• A model for planetesimal meltdown by 26Al and its implications for meteorite parent bodies

      Hevey, Phonsie J.; Sanders, Ian S. (The Meteoritical Society, 2006-01-01)
      The melting of planetesimals heated by 26Al has been modelled using a new finite difference method that incorporates convection. As an example, we consider a planetesimal with a radius of 64 km, which accretes instantaneously at t = 0.75 Myr (after the formation of calcium-aluminum-rich inclusions) from cold (250 K) nebular dust with 50% porosity. At t = 0.9 Myr (T = 700 K), the planetesimal shrinks to a radius of 50 km due to sintering. At t = 1.2 Myr (T = 1425 K), the fully insulated interior, deeper than a few kilometers, starts to melt, and at t = 1.5 Myr (T = 1725 K), with 50% melting, convection starts. By t = 2 Myr, the planetesimal is a globe of molten, convecting slurry inside a thin residual crust. From about t = 2.5 Myr, the crust thickens rapidly as the power of 26Al fades.Planetesimals probably melt in this manner when they accrete before t = 1.3 Myr and are large enough to insulate themselves (R >20 km for accretion at t = 0, rising to >80 km at t = 1.3 Myr). Melting behavior will also be affected by the level of 60Fe in nebular dust, by the extent of devolatilization reactions and basalt segregation during heating, and by gradual accretion.The model suggests that a) the parent bodies of differentiated meteorites had accreted before about t = 1.5 to 2 Myr and before most chondritic parent bodies had formed, and b) that molten planetesimals may be a source for chondrule melt droplets.
    • A petrological, mineralogical, and chemical analysis of the lunar mare basalt meteorite LaPaz Icefield 02205, 02224, and 02226

      Joy, Katherine H.; Crawford, Ian A.; Downes, Hilary; Russell, Sara S.; Kearsley, Anton T. (The Meteoritical Society, 2006-01-01)
      LaPaz Icefield (LAP) 02205, 02226, and 02224 are paired stones of a crystalline basaltic lunar meteorite with a low-Ti (3.21-3.43% TiO2) low-Al (9.93-10.45% Al2O3), and low-K (0.11-0.12% K2O) composition. They consist mainly of zoned pyroxene and plagioclase grains, with minor ilmenite, spinel, and mesostasis regions. Large, possibly xenocrystic, forsteritic olivine grains (<3% by mode) contain small trapped multiphase melt inclusions. Accessory mineral and mesostasis composition shows that the samples have experienced residual melt crystallization with silica oversaturation and late-stage liquid immiscibility. Our section of LAP 02224 has a vesicular fusion crust, implying that it was at one time located sufficiently close to the lunar surface environment to have accumulated solar-wind-implanted gases. The stones have a comparable major element composition and petrography to low-Ti, low-Albasalts collected at the Apollos 12 and 15 landing sites. However, the LAP stones also have an enriched REE bulk composition and are more ferroan (Mg numbers in the range of 31 to 35) than similar Apollo samples, suggesting that they represent members of a previously unsampled fractionated mare basalt suite that crystallized from a relatively evolved lunar melt.
    • A relict-grain-bearing porphyritic olivine compound chondrule from LL3.0 Semarkona that experienced limited remelting

      Rubin, Alan E. (The Meteoritical Society, 2006-01-01)
      Chondrule D8n in LL3.0 Semarkona is a porphyritic olivine (PO) chondrule, 1300 1900 m in size, with a complicated thermal history. The oldest recognizable portion of D8n is a moderately high-FeO, PO chondrule that is modeled as having become enmeshed in a dust ball containing a small, intact, low-FeO porphyritic chondrule and fine-grained material consisting of forsterite, kamacite, troilite, and possibly reduced degrees C. The final chondrule melting event may have been a heat pulse that preferentially melted the low-FeO material and produced a low-FeO, opaquerich, exterior region, 45-140 m in thickness, around the original chondrule. At one end of the exterior region, a kamacite- and troilite-rich lump 960 micrometers in length formed. During the final melting event, the coarse, moderately ferroan olivine phenocrysts within the original chondrule appear to have been partly resorbed (These relict phenocrysts have the highest concentrations of FeO, MnO, and Cr2O3--7.5, 0.20, and 0.61 wt%, respectively--in D8n.). Narrow olivine overgrowths crystallized around the phenocrysts following final chondrule melting; their compositions seem to reflect mixing between melt derived from the exterior region and the resorbed margins of the phenocrysts. During the melting event, FeO in the relict phenocrysts was reduced, producing numerous small blebs of Ni-poor metallic Fe along preexisting curvilinear fractures. The reduced olivine flanking the trails of metal blebs has lower FeO than the phenocrysts but virtually identical MnO and Cr2O3 contents. Subsequent parent-body aqueous alteration in the exterior region of the chondrule formed pentlandite and abundant magnetite.
    • A study of Mg and K isotopes in Allende CAIs: Implications to the time scale for the multiple heating processes

      Ito, M.; Nagasawa, H.; Yurimoto, H. (The Meteoritical Society, 2006-01-01)
      The measurements of magnesium and potassium isotopic compositions of refractory minerals in Allende calcium-aluminum-rich inclusions (CAIs), 7R-19-1, HN3-1, and EGG3 were taken by secondary ion mass spectrometry (SIMS). The 7R-19-1 contains 16O-rich and 16O-poor melilite grains and define a single isochron corresponding to an initial 26Al/27Al ratio of (6.6 +/- 1.3) x 10^(-5). The Al-Mg isochron, O isotope measurements and petrography of melilite in 7R-19-1 indicate that 16O-poor melilite crystallized within 0.4 Myr after crystallization of 16O-rich melilite, suggesting that oxygen isotopic composition of the CAI-forming region changed from 16O-rich to 16O-poor within this time interval. The 16O-poor melilite is highly depleted in K compared to the adjacent 16Orich melilite, indicating evaporation during remelting of 7R-19-1. We determined the isochron for 41Ca-41K isotopic systematics in EGG3 pyroxene with (4.1 +/- 2.0) x 10^(-9) (2-sigma) as an initial ratio of 41Ca/40Ca, which is at least two times smaller than the previous result (Sahijipal et al. 2000). The ratio of 41Ca/40Ca in the EGG3 pyroxene grain agrees within error with the value obtained by Hutcheon et al. (1984). No evidence for the presence of 41K excess (decay product of a short-lived radionuclide 41Ca) was found in 7R-19-1 and HN3-1. We infer that the CAI had at least an order of magnitude lower than canonical 41Ca/40Ca ratio at the time of the CAI formation.
    • Abstracts

      The Meteoritical Society, 2006-01-01
    • Acrylic embedding of Stardust particles encased in aerogel

      Matrajt, G.; Brownlee, D. E. (The Meteoritical Society, 2006-01-01)
      Ultramicrotomy of samples embedded in epoxy resin is a standard method for preparing ultra-thin sections for electron microscopy. In this report we describe a new embedding technique that uses acrylic resin instead of epoxy. This method offers several important advantages for sectioning small extraterrestrial samples. One is that the acrylic resin is soluble and can be removed after ultramicrotomy to leave a sample that is free of the mounting media. This is important for studying carbon and insoluble organic components. A second major advantage of acrylic is that, when combined with pre-embedding compression, it provides a very effective method of mounting samples collected in silica aerogel. Acrylic embedding is currently being used to mount comet particles collected by NASA's Stardust mission. Combined with a flattening process, the acrylic embedding and sectioning preserves all pieces of collected samples in their collection matrix. In addition to Stardust, acrylic may be applied to other samples collected in aerogel such as those from the Russian Mir space station (Hörz et al. 2000) and future missions such as Sample Collection for Investigation of Mars (SCIM) (Leshin 2003), a proposed mission to collect atmospheric dust particles from Mars.
    • Ages of rampart craters in equatorial regions on Mars: Implications for the past and present distribution of ground ice

      Reiss, D.; Van Gasselt, S.; Hauber, E.; Michael, G.; Jaumann, R.; Neukum, G. (The Meteoritical Society, 2006-01-01)
      We are testing the idea of Squyres et al. (1992) that rampart craters on Mars may have formed over a significant time period and therefore the onset diameter (minimum diameter of a rampart crater) only reflects the ground ice depth at a given time. We measured crater size frequencies on the layered ejecta of rampart craters in three equatorial regions to derive absolute model ages and to constrain the regional volatile history. Nearly all rampart craters in the Xanthe Terra region are ~3.8 Gyr old. This corresponds to the Noachian fluvial activity that region. Rampart crater formation declines in the Hesperian, whereas onset diameters (minimum diameter) increase. No new rampart craters formed after the end of the Hesperian (~3 Gyr). This indicates a lowering of the ground ice table with time in the Xanthe Terra region. Most rampart craters in the Valles Marineris region are around 3.6 Gyr old. Only one large, probably Amazonian-aged (~2.5 Gyr), rampart crater exists. These ages indicate a volatile-rich period in the Early Hesperian and a lowering of the ground ice table with time in the Valles Marineris study region. Rampart craters in southern Chryse Planitia,which are partly eroded by fluvial activity, show ages around 3.9 Gyr. Rampart craters superposed on channels have ages between ~1.5 and ~0.6 Gyr. The onset diameter (3 km at ~1.5 Gyr) in this region may indicate a relatively shallow ground ice table. Loss of volatiles due to diffusion and sublimation might have lowered the ground ice table even in the southern Chryse Planitia region afterwards. In general, our study implies a formation of the smallest rampart craters within and/or shortly after periods of fluvial activity and a subsequent lowering of the ground ice table indicated by increasing onset diameter to the present. These results question the method to derive present equatorial ground ice depths from the onset diameter of rampart craters without information about their formation time.
    • Alastair Graham Walter Cameron (1925-2005)

      Consolmagno, Guy; Fegley, Bruce; Kring, David (The Meteoritical Society, 2006-01-01)
    • Alkali-feldspathic material entrained in Fe,S-rich veins in a monomict ureilite

      Warren, Paul H.; Huber, Heinz; Ulff-Møller, Finn (The Meteoritical Society, 2006-01-01)
      The Elephant Moraine (EET) 96001 ureilite contains a remarkable diversity of feldspars, which occur as tiny (no more than 60 micrometers maximum dimension) grains within a few Fe,S-rich (now weathered to mostly Fe oxide) veins. Molar S:Fe ratio in the veins averages 0.08 0.02. The veins meander and feature large fluctuations in apparent width; they appear to have entered this monomict breccia by a gentle, percolative process, not by violent impact injection. The feldspars are accompanied by a diverse suite of K-rich (and generally also Ti-rich) feldspathic glasses, and also major proportions of silica and pyroxene, which is largely fassaitic. A rhnite-like phase is also found, and, as inclusions in one of the fassaites, a Cr-poor spinel-like phase. The feldspars mostly feature remarkably high K/Na compared to feldspars of comparable An from polymict ureilites. The EET 96001 feldspathic component was probably once part of a thin basaltic crust on a ureilite asteroid. The spinel included in one of the fassaites formed at remarkably high fO2 (apparent oxidation state of iron: 41 atom% Fe3+), suggesting that the parent magma possibly assimilated near-surface water (however, the Fe3+was not directly measured, and has conceivably been affected by terrestrial weathering; also, there is no assurance that this fassaite originated together with the typical feldspar). We speculate that the feldspathic component was mixed into the dense, Fe,S-rich vein material, and very soon thereafter the Fe,S-rich vein material was emplaced adjacent to the EET 96001 host ureilite, at an advanced stage in a chaotic catastrophic disruption and partial reassembly process that affected all ureilites. The high-K nature of the EET 96001 feldspathic component, including the feldspathic glasses, suggests that fractional fusion may not have been as common during ureilite anatexis as has been inferred from recent studies of clast assemblages in polymict ureilites.
    • Amino acid analyses of Antarctic CM2 meteorites using liquid chromatography-time of flight-mass spectrometry

      Glavin, Daniel P.; Dworkin, Jason P.; Aubrey, Andrew; Botta, Oliver; Doty, James H.; Martins, Zita; Bada, Jeffrey L. (The Meteoritical Society, 2006-01-01)
      Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography-time of flight-mass spectrometry (LC-ToF-MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, Beta-alanine, gamma-amino-n-butyric acid (gamma-ABA), and alpha-aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440-3200 ppb). In addition, ALH 83100 was found to have lower abundances of the -dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of epsilon-amino-n-caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon-6 contamination from the Antarctic sample storage bags may have occurred during collection.
    • An anomalous eucrite, Dhofar 007, and a possible genetic relationship with mesosiderites

      Yamaguchi, Akira; Setoyanagi, Takehiko; Ebihara, Mitsuru (The Meteoritical Society, 2006-01-01)
      We studied the texture, mineralogy, and bulk chemical composition of Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict breccia that is mostly composed of coarse-grained granular (CG) clasts with a minor amount of xenolithic components, such as a fragment of Mg-rich pyroxene. The coarse-grained, relict gabbroic texture, mineral chemistry, and bulk chemical data of the coarse-grained clast indicate that the CG clasts were originally a cumulate rock crystallized in a crust of the parent body. However, in contrast to monomict eucrites, the siderophile elements are highly enriched and could have been introduced by impact events. Dhofar 007 appears to have experienced a two-stage postcrystallization thermal history: rapid cooling at high temperatures and slow cooling at lower temperatures. The presence of pigeonite with closely spaced, fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures (>0.5 degrees C/yr at ~1000 degrees C) than typical cumulate eucrites. However, the presence of the cloudy zone in taenite and the Ni profile across the kamacite-taenite boundaries indicates that the cooling rate was very slow at lower temperatures (~1-10 degrees C/Myr at <600-700 degrees C). The slow cooling rate is comparable to those in mesosiderites and pallasites. The two-stage thermal history and the relative abundance of siderophile elements similar to those for metallic portions in mesosiderites suggest that Dhofar 007 is a large inclusion of mesosiderite. However, we cannot rule out a possibility that Dhofar 007 is an anomalous eucrite.
    • An atmospheric blast/thermal model for the formation of high-latitude pedestal craters

      Wrobel, Kelly; Schultz, Peter; Crawford, David (The Meteoritical Society, 2006-01-01)
      Although tenuous, the atmosphere of Mars affects the evolution of impact-generated vapor. Early-time vapor from a vertical impact expands symmetrically, directly transferring a small percentage of the initial kinetic energy of impact to the atmosphere. This energy, in turn, induces a hemispherical shock wave that propagates outward as an intense airblast (due to high-speed expansion of vapor) followed by a thermal pulse of extreme atmospheric temperatures (from thermal energy of expansion). This study models the atmospheric response to such early-time energy coupling using the CTH hydrocode written at Sandia National Laboratories. Results show that the surface surrounding a 10 km diameter crater (6 km "apparent" diameter) on Mars will be subjected to intense winds (~200 m/s) and extreme atmospheric temperatures. These elevated temperatures are sufficient to melt subsurface volatiles at a depth of several centimeters for an ice-rich substrate. Ensuing surface signatures extend to distal locations (~4 apparent crater diameters for a case of 0.1% energy coupling) and include striations, thermally armored surfaces, and/or ejecta pedestals--all of which are exhibited surrounding the freshest high-latitude craters on Mars. The combined effects of the atmospheric blast and thermal pulse, resulting in the generation of a crater-centered erosion-resistant armored surface, thus provide a new, very plausible formation model for high-latitude Martian pedestal craters.
    • An experimental study on Fischer-Tropsch catalysis: Implications for impact phenomena and nebular chemistry

      Sekine, Yasuhito; Sugita, Seiji; Shido, Takafumi; Yamamoto, Takashi; Iwasawa, Yasuhiro; Kadono, Toshihiko; Matsui, Takafumi (The Meteoritical Society, 2006-01-01)
      Fischer-Tropsch catalysis, by which CO and H2 are converted to CH4 on the surface of transition metals, has been considered to be one of the most important chemical reactions in many planetary processes, such as the formation of the solar and circumplanetary nebulae, the expansion of vapor clouds induced by cometary impacts, and the atmospheric re-entry of vapor condensate due to asteroidal impacts. However, few quantitative experimental studies have been conducted for the catalytic reaction under conditions relevant to these planetary processes. In this study, we conduct Fischer-Tropsch catalytic experiments at low pressures (1.3 10^(4) bar is less than or equal to P which is less than or equal to 5.3 10^(1) bar) over a wide range of H2/CO ratios (0.25-1000) using pure iron, pure nickel, and iron-nickel alloys. We analyze what gas species are produced and measure the CH4 formation rate. Our results indicate that the CH4 formation rate for iron catalysts strongly depends on both pressure and the H2/CO ratio, and that nickel is a more efficient catalyst at lower pressures and lower H2/CO ratios. This difference in catalytic properties between iron and nickel may come from the reaction steps concerning disproportionation of CO, hydrogenation of surface carbon, and the poisoning of the catalyst. These results suggest that nickel is important in the atmospheric re-entry of impact condensate, while iron is efficient in circumplanetary subnebulae. Our results also indicate that previous numerical models of iron catalysis based on experimental data at 1 bar considerably overestimate CH4 formation efficiency at lower pressures, such as the solar nebula and the atmospheric re-entry of impact condensate.
    • An improved extraction system to measure carbon-14 terrestrial ages of meteorites and pairing of the Antarctic Yamato-75097 group chondrites

      Minami, M.; Terui, A.; Takaoka, N.; Nakamura, T. (The Meteoritical Society, 2006-01-01)
      We examined an improved system for extraction of carbon from meteorites, using a vacuum-tight RF melting method. Meteorite samples mixed with an iron combustion accelerator, including a specific amount of carbon (0.052%), were combusted in a RF furnace (LECO HF-10). 14CO2 extracted from the meteorite was diluted with a known amount of nearly 14C-free CO2, evolved from the iron accelerator on combustion. The 14C activities of the recently fallen Holbrook (L6) and Mt. Tazerzait (L5) meteorites were measured by this method. The mean value was 56.5 +/- 3.0 dpm/kg, which is similar to the values reported for recently fallen L6 chondrites. Furthermore, terrestrial ages were measured for four Antarctic meteorites: 1.8 +/- 0.5 kyr for Yamato (Y-) 75097 (L6), 1.8 +/- 0.5 kyr for Y-75108 (L6), and 0.1 +/- 0.1 kyr for Y-74192 (H5). For Y-74190 (L6), an apparent age of 0.8 +/- 0.5 kyr was calculated. After consideration of the shielding effect by using 22Ne/21Ne values, we obtained about 1.8 kyr for the terrestrial age of this chondrite. The five samples Y-74190, Y-75097, and Y-75108, together with Y-75102 (L6) and Y-75271 (L6), have been reported to be paired and fragments of an L-chondrite shower (Honda 1981; Takaoka 1987). The result of this work and literature data for the latter two samples confirmed that they are paired. More discussion and experimental work are needed for other recently fallen meteorites, both for L and H chondrites, and a correction for the shielding effect should be done to determine a more reliable terrestrial age.
    • Analysis and survival of amino acids in Martian regolith analogs

      Garry, James R. C.; ten Kate, Inge Loe; Martins, Zita; Nørnberg, Per; Ehrenfreund, Pascale (The Meteoritical Society, 2006-01-01)
      We have investigated the native amino acid composition of two analogs of Martian soil, JSC Mars-1 and Salten Skov. A Mars simulation chamber has been built and used to expose samples of these analogs to temperature and lighting conditions similar to those found at low latitudes on the Martian surface. The effects of the simulated conditions have been examined using high-performance liquid chromatography (HPLC). Exposure to energetic ultraviolet (UV) light in vacuum appears to cause a modest increase in the concentration of certain amino acids within the materials, which is interpreted as resulting from the degradation of microorganisms. The influence of low temperatures shows that the accretion of condensed water on the soils leads to the destruction of amino acids, supporting the idea that reactive chemical processes involving H2O are at work within the Martian soil. We discuss the influence of UV radiation, low temperatures, and gaseous CO2 on the intrinsic amino acid composition of Martian soil analogs and describe, with the help of a simple model, how these studies fit within the framework of life detection on Mars and the practical tasks of choosingand using Martian regolith analogs in planetary research.
    • Announcement: Biological Processes in Impact Craters

      Zolensky, Michael (The Meteoritical Society, 2006-01-01)
    • Annual Author Index

      The Meteoritical Society, 2006-01-01
    • Annual Subject Index

      The Meteoritical Society, 2006-01-01
    • Asteroid 3628 Božněmcová: Covered with angrite-like basalts?

      Cloutis, E. A.; Binzel, R. P.; Burbine, T. H.; Gaffey, M. J.; McCoy, T. J. (The Meteoritical Society, 2006-01-01)
      A detailed analysis of the reflectance spectrum of asteroid 3628 Božněmcová, previously identified as a possible ordinary chondrite parent body, indicates that its surface consists of an assemblage dominated by clinopyroxene and plagioclase feldspar. The clinopyroxene is Fe2+-bearing (likely in the range Fs~10-20), with >90% of the Fe2+ being present in the M1 crystallographic site (spectral type A). The clinopyroxene:plagioclase feldspar ratio is between ~2 and 3 (~55-75% clinopyroxene, ~20-33% plagioclase feldspar). If olivine is present, the clinopyroxene:olivine ratio is >~3 (<20% olivine). The derived mineralogy of Božněmcov is most similar, but not identical, to the known angrite meteorites. The data suggest that Božněmcov formed by melting and differentiation of an oxidized chondritic precursor and probably represents an unsampled angrite-like body.
    • Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

      Glass, Billy P.; Koeberl, Christian (The Meteoritical Society, 2006-01-01)
      Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95-17957-2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock-metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine-grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm^2) of any known Australasian microtektite-bearing site and may be closer to the source crater than any previously identified Australasian microtektite-bearing site. A source crater in the vicinity of 22 degrees N and 10 degrees E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 +/- 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ~800 ka ago and that it is spread over an area of at least 3.7 x 10^7 km^2.