• Erratum

      The Meteoritical Society, 2004-01-01
    • Book Review: The Cambridge Guide to the Solar System, K. R. Lang

      Bleamaster, L. F. (The Meteoritical Society, 2004-01-01)
    • Denis Martin Shaw, 1923-2003

      Crocket, J. (The Meteoritical Society, 2004-01-01)
    • Notice of Grant Funding Availability

      The Meteoritical Society, 2004-01-01
    • Noble gases in chondrules and associated metal-sulfide-rich samples: Clues on chondrule formation and the behavior of noble gas carrier phases

      Vogel, N.; Leya, I.; Bischoff, A.; Baur, H.; Wieler, R. (The Meteoritical Society, 2004-01-01)
      Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide- rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial 20Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial 36Ar. Our preferred interpretation is that--in contrast to CAIs--the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial 20Ne and 36Ar show low presolar-diamond-like 36Ar/20Ne ratios. In contrast, the metal-sulfide-rich coatings generally show higher gas concentrations and Q-like 36Ar/20Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar-carrying phase Q became enriched in the metal-sulfide-rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne-carrying presolar diamonds survived the melting event leading to the low observed 36Ar/20Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar-type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.
    • Los Angeles: A tale of two stones

      Warren, P. H.; Greenwood, J. P.; Rubin, A. E. (The Meteoritical Society, 2004-01-01)
      We compare and contrast the mineralogy and petrology of the 2 stones of the extremely ferroan and Cr-poor martian meteorite, Los Angeles. The 2 stones are similar in many characteristics, strongly suggesting that they originated from a single flow or shallow intrusion. However, stone 2 is more ferroan and enriched in late-stage materials than its larger, and more widely studied, sibling. Stone 2 has a far higher abundance (~25 vol%) than stone 1 (10 vol%) of combined opaques, meaning not only conventional opaque minerals but also, and more abundantly, fine-grained symplectitic intergrowths of fayalite + ferroan augite + silica (interpreted as pyroxferroite breakdown material, PBM). The bulk composition of the PBM is close to that of stoichiometric pyroxferroite, with roughly 45 wt% FeO. Extensive zonation within the pyroxenes of both stones is consistent with origin by closed-system fractional crystallization of the parent basaltic melt(s). However, the compositional and modal disparity between the two stones suggests that they formed in an environment where at least mild multi-cm-scale differentiation occurred. Probably, in both stones, crystallization began from similar melts with mg ~27-28 mol%, but during crystallization, significant migration of the melt component occurred, perhaps by crystal settling and/or filter pressing. Stone 2 acquired an enhanced proportion of residual melt and, thus, higher proportions of late-stage materials such as PBM, oxides, and phosphates. Within the PBM, clinopyroxene poikiloblastically encloses fayalite and silica. At least some of the PBM had already formed by decomposition of pyroxferroite before the major shock that caused the very scarce brecciation within Los Angeles. However, the low abundance of fractures within PBM, in comparison to pyroxene and some other minerals, may be an indication that the textures of PBM regions typically did not assume their final detailed configuration until after the last major shock. The steep slope of a pyroxene mg-Cr correlation suggests that igneous crystallization occurred at higher fO2 in Los Angeles than in otherwise similar shergottites such as QUE 94201, Shergotty, and Zagami.
    • Book Review: Meteorites, Ice, and Antarctica, William A. Cassidy

      Genge, M. (The Meteoritical Society, 2004-01-01)
    • Distribution of noble gases in Chinese tektites: Implications for neon solubility in natural gases

      Pinti, D. L.; Matsumoto, T.; Matsuda, J.-I.; Fang, Z. (The Meteoritical Society, 2004-01-01)
      Five indochinites from Hainan Island and the Leizhou Peninsula, China were analyzed for noble gas abundances and isotopic ratios. These splash-form tektites show vesiculation ranging from 0.4 vol% to 8 vol%, as determined by digital image analysis (software SXM(R)) on thin section photographs. To study the distribution of noble gases in vesicles and in glass, the gases were extracted by heating and by crushing, respectively, on 2 aliquots of the same sample. The results show that 5 to 53% of the total measured 20Ne resides in vesicles. The calculated concentration of neon dissolved in the glass is higher (0.7-1 x 10^(-7) cm3STP/g) than that expected from solubility equilibrium (1 x 10^(-8) cm3STP/g), assuming solubility data from MORB glasses. The neon concentration of splash-form tektites, those analyzed in this work and those from other strewn-fields worldwide, is correlated with the SiO2 content of glass and with the nonbridging oxygen per tetrahedral cation in the melt (hereafter NBO/T ratio), the latter being an index of the free-volume in the silica network where neon could be dissolved. These correlations suggest that the glass structure of tektite has a larger free-volume available for dissolving noble gases than MORB glasses.
    • Empirical and theoretical comparisons of the Chicxlub and Sudbury impact structures

      Pope, K. O.; Kieffer, S. W.; Ames, D. E. (The Meteoritical Society, 2004-01-01)
      Chicxulub and Sudbury are 2 of the largest impact structures on Earth. Research at the buried but well-preserved Chicxulub crater in Mexico has identified 6 concentric structural rings. In an analysis of the preserved structural elements in the eroded and tectonically deformed Sudbury structure in Canada, we identified ring-like structures corresponding in both radius and nature to 5 out of the 6 rings at Chicxulub. At Sudbury, the inner topographic peak ring is missing, which if it existed, has been eroded. Reconstructions of the transient cavities for each crater produce the same range of possible diameters: 80110 km. The close correspondence of structural elements between Chicxulub and Sudbury suggests that these 2 impact structures are approximately the same size, both having a main structural basin diameter of ~150 km and outer ring diameters of ~200 km and ~260 km. This similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt (melt produced directly upon decompression from high pressure impact) and impact melt (shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall) in large impacts. Our empirical comparisons suggest that Sudbury has ~70% more impact melt than does Chicxulub (~31,000 versus ~18,000 km3) and 85% more shock melt (27,000 km3 versus 14,500 km3). To examine possible causes for this difference, we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts. We use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models. The results demonstrate that the differences in melt volumes can be readily explained if Chicxulub was an asteroid impact and Sudbury was a comet impact. The estimated 70% difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked. Preheating of the target rocks at Sudbury by the Penokean Orogeny cannot explain the excess melt at Sudbury, the majority of which resides in the suevite. The greater amount of suevite at Sudbury compared to Chicxulub may be due to the dispersal of shock melt by cometary volatiles at Sudbury.
    • Geochemistry and shock petrography of the Crow Creek Member, South Dakota, USA:Ejecta from the 74 Ma Manson impact structure

      Katongo, C.; Koeberl, C.; Witzke, B. J.; Hammond, R. H.; Anderson, R. R. (The Meteoritical Society, 2004-01-01)
      The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale Formation of eastern South Dakota and northeastern Nebraska, but it is the only unit that contains shock-metamorphosed minerals. The shocked minerals represent impact ejecta from the 74-Ma Manson impact structure (MIS). This study was aimed at determining the bulk chemical compositions and analysis of planar deformation features (PDFs) of shocked quartz; for the basal and marly units of the Crow Creek Member. We studied samples from the Gregory 84-21 core, Iroquois core and Wakonda lime quarry. Contents of siderophile elements are generally high, but due to uncertainties in the determination of Ir and uncertainties in compositional sources for Cr, Co, and Ni, we could not confirm an extraterrestrial component in the Crow Creek Member. We recovered several shocked quartz grains from basal-unit samples, mainly from the Gregory 84-21 core, and results of PDF measurements indicate shock pressures of at least 15 GPa. All the samples are composed chiefly of SiO2 (29-58 wt%), Al2O3 (6-14 wt%), and CaO (7-30 wt%). When compared to the composition of North American Shale Composite, the samples are significantly enriched in CaO, P2O5, Mn, Sr, Y, U, Cr, and Ni. The contents of rare earth elements (REE), high field strength elements (HFSE), Cr, Co, Sc, and their ratios and chemical weathering trends, reflect both felsic and basic sources for the Crow Creek Member, an inference, which is consistent with the lithological compositions in the environs of the MIS. The high chemical indices of alteration and weathering (CIA and CIW: 7599), coupled with the Al2O3-(CaO*+Na2O)-K2O (A-CN-K) ratios, indicate that the Crow Creek Member and source rocks had undergone high degrees of chemical weathering. The expected ejecta thicknesses at the sampled locations (409 to 219 km from Manson) were calculated to range from about 1.9 to 12.2 cm (for the present-day crater radius of Manson), or 0.4 to 2.4 cm (for the estimated transient cavity radius). The trend agrees with the observed thicknesses of the basal unit of the Crow Creek Member, but the actually observed thicknesses are larger than the calculated ones, indicating that not all of the basal unit comprises impact ejecta.
    • Multi-generational carbonate assemblages in martian meteorite Allan Hills 84001: Implications for nucleation, growth, and alteration

      Corrigan, C. M.; Harvey, R. P. (The Meteoritical Society, 2004-01-01)
      The carbonate mineralogy of several complex carbonate-rich regions in Allan Hills (ALH) 84001 has been examined. These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform "slab" carbonates, distinct "post-slab" magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite-rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite-siderite layers on the exterior surfaces of the carbonate. Post-slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts.
    • Enstatite aggregates with niningerite, heideite, and oldhamite from the Kaidun carbonaceous chondrite: Relatives of aubrites and EH chondrites?

      Kurat, G.; Zinner, E.; Brandstätter, F.; Ivanov, A. V. (The Meteoritical Society, 2004-01-01)
      We studied 2 enstatite aggregates (En >99), with sizes of 0.5 and 1.5 mm, embedded in the carbonaceous matrix of Kaidun. They contain sulfide inclusions up to 650 micrometers in length, which consist mainly of niningerite but contain numerous grains of heideite as well as oldhamite and some secondary phases (complex Fe, Ti, S hydroxides and Ca carbonate). Both niningerite and heideite are enriched in all trace elements relative to the co-existing enstatite except for Be and Sc. The niningerite has the highest Ca content (about 5 wt%) of all niningerites analyzed so far in any meteorite and is the phase richest in trace elements. The REE pattern is fractionated, with the CI-normalized abundance of Lu being higher by 2 orders of magnitude than that of La, and has a strong negative Eu anomaly. Heideite is, on average, poorer in trace elements except for Zr, La, and Li. Its REE pattern is flat at about 0.5 x CI, and it also has a strong negative Eu anomaly. The enstatite is very poor in trace elements. Its Ce content is about 0.01 that of niningerite, but Li, Be, Ti, and Sc have between 0.1 and 1 x CI abundances. The preferential partitioning of typical lithophile elements into sulfides indicates highly O-deficient and S-dominated formation conditions for the aggregates. The minimum temperature of niningerite formation is estimated to be ~850-900 degrees C. The texture and the chemical characteristics of the phases in the aggregates suggest formation by aggregation and subsequent sintering before incorporation into the Kaidun breccia. The trace element data obtained for heideite, the first on record, show that this mineral, in addition to oldhamite and niningerite, is also a significant carrier of trace elements in enstatite meteorites
    • Evolution and classification of acapulcoites and lodranites from a chemical point of view

      Patzer, A.; Hill, D. H.; Boynton, W. V. (The Meteoritical Society, 2004-01-01)
      We examined 15 bulk samples of the acapulcoite-lodranite clan for their major, minor, and trace element concentrations using INAA techniques. Among the analyzed meteorites are 2 new acapulcoites (Dhofar [Dho] 290, Thiel Mountains [TIL] 99002) as well as an additional acapulcoite that has been described previously only in very brief form (Graves Nunataks [GRA] 98028). The petrographic attributes of these 3 samples are addressed thoroughly. We also include petrographic information on 2 acapulcoites from Africa: Northwest Africa (NWA) 725 and NWA 1058. In general, our study strongly supports the widely accepted idea that acapulcoites and lodranites evolved through partial melting and melt migration of metal/sulfide phases and plagioclase. Furthermore, we concur with previous researchers that the original bimodal classification scheme for acapulcoites and lodranites proves to be too simple. Based on our data set, we introduce an alternative, extended scheme. With respect to their elemental distribution patterns, we distinguish 5 subtypes comprising primitive, typical, transitional, and enriched acapulcoites on one hand and lodranites on the other. The chemical distinction between the primitive, typical, and transitional acapulcoites is rather subtle and gradual. It stands in contrast to the clear modifications observed for the signatures of the enriched acapulcoites and the lodranites. The definition of subcategories basically reflects the concentrations of 2 key elements: K and Se. We note, however, that the assignment of subgroups may not be exclusively inferred from elemental abundances but should also consider additional petrographic information.
    • Modal mineralogy of carbonaceous chondrites by X-ray diffraction and Mössbauer spectroscopy

      Bland, P. A.; Cressey, G.; Menzies, O. N. (The Meteoritical Society, 2004-01-01)
      Carbonaceous chondrites are among the most analyzed geological materials on Earth. However, despite this attention, and unlike most terrestrial rocks, little is known on the abundance of individual phases within them. Here, we show how a combination of several novel X-ray diffraction (XRD) techniques (including a high-brightness X-ray MicroSource(R)), and Mössbauer spectroscopy, allows a complete modal mineralogy to be ascertained from even the most highly unequilibrated, fine-grained chondrites for all minerals of abundance >1 wt%. Knowledge of the modal mineralogy of a sample also allows us to calculate grain density. We analyzed Allende, Murchison, Tagish Lake, and Orgueil. Based on our modal data, the grain density estimates for Allende, Murchison, and Orgueil are close to literature values. In the case of Tagish Lake, there is no published grain density, although a bulk density measurement does exist. Taking our estimate of grain density, and the measured bulk density, we calculate an exceptionally high porosity of 41% for this meteorite, similar to some chondritic IDPs and in line with a porosity calculated from an entry model for the Tagish Lake fireball. Although it is an oxidized CV, magnetite is present in Allende at a level of <0.5 wt% or <0.3 vol%, a result that is substantiated by several other instrumental studies. This may be an oxidized meteorite, but that oxidation is not manifested in abundant magnetite. In addition, we note appreciable fayalitic olivine in Orgueil, detected by both XRD and Mössbauer. We employed MicroSource(R) XRD to look at heterogeneity in mineral abundance in Orgueil and found substantial variation, with phyllosilicates varying inversely with olivine. The data suggest that Orgueil was initially composed primarily of anhydrous materials, which have been partially, but not completely, altered. Although the data are preliminary, comparison between our XRD modal assessment, bulk chemistry, grain density, and Mössbauer data, suggests that our estimates of mineral abundance are robust. The advent of MicroSource(R) XRD allows similar modal data to be acquired from samples as small as a few hundred micrograms.