• Book Review: Comets II, M. C. Festou, H. U. Keller, H. A. Weaver (Eds.)

      Trigo‐Rodríguez, J. M. (The Meteoritical Society, 2005-01-01)
    • Chemical compositions of impact melt breccias and target rocks from the Tenoumer impact crater, Mauritania

      Pratesi, Giovanni; Morelli, Marco; Rossi, Angelo Pio; Ori, Gian Gabriele (The Meteoritical Society, 2005-01-01)
      The impact melt breccias from the Tenoumer crater (consisting of a fine-grained intergrowth of plagioclase laths, pyroxene crystals, oxides, and glass) display a wide range of porosity and contain a large amount of target rock clasts.Analyses of major elements in impact melt rocks show lower contents of SiO2, Al2O3, and Na2O, and higher contents of MgO, Fe2O3, and CaO, than the felsic rocks (i.e., granites and gneisses) of the basement. In comparison with the bulk analyses of the impact melt, the glass is strongly enriched in Si-Al, whereas it is depleted both in Mg and Fe; moreover, the impact melt rocks are variably enriched or depleted in some REE with respect to the felsic and mafic bedrock types. Gold is slightly enriched in the impact melt, and Co, Cr, and Ni abundances are possibly due to a contribution from mafic bedrock.Evidences of silicate-carbonate liquid immiscibility, mainly as spherules and globules of calcite within the silicate glass, have been highlighted.HMX mixing calculation confirm that the impact melt rocks are derived from a mixing of at least six different target lithologies outcropping in the area of the crater. A large contribution is derived from granitoids (~50%) and mica schist (17-19%), although amphibolites (~15%), cherty limestones (~10%), and ultrabasites (~6%) components are also present. The very low abundances of PGE in the melt rock seem to come mainly from some ultrabasic target rocks; therefore, the contamination from the meteoritic projectile appears to have been negligible.
    • Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6

      Wlotzka, Frank (The Meteoritical Society, 2005-01-01)
      Many equilibrated ordinary chondrites contain (besides chromites of constant composition) Cr spinel with a large spread in Cr/(Cr + Al) ratios. They occur mainly as large grains in chondrules rich in mesostasis, preventing complete equilibration in Cr/Al but not in Fe/Mg. This partially equilibrated Cr spinel turned out to be particularly useful for the selection of an appropriate olivine/spinel thermometer and for the determination of equilibration temperatures. The main results are:1) The H3.7 to 3.8 and the L3.7 to 3.8 chondrites analyzed show temperatures of 625 to 680 C;2) Equilibrated chondrites show a range of olivine/Cr-spinel temperatures between 700 and 820 degrees C, and the same average temperatures for type 4 to 6 (number of analyzed meteorites in brackets): H4 (9) 766 degrees C, H5 (7) 774 degrees C, H6 (3) 775 degrees C, L4 (5) 752 degrees C, L5 (4) 754 degrees C, L6 (1) 754 degrees C. These temperatures are interpreted as equilibration temperatures. One indication is that the measured isotherms are straight lines down to low Cr/(Cr + Al) ratios, which have a higher Fe/Mg interdiffusion coefficient than grains with high ratios. And there is no correlation of measured temperature with grain size of Cr spinel.3) Chromites sensu stricto show temperatures about 50 to 100 degrees C lower than Cr spinel, and a correlation with grain size. This is a closure temperature established during cooling and in situ crystallization.These results can best be interpreted by a "rubble pile" model of parent body evolution. This model cannot explain, however, the absence of type 4 to 6 chondrites with temperatures as low as for type 3.7 to 3.8.
    • Iron oxidation state in impact glass from the K/T boundary at Beloc, Haiti, by high-resolution XANES spectroscopy

      Giuli, Gabriele; Eeckhout, Sigrid Griet; Paris, Eleonora; Koeberl, Christian; Pratesi, Giovanni (The Meteoritical Society, 2005-01-01)
      We examined the local iron environment in nine impact glasses from the Cretaceous- Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples have been analyzed by Fe K-edge high-resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre-edge peak of our high-resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four- and five-coordinated Fe2+) to [4]Fe^(3+) and [5]Fe^(3+), respectively. Thus, the XANES spectra can be interpreted as a mixture of [4]Fe^(2+), [5]Fe^(2+), [4]Fe^(3+), and [5]Fe^(3+). There is no evidence for six-fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash-form tektites, followed by progressive oxidation.
    • Mineralogy and petrology of the LaPaz Icefield lunar mare basaltic meteorites

      Righter, K.; Collins, S. J.; Brandon, A. D. (The Meteoritical Society, 2005-01-01)
      Five basaltic meteorites from the LaPaz ice field are paired on the basis of their mineralogy and texture, and represent a unique basalt type distinct from those in the Apollo or Luna sample collections. LaPaz Icefield (LAP) 02205, LAP 02224, LAP 02226, LAP 02436 and LAP 03632 all contain plagioclase, pyroxene, ilmenite, spinel, olivine, and minor troilite, metal, phosphate, baddeleyite and silica (cristobalite). Brown glassy melt veins are ubiquitous and cross the primary igneous texture. Plagioclase, the major mineral and occurring as laths in a subophitic texture, is of narrow compositional range, from An85-89. Pyroxene, also a major mineral, is strongly zoned, from augite and pigeonite cores to very iron-rich rims. Ilmenite laths comprise approximately 3-5% of the basalts. Spinels show a large compositional range, comparable to that documented in Apollo 15 basalts, indicating an early chromite-rich stage followed by an intermediate to late stage with Cr-rich ulvspinel. Relatively large, subhedral to skeletal olivine crystals (Fo46-62) are sparse, and are too Forich to be in equilibrium with the bulk rock, indicating that these are xenocrysts rather than phenocrysts. The presence of melt veins with a similar composition to the bulk rock, maskelynitized plagioclase feldspar, and metastable cristobalite indicate that these rocks underwent significant shock, between 30 and 50 GPa. Calculated oxygen fugacity, using spinel-ilmenite-iron metal equilibria, is within the range defined by previous studies of lunar materials. The bulk composition (low MgO) and low calculated temperatures, together with modelling calculations, indicate an origin by fractional crystallization of a more primitive low TiO2 parent liquid similar to Apollo 12 olivine basalt.
    • Northwest Africa 1950: Mineralogy and comparison with Antarctic lherzolitic shergottites

      Mikouchi, Takashi (The Meteoritical Society, 2005-01-01)
      NWA 1950 is a new lherzolitic shergottite recently recovered from Morocco and is the first sample of this group found outside Antarctica. Major constituent phases of NWA 1950 are olivine, pyroxenes, and plagioclase glass ("maskelynite") and the rock shows a two distinct textures: poikilitic and non-poikilitic typical of lherzolitic shergottites. In poikilitic areas, several-millimeter-sized pyroxene oikocrysts enclose cumulus olivine and chromite. In contrast, pyroxenes are much smaller in non-poikilitic areas, and olivine and plagioclase glass are more abundant. Olivine in non-poikilitic areas is more Fe-rich (Fa29-31) and shows a narrower distribution than that in poikilitic areas (Fa23-29). Pyroxenes in non-poikilitic areas are also more Fe-rich than those in poikilitic areas that show continuous chemical zoning suggesting fractional crystallization under a closed system. These observations indicate that pyroxene in non-poikilitic areas crystallized from evolved interstitial melts and olivine was re-equilibrated with such melts. NWA 1950 shows similar mineralogy and petrology to previously known lherzolitic shergottites (ALH 77005, LEW 88516, Y-793605 and GRV 99027) that are considered to have originated from the same igneous body on Mars. Olivine composition of NWA 1950 is intermediate between those of ALH 77005-GRV 99027 and those of LEW 88516-Y-793605, but is rather similar to ALH 77005 and GRV 99027. The subtle difference of mineral chemistry (especially, olivine composition) can be explained by different degrees of re-equilibration compared to other lherzolitic shergottites, perhaps due to different location in the same igneous body. Thus, NWA 1950 experienced a high degree of re-equilibration, similar to ALH 77005 and GRV 99027.
    • Petrogenesis of the new lherzolitic shergottite Grove Mountains 99027: Constraints of petrography, mineral chemistry, and rare earth elements

      Lin, Yangting; Guan, Yunbin; Wang, Daode; Kimura, Makoto; Leshin, Laurie A. (The Meteoritical Society, 2005-01-01)
      We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 degrees C, based on a two-pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5-2.5 (av. 2.0 +/- 0.4) log units below the quartz-fayalite-magnetite (QFM) buffer at 960-1360 degrees C, according to the olivine-orthopyroxene-chromite barometer. The ilmenite-chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0-7.0 log unit below QFM) and temperature (1130-480 degrees C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust.Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post-shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re-crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.
    • Petrology of the Yamato nakhlites

      Imae, N.; Ikeda, Y.; Kojima, H. (The Meteoritical Society, 2005-01-01)
      The Yamato nakhlites, Y-000593, Y-000749, and Y-000802, were recovered in 2000 from the bare icefield around the Yamato mountains in Antarctica, consisting of three independent specimens with black fusion crusts. They are paired cumulate clinopyroxenites. We obtained the intercumulus melt composition of the Yamato nakhlites and here call it the Yamato intercumulus melt (YIM). The YIM crystallized to form the augite rims, the olivine rims and the mesostasis phases in the cumulates. The augite rims consist of two layers: inner and outer. The crystallization of the inner rim drove the interstitial melt into the plagioclase liquidus field. Subsequently, the residual melt crystallized pigeonites and plagioclase to form the outer rims and the mesostasis.Three types of inclusions were identified in olivine phenocrysts: rounded vitrophyric, angular vitrophyric, and monomineralic augite inclusions. The monomineralic augite inclusions are common and may have been captured by growing olivine phenocrysts. The rounded vitrophyric inclusions are rare and may represent the composition of middle-stage melts, whereas the angular vitrophyric inclusions seem to have been derived from fractionated late-stage melts. Glass inclusions occur in close association with titanomagnetite and ferroan augite halo in phenocryst core augites and the assemblages may be magmatic inclusions in augites. We compared the YIM with compositions of magmatic inclusions in olivine and augite. The composition of magmatic inclusions in augite is similar to the YIM.Phenocrystic olivines contain exsolution lamellae, augite-magnetite aggregates, and symplectites in the cores. The symplectites often occur at the boundaries between olivine and augite grains. The aggregates, symplectite and lamellae formed by exsolution from the host olivine at magmatic temperatures.We present a formational scenario for nakhlites as follows: (1) accumulation of augite, olivine, and titanomagnetite phenocrysts took place on the floor of a magma chamber; (2) olivine exsolved augite and magnetite as augite-magnetite aggregates, symplectites and lamellae; (3) the overgrowth on olivine phenocrysts formed their rims, and the inner rims crystallized on augite phenocryst cores; and finally, (4) the outer rim formed surrounding the inner rims of augite phenocrysts, and plagioclase and minor minerals crystallized to form mesostasis under a rapid cooling condition, probably in a lava flow or a sill.
    • Radar observations of Itokawa in 2004 and improved shape estimation

      Ostro, Steven J.; Benner, Lance A. M.; Magri, Christopher; Giorgini, Jon D.; Rose, Randy; Jurgens, Raymond F.; Yeomans, Donald K.; Hine, Alice A.; Nolan, Michael C.; Scheeres, Daniel J.; et al. (The Meteoritical Society, 2005-01-01)
      We present June 2004 radar images of asteroid 25143 Itokawa (1998 SF36) that improve upon the longitude-latitude coverage of images obtained in 2001 by Ostro et al. (2004) and use the 2001-2004 data to refine that papers constraints on Itokawas shape. The 2004 images, the first of the asteroids southern side, look distinctly different from the 2001 images, revealing leading edges that are much more curved and rugged than the nearly convex leading edges seen at northern latitudes in 2001. Itokawa is shaped like a slightly asymmetrical, bent, lumpy ellipsoid with dimensions along the principal axes within 10% of 594 x 320 x 288 m. To illustrate the uncertainty space associated with shape reconstruction from images with suboptimal orientational coverage, we present two alternative three-dimensional models of the object.
    • Rapid extraction of dust impact tracks from silica aerogel by ultrasonic microblades

      Ishii, H. A.; Graham, G. A.; Kearsley, A. T.; Grant, P. G.; Snead, C. J.; Bradley, J. P. (The Meteoritical Society, 2005-01-01)
      In January 2006, NASA's Stardust mission will return with its valuable cargo of the first cometary dust particles captured at hypervelocity speeds in silica aerogel collectors and brought back to Earth. Aerogel, a proven capture medium, is also a candidate for future sample return missions and low-Earth orbit (LEO) deployments. Critical to the science return of Stardust as well as future missions that will use aerogel is the ability to efficiently extract impacted particles from collector tiles. Researchers will be eager to obtain Stardust samples as quickly as possible; tools for the rapid extraction of particle impact tracks that require little construction, training, or investment would be an attractive asset. To this end, we have experimented with diamond and steel microblades. Applying ultrasonic frequency oscillations to these microblades via a piezo-driven holder produces rapid, clean cuts in the aerogel with minimal damage to the surrounding collector tile. With this approach, intact impact tracks and associated particles in aerogel fragments with low-roughness cut surfaces have been extracted from aerogel tiles flown on NASA's Orbital Debris Collector (ODC) experiment. The smooth surfaces produced during cutting reduce imaging artifacts during analysis by scanning electron microscopy (SEM). Some tracks have been dissected to expose the main cavity for eventual isolation of individual impact debris particles and further analysis using techniques such as transmission electron microscopy (TEM) and nano-secondary ion mass spectrometry (nanoSIMS).
    • Spallation recoil II: Xenon evidence for young SiC grains

      Ott, U.; Altmaier, M.; Herpers, U.; Kuhnhenn, J.; Merchel, S.; Michel, R.; Mohapatra, R. K. (The Meteoritical Society, 2005-01-01)
      We have determined the recoil range of spallation xenon produced by irradiation of Ba glass targets with ~1190 and ~268 MeV protons, using a catcher technique, where spallation products are measured in target and catcher foils. The inferred range for 126Xe produced in silicon carbide is ~0.19 micrometers, which implies retention of ~70% for 126Xe produced in "typical" presolar silicon carbide grains of 1 micrometer size. Recoil loss of spallation xenon poses a significantly smaller problem than loss of the spallation neon from SiC grains. Ranges differ for the various Xe isotopes and scale approximately linearly as function of the mass difference between the target element, Ba, and the product. As a consequence, SiC grains of various sizes will have differences in spallation Xe composition. In an additional experiment at ~66 MeV, where the recoil ranges of 22Na and 127Xe produced on Ba glass were determined using gamma-spectrometry, we found no evidence for recoil ranges being systematically different at this lower energy. We have used the new data to put constraints on the possible presolar age of the SiC grains analyzed for Xe by Lewis et al. (1994). Uncertainties in the composition of the approximately normal Xe component in SiC (Xe-N) constitute the most serious problem in determining an age, surpassing remaining uncertainties in Xe retention and production rate. A possible interpretation is that spallation contributions are negligible and that trapped 124Xe/126Xe is ~5% lower in Xe-N than in Q-Xe. But also for other reasonable assumptions for the 124Xe/126Xe ratio in Xe-N (e.g., as in Q-Xe), inferred exposure ages are considerably shorter than theoretically expected lifetimes for interstellar grains. A short presolar age is in line with observations by others (appearance, grain size distribution) that indicate little processing in the interstellar medium (ISM) of surviving (crystalline) SiC. This may be due to amorphization of SiC in the ISM on a much shorter time scale than destruction, with amorphous SiC not surviving processing in the early solar system. A large supply of relatively young grains may be connected to the proposed starburst origin (Clayton 2003) for the parent stars of the mainstream SiC grains.
    • Structural characteristics of the Sudbury impact structure, Canada: Impact-induced versus orogenic deformation—A review

      Riller, U. (The Meteoritical Society, 2005-01-01)
      Orogenic deformation, both preceding and following the impact event at Sudbury, strongly hinders a straightforward assessment of impact-induced geological processes that generated the Sudbury impact structure. Central to understanding these processes is the state of strain of the Sudbury Igneous Complex, the solidified impact melt sheet, its underlying target rocks, overlying impact breccias and post-impact sedimentary rocks. This review addresses (1) major structural, metamorphic and magmatic characteristics of the impact melt sheet and associated dikes, (2) attempts that have been made to constrain the primary geometry of the igneous complex, (3) modes of impact-induced deformation as well as (4) mechanisms of pre- and post-impact orogenic deformation. The latter have important consequences for estimating parameters such as magnitude of structural uplift, tilting of pre-impact (Huronian) strata and displacement on major discontinuities which, collectively, have not yet been considered in impact models. In this regard, a mechanism for the emplacement of Offset Dikes is suggested, that accounts for the geometry of the dikes and magmatic characteristics, as well as the occurrence of sulfides in the dikes. Moreover, re-interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact. Uncertainties still exist as to whether the Sudbury impact structure was a peak-ring or a multi-ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification.