-01-01)
    • Constraints on the depth and variability of the lunar regolith

      Wilcox, B. B.; Robinson, M. S.; Thomas, P. C.; Hawke, B. R. (The Meteoritical Society, 2005-01-01)
      Knowledge of regolith depth structure is important for a variety of studies of the Moon and other bodies such as Mercury and asteroids. Lunar regolith depths have been estimated using morphological techniques (i.e., Quaide and Oberbeck 1968; Shoemaker and Morris 1969), crater counting techniques (Shoemaker et al. 1969), and seismic studies (i.e., Watkins and Kovach 1973; Cooper et al. 1974). These diverse methods provide good first order estimates of regolith depths across large distances (tens to hundreds of kilometers), but may not clearly elucidate the variability of regolith depth locally (100 m to km scale). In order to better constrain the regional average depth and local variability of the regolith, we investigate several techniques. First, we find that the apparent equilibrium diameter of a crater population increases with an increasing solar incidence angle, and this affects the inferred regolith depth by increasing the range of predicted depths (from ~-15 m depth at 100 m equilibrium diameter to ~8-40 m at 300 m equilibrium diameter). Second, we examine the frequency and distribution of blocky craters in selected lunar mare areas and find a range of regolith depths (831 m) that compares favorably with results from the equilibrium diameter method (833 m) for areas of similar age (~2.5 billion years). Finally, we examine the utility of using Clementine optical maturity parameter images (Lucey et al. 2000) to determine regolith depth. The resolution of Clementine images (100 m/pixel) prohibits determination of absolute depths, but this method has the potential to give relative depths, and if higher resolution spectral data were available could yield absolute depths.
    • Editor's Note

      Jull, A. J. Timothy (The Meteoritical Society, 2005-01-01)
    • Geochemistry and 40Ar-39Ar geochronology of impact-melt clasts in feldspathic lunar meteorites: Implications for lunar bombardment history

      Cohen, B. A.; Swindle, T. D.; Kring, D. A. (The Meteoritical Society, 2005-01-01)
      We studied 42 impact-melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impactmelt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The 40Ar- 39Ar ages on 31 of the impact melts, the first ages on impact-melt samples from outside the region of the Apollo and Luna sampling sites, range from ~4 to ~2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact-melt rock age cluster at 3.9 Ga, but the meteorite impact-melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact-melt clast ages is a lunar-wide phenomenon. No meteorite impact melts have ages more than 1 older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.
    • Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission

      Mittlefehldt, D. W. (The Meteoritical Society, 2005-01-01)
      I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 +/- 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguish it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and will perform detailed mapping and mineralogical, compositional, and geophysical studies of the asteroid. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid.
    • In situ survey of graphite in unequilibrated chondrites: Morphologies, C, N, O, and H isotopic ratios

      Mostefaoui, S.; Zinner, E.; Hoppe, P.; Stadermann, F. J.; El Goresy, A. (The Meteoritical Society, 2005-01-01)
      We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezö-Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine-grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, degrees C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The delta-15N and delta-13C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15N excesses are found, with delta-15Nmax ~+955 ppm, confirming previous results. Excesses in 15N are also detected in fine-grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with delta-15N ranging up to +440 ppm. The 15N excesses are attributed to ion-molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in delta-13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small-and large-scale carbon isotopic variations in the solar nebula.
    • Major element and primary sulfur concentrations in Apollo 12 mare basalts: The view from melt inclusions

      Bombardieri, D. J.; Norman, M. D.; Kamenetsky, V. S.; Danyushevsky, L. V. (The Meteoritical Society, 2005-01-01)
      Major element and sulfur concentrations have been determined in experimentally heated olivine-hosted melt inclusions from a suite of Apollo 12 picritic basalts (samples 12009, 12075, 12020, 12018, 12040, 12035). These lunar basalts are likely to be genetically related by olivine accumulation (Walker et al. 1976a, b). Our results show that major element compositions of melt inclusions from samples 12009, 12075, and 12020 follow model crystallization trends from a parental liquid similar in composition to whole rock sample 12009, thereby partially confirming the olivine accumulation hypothesis. In contrast, the compositions of melt inclusions from samples 12018, 12040, and 12035 fall away from model crystallization trends, suggesting that these samples crystallized from melts compositionally distinct from the 12009 parent liquid and therefore may not be strictly cogenetic with other members of the Apollo 12 picritic basalt suite. Sulfur concentrations in melt inclusions hosted in early crystallized olivine (Fo75) are consistent with a primary magmatic composition of 1050 ppm S, or about a factor of 2 greater than whole rock compositions with 400 600 ppm S. The Apollo 12 picritic basalt parental magma apparently experienced outgassing and loss of S during transport and eruption on the lunar surface. Even with the higher estimates of primary magmatic sulfur concentrations provided by the melt inclusions, the Apollo 12 picritic basalt magmas would have been undersaturated in sulfide in their mantle source regions and capable of transporting chalcophile elements from the lunar mantle to the surface. Therefore, the measured low concentration of chalcophile elements (e.g., Cu, Au, PGEs) in these lavas must be a primary feature of the lunar mantle and is not related to residual sulfide remaining in the mantle during melting. We estimate the sulfur concentration of the Apollo 12 mare basalt source regions to be ~75 ppm, which is significantly lower than that of the terrestrial mantle.
    • 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.