• Porosity and density of ordinary chondrites: Clues to the formation of friable and porous ordinary chondrites

      Wilkison, S. L.; McCoy, T. J.; McCamant, J. E.; Robinson, M. S.; Britt, D. T. (The Meteoritical Society, 2003-01-01)
      Densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors. We report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study. Based on our measurements, we find that no significant correlation exists between porosity and petrologic grade, chemical group, sample mass, bulk and grain density, or shock level. To investigate variations in porosity and density between pieces of a meteorite, we examined stones from two showers, Holbrook and Pultusk. Examination of nine samples of Holbrook suggests relative homogeneity in porosity and density between pieces of this shower. Measurements of three samples of Pultusk show homogeneity in bulk density, in contrast to Wilkison and Robinson (2000), a study that reported significant variations in bulk density between 11 samples of Pultusk. Finally, examination of two friable ordinary chondrites, Bjurböle and Allegan, reveal variability in friability and porosity among pieces of the same fall. We suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid.
    • Book Review: Stable Isotope Geochemistry, J. Valley, D. R. Cole

      Thiemens, M. H. (The Meteoritical Society, 2003-01-01)
    • Impact-induced frictional melting in ordinary chondrites: A mechanism for deformation, darkening, and vein formation

      Van der Bogert, C. H.; Schultz, P. H.; Spray, J. G. (The Meteoritical Society, 2003-01-01)
      High speed friction experiments have been performed on the ordinary chondrites El Hammami (H5, S2) and Sahara 97001 (L6, S3) using an axial friction-welding apparatus. Each sample was subjected to a strain rate of 103 to 104 s-1, which generated 250 to 500 micrometer-deep darkened zones on each sample cube. Thin section analyses reveal that the darkened areas are composed of silicate glass and mineral fragments intermingled with dispersed submicron-size FeNi and FeS blebs. Fracturing of mineral grains and the formation of tiny metallic veins define the extent of deformation beyond the darkened shear zone. These features are not present in the original meteorites. The shear zones and tiny veins are quite similar to certain vein systems seen in naturally deformed ordinary chondrites. The experiments show that shock deformation is not required for the formation of melt veins and darkening in ordinary chondrites. Therefore, the presence of melt veins and darkening does not imply that an ordinary chondrite has undergone severe shock deformation. In fact, high strain rate deformation and frictional melting are especially important for the formation of veins at low shock pressures.
    • Lunar mare volcanism in the eastern nearside region derived from Clementine UV/VIS data

      Kodama, S.; Yamaguchi, Y. (The Meteoritical Society, 2003-01-01)
      Clementine UV/VIS multi-spectral data were used to map mare deposits in the eastern lunar nearside region (Mare Tranquillitatis, Mare Fecunditatis, Mare Serenitatis, Mare Crisium, Mare Nectaris) to understand the volcanic history of this region. An array of Clementine and Clementine-derived data were used to classify mare basalts; these include: 750 nm albedo, UV/VIS ratio, 1 micrometer absorption signatures, and Clementine derived FeO and TiO2 contents. We have successfully identified several new geological units and have determined their spectral characteristics. For example, the relatively younger low-Ti basalts were recognized in the eastern part of Mare Tranquillitatis. The central low-Ti basalts in Mare Serenitatis, which had been classed as mISP, were divided into 2 groups. In Mare Nectaris, 2 types of mare basalts were identified, while only 1 group was recognized in the previous study. The stratigraphy constructed from the spectral analysis indicates that the mare deposits tend to become younger in the northern maria, including Serenitatis and Crisium, and older in the southern maria, including Tranquillitatis, Fecunditatis, and Nectaris. According to the relationship between the titanium contents of the mare units and their stratigraphy, the titanium content decreases with time in the early stage but increases toward the end of volcanism in the Serenitatis and Crisium region, while it increases with time but finally decreases in the Tranquillitatis and Fecunditatis region. In connection with the distribution of mare basalts, a large amount of high-Ti mare basalts are found in Mare Tranquillitatis, especially in the western part, while other maria are covered by low-Ti basalts. The iron contents show a similar distribution to that of titanium.
    • New noble gas data of primitive and differentiated achondrites including Northwest Africa 011 and Tafassasset

      Patzer, A.; Schultz, L.; Franke, L. (The Meteoritical Society, 2003-01-01)
      This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite-like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset. We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg-rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q-like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar 36Ar/ 132Xe ratio of ~400 along with a slightly elevated 84Kr/132Xe ratio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration. Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low 132Xe concentrations, a low trapped 36Ar/132Xe ratio of ~30, and a noticeable amount of radiogenic 129Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassassets noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite).
    • Spade: An H chondrite impact-melt breccia that experienced post-shock annealing

      Rubin, A. E.; Jones, R. H. (The Meteoritical Society, 2003-01-01)
      The low modal abundances of relict chondrules (1.8 vol%) and of coarse (i.e., greater than or equal to 200 micrometer-size) isolated mafic silicate grains (1.8 vol%) in Spade relative to mean H6 chondrites (11.4 and 9.8 vol%, respectively) show Spade to be a rock that has experienced a significant degree of melting. Various petrographic features (e.g., chromite-plagioclase assemblages, chromite veinlets, silicate darkening) indicate that melting was caused by shock. Plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains. During crystallization, plagioclase developed igneous zoning. Low-Ca pyroxene that crystallized from the melt (or equilibrated with the melt at high temperatures) acquired relatively high amounts of CaO. Metallic Fe-Ni cooled rapidly below the Fe-Ni solvus and transformed into martensite. Subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite. Ambiguities exist in the shock stage assignment of Spade. The extensive silicate darkening, the occurrence of chromite-plagioclase assemblages, and the impact-melted characteristics of Spade are consistent with shock stage S6. Low shock (stage S2) is indicated by the undulose extinction and lack of planar fractures in olivine. This suggests that Spade reached a maximum prior shock level equivalent to stage S6 and then experienced post-shock annealing (probably to stage S1). These events were followed by a less intense impact that produced the undulose extinction in the olivine, characteristic of shock stage S2. Annealing could have occurred if Spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket. Spade firmly establishes the case for post-shock annealing. This may have been a common process on OC asteroids.
    • Northwest Africa 428: Impact-induced annealing of an L6 chondrite breccia

      Rubin, Alan E. (The Meteoritical Society, 2003-01-01)
      Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type-6, shocked to stage S4-S5, brecciated, and annealed to approximately petrologic type-4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 9930, which formed on a different asteroid. The petrologic type-6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 +/- 15 micrometers), troilite (100 +/- 70 micrometers), and plagioclase (20-60 micrometers) grains, and relatively homogeneous olivine (Fa24.4 +/- 0.6), low-Ca pyroxene (Fs20.5 +/- 0.4), and plagioclase (Ab84.2 +/- 0.4) compositions. The petrographic criteria that indicate shock stage S4-S5 include the presence of chromite veinlets, chromite-plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe-Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low-Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low-Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post-shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post-shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type-4 (~600-700 degrees C) during annealing, the low-Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post-metamorphic, post-shock annealing because any 26Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.
    • From the Editor

      Jull, A. J. T. (The Meteoritical Society, 2003-01-01)
    • The clay mineralogy of sediments related to the marine Mjølnir impact crater

      Dypvik, H.; Ferrell, R. E.; Sandbakken, P. T. (The Meteoritical Society, 2003-01-01)
      The 40 km diameter Mjølnir Crater is located on the central Barents Sea shelf, north of Norway. It was formed about 142 +/- 2.6 Myr ago by the impact of a 1-2 km asteroid into the shallow shelf clays of the Hekkingen Formation and the underlying Triassic to Jurassic sedimentary strata. A core recovered from the central high within the crater contains slump and avalanche deposits from the collapse of the transient crater and central high. These beds are overlain by gravity flow conglomerates, with laminated shales and marls on top. Here, impact and post-impact deposits in this core are studied with focus on clay mineralogy obtained from XRD decomposition and simulation analysis methods. The clay-sized fractions are dominated by kaolinite, illite, mixed-layered clay minerals and quartz. Detailed analyses showed rather similar composition throughout the core, but some noticeable differences were detected, including varying crystal size of kaolinite and different types of illites and illite/smectite. These minerals may have been formed by diagenetic changes in the more porous/fractured beds in the crater compared to time-equivalent beds outside the crater rim. Long-term post-impact changes in clay mineralogy are assumed to have been minor, due to the shallow burial depth and minor thermal influence from impact-heated target rocks. Instead, the clay mineral assemblages, especially the abundance of chlorite, reflect the impact and post-impact reworking of older material. Previously, an ejecta layer (the Sindre Bed) was recognized in a nearby well outside the crater, represented by an increase in smectite-rich clay minerals, genetically equivalent to the smectite occurring in proximal ejecta deposits of the Chicxulub crater. Such alteration products from impact glasses were not detected in this study, indicating that little, if any, impact glass was deposited within the upper part of the crater fill. Crater-fill deposits inherited their mineral composition from Triassic and Jurassic sediments underlying the impact site.
    • The parameterization of solid metal-liquid metal partitioning of siderophile elements

      Chabot, N. L.; Jones, J. H. (The Meteoritical Society, 2003-01-01)
      Many solar system processes involve a metallic liquid, and the composition of the metallic liquid, such as the liquid's concentrations of S, P, and degrees C, will influence the partitioning of elements during such processes. We present a method for parameterizing solid metal-liquid metal partition coefficients for siderophile (metal-loving) elements as a function of the metallic liquid composition. Our parameterization method is based on an older theory of Jones and Malvin (1990), which stated that the metallic liquid is composed of metal and non-metal-bearing domains, and the domains are the dominant influence on the partitioning behavior. By revising the means by which the metal domains are calculated, our revised parameterization method is able to match experimental partitioning data from the Fe-Ni-S, Fe-Ni-P, Fe-Ni-S-P, and Fe-Ni-C systems. Mathematical expressions were derived for the solid metal-liquid metal partitioning of 13 siderophile elements. Elements that are chalcophile (S-loving), P-loving, or C-loving prefer the non-metal-bearing domains in the metallic liquid and, consequently, aren't fit by the parameterization method presented here. Possible applications for our parameterization method include modeling the crystallization of iron meteorites, planetary differentiation, and the solidification of Earth's inner core.
    • Numerical simulation of temperature effects at fissures due to shock loading

      Heider, N.; Kenkmann, T. (The Meteoritical Society, 2003-01-01)
      The localized appearance of specific shock features in target rocks and meteorites such as melt veins and high pressure polymorphs suggests that regions with a local increase in pressure and temperature exist as a shock wave propagates through an inhomogeneous rock. In this paper, we investigate the effect of planar fissures on the local temperature distribution using numerical simulations. Time-dependent parameters such as temperature, pressure, and displacement are evaluated. The simulation model is based on a shock equation of state for the involved materials, dunite and quartzite, and simulates geometries that were also used in shock-loading experiments. An artificial gap between the materials simulates an open fissure at the interface. A strong temperature increase occurs at a gap size of 0.1 mm, which potentially can cause melting in a thin layer at the interfaces. The temperature decreases with decreasing gap size. Temperature and pressure excursions at the interface are induced by the closure of the gap, which causes a second shock wave to superpose the primary wave. Open fissures and fractures, which occur ubiquitously in shallow-buried target rocks and projectiles, thus, act as local pressure and temperature amplyfiers and may be responsible for thin melt vein formation in shocked rocks.