• The 410,000 year terrestrial age of eucrite Rio Cuarto 001

      Levine, Jonathan; Arazi, Andrés; Faestermann, Thomas; Fernández Niello, Jorge O.; Korschinek, Gunther; La Gamma, Ana María; Negri, Agustín; Rugel, Georg; Steier, Peter; Wallner, Anton (The Meteoritical Society, 2008-01-01)
      We have measured a surprisingly long terrestrial age of 410,000 +20,000/-45,000 years (410 +20/-45 ka) for basaltic eucrite Ro Cuarto 001 using accelerator mass spectrometry of 26Al, 36Cl, and 41Ca. Though many meteorites are known to have survived for tens or hundreds of ka in Antarctica or hot deserts, the mean annual precipitation of 815 mm in Ro Cuarto, Cordoba Province, Argentina, makes the long survival of this meteorite remarkable. We propose two explanations for the exceptional preservation of Ro Cuarto 001. First, the meteorite contains only trace amounts of metal, so the weathering and oxidation of metallic Fe, which commonly destroys chondrites, is ineffective in this case. Second, the meteorite was found in a relatively young deflation basin, and may have been exhumed only recently from beneath a protective layer of soil. Insofar as the survival on Earth of Ro Cuarto 001 is due to environmental factors, there may be other meteorites with comparably long terrestrial ages still to be discovered in the vicinity.
    • Radial transport in the solar nebula: Implications for moderately volatile element depletions in chondritic meteorites

      Ciesla, Fred J. (The Meteoritical Society, 2008-01-01)
      In this paper, we explore the possibility that the moderately volatile element depletions observed in chondritic meteorites are the result of planetesimals accreting in a solar nebula that cooled from an initially hot state (temperatures >1350 K out to ~2-4 AU). A model is developed to track the chemical inventory of planetesimals that accrete in a viscously evolving protoplanetary disk, accounting for the redistribution of solids and vapor by advection, diffusion, and gas drag. It is found that depletion trends similar to those observed in the chondritic meteorites can be reproduced for a small range of model parameters. However, the necessary range of parameters is inconsistent with observations of disks around young stars and other constraints on meteorite parent body formation. Thus, counter to previous work, it is concluded that the global scale evolution of the solar nebula is not the cause for the observed depletion trends. Instead, it appears that localized processing must be considered.
    • Evolution of the winonaite parent body: Clues from silicate mineral trace element distributions

      Floss, C.; Crozaz, G.; Jolliff, B.; Benedix, G.; Colton, S. (The Meteoritical Society, 2008-01-01)
      We have measured the trace element compositions of individual plagioclase, pyroxene, and olivine grains in 6 different winonaites that span the range of textures and mineralogies observed in these meteorites. Textural evidence in these meteorites, including the presence of a plagioclase/ clinopyroxene-rich lithology and coarse-grained olivine lithologies, suggests that they may have experienced some silicate partial melting. However, trace element distributions in these lithologies do not show any clear signatures for such an event. Pyroxene trace element compositions do exhibit systematic trends, with abundances generally lowest in Pontlyfni and highest in Winona. The fact that the same trends are present for both incompatible and compatible trace elements suggests, however, that the systematics are more likely the result of equilibration of minerals with initially heterogeneous and distinct compositions, rather than partial melting of a compositionally homogeneous precursor. The winonaites have experienced brecciation and mixing of lithologies, followed by varying degrees of thermal metamorphism on their parent body. These factors probably account for the variable bulk rare earth element (REE) patterns noted for these meteorites and may have led to re-equilibration of trace elements in different lithologies.
    • PDF orientations in shocked quartz grains around the Chicxulub crater

      Nakano, Yoichiro; Goto, Kazuhisa; Matsui, Takafumi; Tada, Ryuji; Tajika, Eiichi (The Meteoritical Society, 2008-01-01)
      We measured 852 sets of planar deformation features (PDFs) in shocked quartz grains in impactite samples of the Yaxcopoil (YAX-1) core and from 4 Cretaceous/Tertiary (K/T) boundary deposits: the Monaca, the Cacarajcara, and the Pealver formations in Cuba, and DSDP site 536, within 800 km of the Chicxulub crater, in order to investigate variations of PDF orientations in the proximity of the crater. Orientations of PDFs show a broad distribution with peaks at omega {1013}, pi {1012}, and xi {1112}, plus r, z {1011} orientations with minor c(0001), s{1121}, t{2241} plus x{5161}, and m{1010} plus a{11-20} orientations. Planar deformation features with c(0001) orientation are relatively more abundant in the proximity of the Chicxulub crater than in distal sitessuch as North America, the Pacific Ocean, and Europe. This feature indicates that in the proximity ofthe crater, part of the shocked quartz grains in the K/T boundary deposits were derived from the lowshock pressure zones. Moreover, the orientations of PDFs with xi {1122} plus r, z {1011} are high in our studied sites, and frequencies of these orientations decrease with increasing distance from the crater. On the other hand, absence of c(0001) and the rare occurrence of PDFs with xi {1122} plus r, z {1011} orientations in the sample from the YAX-1 core that was taken at the top of the impactitelayer of the Chicxulub crater suggests that the sampling horizon that reflects a certain cratering stageis also an important factor for variations in shocked quartz.
    • Structural evolution of the 40 km wide Araguainha impact structure, central Brazil

      Lana, C.; Souza Filho, C. R.; Marangoni, Y. R.; Yokoyama, E.; Trindade, R. I. F.; Tohver, E.; Reimold, W. U. (The Meteoritical Society, 2008-01-01)
      The 40 km wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward-verging inclined folds and a km-scale anticline in the outer ring of the structure. The folding stage was followed by radial and concentric faulting, with downward displacement of kilometer-scale blocks around the crater rim. The central uplift records evidence for km-scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached from the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km-scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward-plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets. Each individual sheet records an early stage of folding and thickening due to non-coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer-scale inward movement of the target strata from the transient cavity walls to the central uplift. The outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 1012 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks.
    • Noble gases in Grant and Carbo and the influence of S- and P-rich mineral inclusions on the 41K-40K dating system

      Ammon, Katja; Masarik, Jozef; Leya, Ingo (The Meteoritical Society, 2008-01-01)
      Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re-determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur- and/or phosphorus-rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (~10 cm) from earlier results (~165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36Cl data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21Ne was produced from these minor elements. Using measured 21Ne concentrations and endmember 22Ne/21Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21Ne produced from S and/or P. The thus corrected data are then used to calculate new 41K-40K exposure ages--using published K data--which results in 564 +/- 78 Ma for Grant and 725 +/- 100 Ma for Carbo. The correction always lowers the 21Ne concentrations and consequently decreases the 41K-40K exposure ages. The discrepancies between 36Cl-36Ar and 41K-40K ages are accordingly reduced. The existence of a significant long-term variation of the GCR, which is based on a former 30-50% difference between 41K-40K and 36Cl-36Ar ages, may warrant re-investigation.
    • Ararki (L5) chondrite: The first meteorite find in Thar Desert of India

      Bhandari, N.; Murty, S. V. S.; Shukla, P. N.; Mahajan, R. R.; Shukla, A. D.; Lashkari, G.; Sisodia, M. S.; Tripathi, R. P.; Parthasarathy, G.; Verma, H. C.; et al. (The Meteoritical Society, 2008-01-01)
      We report here a chance find of a meteorite in the sand dunes of Ararki village of Hanumangarh district in the Rajasthan desert of northwest India. Chemical and petrological evidence in conjunction with isotopic composition of oxygen indicate that it is an L5 chondrite. The fayalite content of olivines is 26.3 mol%. The meteorite has some serpentinized olivines and 0.3% carbon having a terrestrial isotopic composition, indicating that it is moderately weathered. The absence of 22Na indicate that the meteorite fell to Earth more than a decade ago. The cosmic-ray exposure age based on cosmogenic 21Ne is 7.2 Ma. Low density of cosmic-ray heavy nuclei tracks, low 26Al activity, the shielding parameter [(22Ne/21Ne)C = 1.094] and absence of neutron capture effects indicate cosmic-ray shielding in a meteoroid having radius of about 16 cm, implying a meteoroid mass of about 60 kg and ablation of about 93%. The gas retention ages, based on U/Th-4He and K-40Ar are 1.1 and 0.58 Ga, respectively, suggesting a heating and degassing event late in the history of this meteorite.
    • Experimental impacts into chondritic targets, part I: Disruption of an L6 chondrite by multiple impacts

      Cintala, Mark J.; Hörz, Friedrich (The Meteoritical Society, 2008-01-01)
      A fragment of an L6 chondrite (Allan Hills [ALH] 85017,13) with an initial mass (M0) of 464.1 g was the target in a series of experimental impacts in which the largest remaining fragment (MR) after each shot was impacted by a 3.18 mm ceramic sphere at a nominal speed of 2 km s^(-1). This continued until the mass of the largest remaining piece was less than half the mass of the target presented to that shot (MS). Two chunks of Bushveldt gabbro with similar initial masses were also impacted under the same conditions until MR was less than half M0. The two gabbro targets required a total of 1.51 x 1^07 and 1.75 x 10^7 erg g^(-1) to attain 0.27 and 0.33 MR/M0, respectively; the chondrite, however, was considerably tougher, reaching 0.40 and 0.21 MR/M0 only after receiving 2.37 x 10^7 and 3.10 x 10^7 erg g^(-1), respectively. The combined ejecta and spallation products from the gabbro impacts were coarser than those from the chondrite and in sufficient quantities that the new surface areas exceeded those from the meteorite until the fifth shot in the chondrite series, which was the number of impacts required to disrupt each gabbro target (i.e., MR/M0 is less than or equal to 0.5). Unlike the behavior shown in previous regolith-evolution series, neither gabbro target produced an enhancement in the size fraction reflecting the mean size of the crystals composing the rock (about 3 mm), an effect possibly related to the width of the shock pulse. The original chondrite was so fine-grained and fractured, and the variance in its grain-size distribution so large, that effects related to grain size were relegated to the <63 micrometers fraction. Impacts into ALH 85017 produced abundant, fine-grained debris, but otherwise the slopes of its size distributions were comparable to those from other experiments involving natural and fabricated terrestrial targets. The characteristic slopes of the chondrites size distributions, however, were notably more constant over the entire nine-impact series than those from any of the terrestrial targets, a testament to the control over comminution apparently exerted by pre-existing fractures and other, microscopic damage in the meteorite. The enhancement in the finer fraction of debris from ALH 85017 indicates that ordinary chondrites in solar orbit would be very efficient contributors to the cosmic-dust complex. At the same time, the greater resistance to disruption displayed by ordinary chondrites relative to that exhibited by igneous rocks indicates that a selection effect could be operative between the annealed, ordinary-chondritic breccias and relatively weaker, differentiated meteorites. Preferential survival from their time in the regoliths of their parent bodies through their transit to Earth and passage through the atmosphere suggests that meteorite collections could be biased in favor of the ordinary chondrites.
    • 182Hf-182W chronometry and the early evolution history in the acapulcoite-lodranite parent body

      Lee, D.-C. (The Meteoritical Society, 2008-01-01)
      An acapulcoite, Northwest Africa (NWA) 725, a transitional acapulcoite, Graves Nunataks (GRA) 95209, and a lodranite, NWA 2235, have been studied with the short-lived chronometer 182Hf-182W system in order to better constrain the early evolution history in the acapulcoite-lodranite parent body. Unlike the more evolved achondrites originating from differentiated asteroids--e.g., eucrites and angritesbulk rock acapulcoites and lodranite are characterized by distinct 182W deficits relative to the terrestrial W, as well as to the undifferentiated chondrites, epsilon-w varies from -2.7 to -2.4. This suggests that live-182Hf was present during the formation of acapulcoites and lodranites, and their parent body probably had never experienced a global melting event. Due to the large uncertainties associated with the isochron for each sample, the bulk isochron that regressed through the mineral separates from all 3 samples has provided the best estimate to date for the timing of metamorphism in the acapulcoite-lodranite parent body, 5 (+6/-5) Myr after the onset of the solar system. It is thus inconclusive whether acapulcoites and lodranites have shared the same petrogenetic origin, based on the Hf-W data of this study. Nevertheless, the formation of acapulcoite-lodranite clan appears to have post-dated the metal-silicate segregation in differentiated asteroids. This can be explained by a slower accretion rate for the acapulcoite-lodranite parent body, or that it had never accreted to a critical mass that could allow the metal-silicate segregation to occur naturally.
    • Frontier Mountain meteorite specimens of the acapulcoite-lodranite clan: Petrography, pairing, and parent-rock lithology of an unusual intrusive rock

      Burroni, Alessandro; Folco, Luigi (The Meteoritical Society, 2008-01-01)
      In this paper we reconstruct the heterogeneous lithology of an unusual intrusive rock from the acapulcoite-lodranite (AL) parent asteroid on the basis of the petrographic analysis of 5 small (<8.3 g) meteorite specimens from the Frontier Mountain ice field (Antarctica). Although these individual specimens may not be representative of the parent-rock lithology due to their relatively large grain size, by putting together evidence from various thin sections and literature data we conclude that Frontier Mountain (FRO) 90011, FRO 93001, FRO 99030, and FRO 03001 are paired fragments of a medium- to coarse-grained igneous rock which intrudes a lodranite and entrains xenoliths. The igneous matrix is composed of enstatite (Fs13.3 0.4 Wo3.1 0.2), Cr-rich augite (Fs6.1 0.7 Wo42.3 0.9), and oligoclase (Ab80.5 3.3 Or3.2 0.6). The lodranitic xenoliths show a fine-grained (average grain size 488 201 m) granoblastic texture and consist of olivine Fa9.5 0.4 and Fe,Ni metal and minor amounts of enstatite Fs12.7 0.4 Wo1.8 0.1, troilite, chromite, schreibersite, and Ca-phosphates. Crystals of the igneous matrix and lodranitic xenoliths are devoid of shock features down to the scanning electron microscope scale. From a petrogenetic point of view, the lack of shock evidence in the lodranitic xenoliths of all the studied samples favors the magmatic rather than the impact melting origin of this rock. FRO 95029 is an acapulcoite and represents a separate fall from the AL parent asteroid, i.e., it is not a different clast entrained by the FRO 90011, FRO 93001, FRO 99030, and FRO 03001 melt, as in genomict breccias common in the meteoritic record. The specimen-to-meteorite ratio for the AL meteorites so far found at Frontier Mountain is thus 2.5.
    • Kinetic stability of a melted iron globule during chondrule formation. I. Non-rotating model

      Uesugi, Masayuki; Sekiya, Minoru; Nakamura, Tomoki (The Meteoritical Society, 2008-01-01)
      We have investigated the kinematics of the separation of iron globules from chondrules during chondrule formation. A simple model, which assumes that the system has no angular momentum, was used to calculate the energy of a system with an iron globule and a chondrule. The energies of three different states were calculated: 1) a melted iron globule fully embedded in a melted chondrule, 2) a melted iron globule on the surface of a melted chondrule, and 3) a melted iron globule being separated from a melted chondrule. We also calculated the lowest energy shape for a melted iron globule on the surface of a melted chondrule, and compared our result with the shapes of four natural samples of chondrules and iron globules in thin sections. The shapes were calculated using an assumed value for the interface energy between the four couples of melted chondrules and the iron globules, and agree well with the natural shapes of chondrules and iron globules. The results of our calculations show that the iron globules of these four samples would be strongly bound to the surface of the melted chondrule during chondrule formation, and separation would be difficult, if the iron globules had been on the surface of precursors of these chondrules. Our results also show that if these iron globules were initially inside and transported to the surface of the melted chondrule, most of them would be ejected from the inside to outside because of surface tension forces, as long as the energy losses due to viscous dissipation when the globules pass through the surface of melted chondrules were sufficiently small. Although further improvement of the model is required, our results demonstrate that this ejection process may be responsible for the depletion of siderophile elements in natural chondrules.
    • Comment on: Constraints on the depth and variability of the lunar regolith, by B. B. Wilcox, M. S. Robinson, P. C. Thomas, and B. R. Hawke

      Oberbeck, Verne R. (The Meteoritical Society, 2008-01-01)
      Any permanent presence on the Moon will require use of materials from the lunar regolith, the surface soil layer on the Moon. Thus, knowledge of the thickness of the lunar regolith is essential. It has been proposed that crater counts obtained from high Sun angle photography give larger estimates of impact crater equilibrium diameters than for low Sun angle photography, and thus deeper estimates of lunar surface regolith than were previously made using crater morphology, size of blocky rimmed craters, and equilibrium diameters determined on low Sun angle images. The purpose of this comment is to evaluate this result as a means of resolving this important question before planning for future lunar missions is undertaken.