• 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.
    • Bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 °C

      Nozaki, Wataru; Nakamura, Tomoki; Noguchi, Takaaki (The Meteoritical Society, 2006-01-01)
      Small particles 200 micrometers in diameter from the hydrous carbonaceous chondrites Orgueil CI, Murchison CM2, and Tagish Lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry. The three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures, and thus the brief heating up to 1000 degrees C drastically changed the mineralogy. Changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400-600 degrees C, serpentine and saponite decomposition to amorphous phases at 600 and 700 degrees C, respectively, decomposition of Mg- Fe carbonate at 600 degrees C, recrystallization of secondary olivine and Fe oxide or metal at 700-800 degrees C, and recrystallization of secondary low-Ca pyroxene at 800 degrees C. The ambient atmospheric pressures controlled species of secondary Fe phase: taenite at pressures lower than 10^(-2) torr, magnesiowstite from 10^(-3) to 10^(-1) torr, and magnetite from 10^(-2) to 1 torr. The abundance of secondary low-Ca pyroxene increases in the order of Murchison, Orgueil, and Tagish Lake, and the order corresponds to saponite abundance in samples prior to heating. Mineralogy of the three unmelted micrometeorites F96CI024, kw740052, and kw740054 were investigated in detail in order to estimate heating conditions. The results showed that they might have come from different parental objects, carbonaterich Tagish Lake type, carbonate-poor Tagish Lake or CI type, and CM type, respectively, and experienced different peak temperatures, 600, 700, and 800~900 degrees C, respectively, at 60-80 km altitude upon atmospheric entry.
    • Erratum

      The Meteoritical Society, 2006-01-01
    • Extent of chondrule melting: Evaluation of experimental textures, nominal grain size, and convolution index

      Nettles, Jeffrey W.; Lofgren, Gary E.; Carlson, William D.; McSween, Harry Y. (The Meteoritical Society, 2006-01-01)
      Dynamic crystallization experiments on the ordinary chondrite Queen Alexandra Range (QUE) 97008 document textural features that occur in partially melted chondrules with changes in the degree of partial melting and cooling rate. We carried out a matrix of experiments, at peak temperatures of 1250, 1350, 1370, and 1450 degrees C, and cooling rates of 1000, 100, and 10 degrees C/h, and quenched. All experimentally produced textures closely resemble textures of porphyritic chondrules. Because peak temperatures were well below the liquidi for typical chondrule compositions, the textural similarities support an incomplete melting origin for most porphyritic chondrules. Our experiments can be used to determine the extent of melting of natural chondrules by comparing textural relationships among the experimental results with those of natural chondrules. We used our experiments along with X-ray computerized tomography scans of a Semarkona chondrule to evaluate two other methods that have been used previously to quantify the degree of melting: nominal grain size and convolution index. Proper applications of these methods can result in valid assessments of a chondrule's degree of melting, but only if accompanied by careful interpretation, as chondrule textures are controlled by more than just the extent of melting. Such measurements of single aspects of chondrule textures might be coupled with qualitative analysis of other textural aspects to accurately determine degree of melting.
    • Fabric analysis of Allende matrix using EBSD

      Watt, Lauren E.; Bland, Phil A.; Prior, Dave J.; Russell, Sara S. (The Meteoritical Society, 2006-01-01)
      Fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution. Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (~25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events.
    • Free dicarboxylic and aromatic acids in the carbonaceous chondrites Murchison and Orgueil

      Martins, Z.; Watson, J. S.; Sephton, M. A.; Botta, O.; Ehrenfreund, P.; Gilmour, I. (The Meteoritical Society, 2006-01-01)
      We have analyzed an important fraction of the free carboxylic acids present in water extracts of the CM2 chondrite Murchison and the CI1 chondrite Orgueil using gas chromatography-mass spectrometry (GC-MS). The free nature of the carboxylic acids analyzed was ensured by employing a single-step water extraction. Analyses revealed the presence of a structurally diverse suite of both aliphatic and aromatic acids in Murchison, while Orgueil exhibits a simpler distribution of exclusively aromatic acids. Within the Murchison aromatic acids, there are previously unreported phthalic acids, methyl phthalic acids, and hydroxybenzoic acids. In Orgueil, benzoic acid and very small amounts of methylbenzoic acids and methylhydroxybenzoic acids were detected. For the aromatic acids in both Murchison and Orgueil, most structural isomers were identified, suggesting an origin by abiotic processes. Quantitative differences are evident between acids in the two meteorites; carboxylic acids are much more abundant in Murchison than in Orgueil. The data suggest that differing levels of aqueous alteration on the meteorite parent body(ies) has produced dissimilar distributions of carboxylic acids.
    • Microtextures and crystal chemistry of pigeonite in the ureilites ALHA77257, RKPA80239, Y-791538, and ALHA81101

      Tribaudino, Mario (The Meteoritical Society, 2006-01-01)
      The microtextures of pigeonite in four ureilites, Allan Hills (ALH) 77257, Reckling Peak (RKP) A80239, Yamato (Y-) 791538, and Allan Hills A81101, chosen to span a range of composition and shock level, were investigated by transmission electron microscopy (TEM); two of the samples were also investigated by single crystal X-ray diffraction to determine Fe2+-Mg cation site partitioning.The low-shock and compositionally homogeneous pigeonites in ALHA77257 and RKPA80329 (Wo 6.4 for both, mg 86.3 and 84.3 respectively) display irregularly spaced, shock-induced stacking faults oriented parallel to (100), and largeantiphase domains (50-100 nm). Antiphase domains have no preferential orientation. No evidence of exsolution was observed.The low-shock Y-791538 pigeonite is homogeneous and has higher Ca and mg (Wo 9.4, mg 91.2). TEM investigation showed spinodal decomposition, indicative of incipient exsolution; small antiphase domains were observed (approximately equal to 5 nm). Single crystal refinement yielded R4-sigma = 5.71%, with Fe2+-Mg partitioning coefficient kD = 0.077(8) and Tc = 658(35) degrees C. ALHA81101 has compositionally heterogeneous pyroxenes, with large local variations in Wo and mg (Wo = 4-13, mg = 86-68). No compositional gradients from core to rim of grains were observed, and the heterogeneity is interpreted as related to cation migration during shock. In one relatively Ca-rich region (Wo is approximately equal to 12), TEM analysis showed augite-pigeonite exsolution lamellae, with spacing 145(20) nm.Results for ALHA77257, RKPA80239, and Y-791538 support a model of rapid cooling following breakup of the ureilite parent body. The presence of exsolution lamellae in ALHA81101 can be related to a local shock-induced Ca enrichment and provides no constraint on the late cooling history.
    • Petrology and geochemistry of the fine-grained, unbrecciated diogenite Northwest Africa 4215

      Barrat, J. A.; Beck, P.; Bohn, M.; Cotten, J.; Gillet, P. H.; Greenwood, R. C.; Franchi, I. A. (The Meteoritical Society, 2006-01-01)
      We report on the petrology and geochemistry of Northwest Africa (NWA) 4215, an unbrecciated diogenite recovered in the Sahara. This single stone, weighing 46.4 g, displays a well-preserved cumulative texture. It consists of zoned xenomorphic orthopyroxene grains on the order of 500 micrometers in size, along with a few large chromite crystals (5 vol%, up to 3 mm). Accessory olivine and scarce diopside grains occur within the groundmass, usually around the chromite crystals. Minor phases are cristobalite, troilite, and metal. Unlike other diogenites, orthopyroxenes (En76.2Wo1.1Fs22.7 to En68.6Wo5.5Fs25.9), olivines (Fo76 to Fo71), and chromites (Mg# = 14.3 44.0, Cr# = 42.2-86.5) are chemically zoned. The minor element behavior in orthopyroxenes and the intricate chemical profiles obtained in chromites indicate that the zonings do not mirror the evolution of the parental melt. We suggest that they resulted from reaction of the crystals with intercumulus melt. In order to preserve the observed zoning profiles, NWA 4215 clearly cooled significantly faster than other diogenites. Indeed, the cooling rate determined from the diffusion of Cr in olivine abutting chromite is in the order of 10-50 degrees C/a, suggesting that NWA 4215 formed within a small, shallow intrusion.The bulk composition of NWA 4215 has been determined for major and trace elements. This meteorite is weathered and its fractures are filled with calcite, limonite, and gypsum, typical of hot desert alteration. In particular, the FeO, CaO abundances and most of the trace element concentrations (Sr, Ba, Pb, and REE among others) are high and indicate a significant contribution from the secondary minerals. To remove the terrestrial contribution, we have leached with HCl a subsample of the meteorite. The residue, made essentially of orthopyroxene and chromite, has similar major and trace element abundances to diogenites as shown by the shape of its REE pattern or by its high Al/Ga ratio. The connection of NWA 4215 with diogenites is confirmed by its O-isotopic composition (delta-17O = 1.431 +/- 0.102 ppm, delta-18O = 3.203 +/- 0.205 ppm, Delta-17O = -0.248 +/- 0.005 ppm).
    • Terrestrial ages, pairing, and concentration mechanism of Antarctic chondrites from Frontier Mountain, Northern Victoria Land

      Welten, K. C.; Nishiizumi, K.; Caffee, M. W.; Hillegonds, D. J.; Johnson, J. A.; Jull, A. J. T.; Wieler, R.; Folco, L. (The Meteoritical Society, 2006-01-01)
      We report concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in the metal phase of 26 ordinary chondrites from Frontier Mountain (FRO), Antarctica, as well as cosmogenic 14C in eight and noble gases in four bulk samples. Thirteen out of 14 selected H chondrites belong to two previously identified pairing groups, FRO 90001 and FRO 90174, with terrestrial ages of ~40 and ~100 kyr, respectively. The FRO 90174 shower is a heterogeneous H3-6 chondrite breccia that probably includes more than 300 individual fragments, explaining the high H/L chondrite ratio (3.8) at Frontier Mountain. The geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50-100 kyr to less than 40 m, supporting the stability of the meteorite trap over the last glacial cycle. The second H-chondrite pairing group, FRO 90001, is much smaller and its geographic distribution is mainly controlled by wind-transport. Most L-chondrites are younger than 50 kyr, except for the FRO 93009/01172 pair, which has a terrestrial age of ~500 kyr. These two old L chondrites represent the only surviving members of a large shower with a similar preatmospheric radius (~80 cm) as the FRO 90174 shower. The find locations of these two paired L-chondrite fragments on opposite sides of Frontier Mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau. The 50 FRO meteorites analyzed so far represent 21 different falls. The terrestrial ages range from 6 kyr to 500 kyr, supporting the earlier proposed concentration mechanism.
    • The origin of dark inclusions in Allende: New evidence from lithium isotopes

      Sephton, Mark A.; James, Rachael H.; Zolensky, Michael E. (The Meteoritical Society, 2006-01-01)
      Aqueous and thermal processing of primordial materials occurred prior to and during planet formation in the early solar system. A record of how solid materials were altered at this time is present in the carbonaceous chondrites, which are naturally delivered fragments of primitive asteroids. It has been proposed that some materials, such as the clasts termed "dark inclusions" found in type III chondrites, suggest a sequence of aqueous and thermal events. Lithium isotopes (6Li and 7Li) can reveal the role of liquid water in dark inclusion history. During aqueous alteration, 7Li passes preferentially into solution leaving 6Li behind in the solid phase and, consequently, any relatively extended periods of interaction with 7Li-rich fluids would have left the dark inclusions enriched in the heavier isotope when compared to the meteorite as a whole. Our analyses of lithium isotopes in Allende and its dark inclusions reveal marked isotopic homogeneity and no evidence of greater levels of aqueous alteration in dark inclusion history.