• Radar observations of asteroid 1999 JM8

      Benner, Lance A. M.; Ostro, Steven J.; Nolan, Michael C.; Margot, Jean-Luc; Giorgini, Jon D.; Hudson, R. Scott; Jurgens, Raymond F.; Slade, Martin A.; Howell, Ellen S.; Campbell, Donald B.; et al. (The Meteoritical Society, 2002-01-01)
      We report results of delay-Doppler observations of 1999 JM8 with the Goldstone 8560 MHz (3.5 cm) and Arecibo 2380 MHz (13 cm) radars over 18 days in July-August 1999. The images place thousands of pixels on the asteroid and achieve range resolutions as fine as 15 m/pixel. The images reveal an asymmetric, irregularly shaped object with a typical overall dimension within 20% of 7 km. If we assume that 1999 JM8's effective diameter is 7 km, then the absolute magnitude, 15.15, and the average Goldstone radar cross section, 2.49 km^2, correspond to optical and radar albedos of 0.02 and 0.06, establishing that 1999 JM8 is a dark object at optical and radar wavelengths. The asteroid is in a non-principal axis spin state that, although not yet well determined, has a dominant periodicity of ~7 days. However, images obtained between July 31 and August 9 show apparent regular rotation of features from day to day, suggesting that the rotation state is not far from principal axis rotation. 1999 JM8 has regions of pronounced topographic relief, prominent facets several kilometers in extent, numerous crater-like features between ~100 m and 1.5 km in diameter, and features whose structural nature is peculiar. Arecibo images provide the strongest evidence to date for a circular polarization ratio feature on any asteroid. Combined optical and radar observations from April 1990 to December 2000 permit computation of planetary close approach times to within 10 days over the interval from 293 to at least 2907, one of the longest spans for any potentially hazardous asteroid. Integration of the orbit into the past and future shows close approaches to Earth, Mars, Ceres, and Vesta, but the probability of the object impacting Earth is zero for at least the next nine centuries.
    • Radar observations of asteroid 25143 Itokawa (1998 SF36)

      Ostro, S. J.; Benner, L. A. M.; Nolan, M. C.; Magri, C.; Giorgini, J. D.; Scheeres, D. J.; Broschart, S. B.; Kaasalainen, M.; Vokrouhlicky, D.; Chesley, S. R.; et al. (The Meteoritical Society, 2004-01-01)
      We observed 25143 Itokawa, the target of Japans Hayabusa (MUSES-C) sample-return mission, during its 2001 close approach at Arecibo on twelve dates during March 18-April 9 and at Goldstone on nine dates during March 20-April 2. We obtained delay-Doppler images with range resolutions of 100 ns (15 m) at Arecibo and 125 ns (19 m) at Goldstone. Itokawas average circular polarization ratio at 13 cm, 0.26 +/- 0.04, is comparable to that of Eros, so its cm-to-m surface roughness probably is comparable to that on Eros. Itokawas radar reflectivity and polarization properties indicate a near-surface bulk density within 20% of 2.5 g cm^(-3). We present a preliminary estimate of Itokawas shape, reconstructed from images with rather limited rotation-phase coverage, using the method of Hudson (1993) and assuming the lightcurve-derived spin period (12 +/-.132 hr) and pole direction (ecliptic long., lat. = 355 degrees, -84 degrees) of Kaasalainen et al. (2003). The model can be described as a slightly asymmetrical, slightly flattened ellipsoid with extents along its principal axes of 548 x 312 x 276 m +/- 10%. Itokawas topography is very subdued compared to that of other asteroids for which spacecraft images or radar reconstructions are available. Similarly, gravitational slopes on our Itokawa model average only 9 degrees and everywhere are less than 27 degrees. The radar-refined orbit allows accurate identification of Itokawas close planetary approaches through 2170. If radar ranging planned for Itokawas 2004 apparition succeeds, then tracking of Hayabusa during its 2005 rendezvous should reveal Yarkovsky perturbation of the asteroids orbit.
    • 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.
    • 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.
    • Radiocarbon on Titan

      Lorenz, Ralph D.; Jull, A. J. Timothy; Swindle, Timothy D.; Lunine, Jonathan I. (The Meteoritical Society, 2002-01-01)
      We explore the likely production and fate of 14C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of 14C might place on Titan's photochemical, atmospheric transport and surface-atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic-ray flux limits the production of 14C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ~9 atom/cm^2/s, ~4x higher than Earth. The fate and detectability of 14C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the 14C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are "freshest".
    • Radiogenic isotope investigation of the St-Robert H5 fall

      Poirier, A.; Doucelance, R.; Gariépy, C. (The Meteoritical Society, 2004-01-01)
      The St-Robert H5 chondrite yields a mineral/whole-rock Pb-Pb age of 4565 +/- 23 Ma (2-sigma) comparable to the accepted age of most chondrites. The regression of chondrule data give a similar age of 4566 +/- 7 Ma (2-sigma). These results imply that no major perturbation affected the Pb-Pb systematics of this meteorites parent body within the first few billion years following its accretion. Re and Os concentrations along with Os isotopic compositions of whole-rock fragments, surface fusion crusts and metal phases are also reported. The whole rock measurements for this ordinary chondrite are characterized by high Re/Os ratio coupled with relatively high 187Os/188Os (compared to average ordinary chondrites), that we interpret as a long term Re enrichment. As for most chondrites, no precise geochronological information could be extracted from the Re/Os systematics, although most data plot near the IIIAB reference isochron (Smoliar et al. 1996). From the fusion crust results, we rule out the possibility that atmospheric entry caused the perturbations in the Re-Os system, since melted crust analysis yields among the most concordant data points. Evidence from metal phases suggests that a very recent process perturbed the isochron, relocating Re from kamacite toward troilite.
    • Raman spectroscopy of olivine in dunite experimentally shocked to pressures between 5 and 59 GPa

      Farrell-Turner, S.; Reimold, W. U.; Nieuwoudt, M.; Erasmus, R. M. (The Meteoritical Society, 2005-01-01)
      Previous Raman investigations on experimentally shocked single-crystal olivine indicated that the olivine Raman bands seemingly shift to a higher wave number with increasing shock pressure. If this effect could be confirmed, Raman analysis of natural shock-metamorphosed minerals could potentially provide an important shock barometric tool. We carried out a Raman spectroscopic study on olivine in a series of natural dunite samples experimentally shocked to pressures between 5 and 59 GPa. In addition, we analyzed olivine grains in a sample of the Cold Bokkeveld C1 meteorite. We studied samples of several dunites with olivine of 90.64-92.00 mole% Fo to determine Raman effects in the region from 200 to 900 cm(-1). Several olivine grains per sample/shock pressure stage were analyzed. Raman analysis, however, showed little or no shift with increasing shock pressure. The shifts to higher or lower frequencies observed were not specific for a given pressure stage, with some grains within a sample showing more shift than others. This finding is unrelated to the crystallographic orientation of analyzed grains and cannot be related systematically to the different degrees of optically determined shock metamorphism of the analyzed grains. We identified an increase in full width at half maximum (FWHM) for the 824 cm^(-1) band with increased shock pressure in the shocked heim samples above 45 GPa and, to a lesser extent, for the 856 cm^(-1) band. Evaluation of band broadening of olivine in the Cold Bokkeveld meteorite showed FWHM values that were much greater (920 cm^(-1)) than those of olivine in the shocked dunite samples (712 cm^(-1)). We concluded that these differences in FWHM are due to differences in chemical composition between the meteoritic and the experimentally shocked olivine. Therefore, using Raman spectroscopy to detect small shifts in wave numbers to higher frequencies with increased shock pressure does not yield consistent effects for polycrystalline dunite. An extra band at 650 cm^(-1) was identified in the Raman spectra of the unshocked Mooihoek dunite and the heim dunite samples shocked to 5, 29.3, and 59 GPa, as well as another at 696 cm^(-1) in all the spectra of the 59 GPa heim sample. The cause of these extra bands is not known. Comparison of these results with Raman spectra of olivine from the Cold Bokkeveld C1 meteorite did not allow us to determine shock pressures for the meteoritic olivine.
    • Rapid contamination during storage of carbonaceous chondrites prepared for micro FTIR measurements

      Kebukawa, Y.; Nakashima, S.; Otsuka, T.; Nakamura-Messenger, K.; Zolensky, M. E. (The Meteoritical Society, 2009-01-01)
      Organic contamination (~2965 and ~1260 cm^(-1) peaks) was found on Tagish Lake (C2) and Murchison (CM2) carbonaceous chondrites containing abundant hydrous minerals by Fourier transform infrared (FTIR) microspectroscopy on the samples pressed on Al plates. On the other hand, anhydrous chondrite (Moss, CO3) was not contaminated. This contamination occurred within one day of storage, when the samples pressed on Al were stored within containers including silicone rubber mats. Volatile molecules having similar peaks to the contaminants were detected by long-path gas cell FTIR measurements for the silicone rubber mat. Rapid adsorption of the volatile contaminants also occurred when silica gel and hydrous minerals such as serpentine were stored in containers including silicone rubber, silicone grease, or adhesive tape. However, they did not show any contamination when stored in glass and polystyrene containers without these compounds. Therefore, precious astronomical samples such as meteorites, interplanetary dust particles (IDPs), and mission-returned samples from comets, asteroids, and Mars, should be measured by micro FTIR within one day of storage in glass containers without silicone rubber, silicone grease, or adhesive tape.
    • 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).
    • Re-evaluating the age of the Haughton impact event

      Sherlock, Sarah C.; Kelley, Simon P.; Parnell, John; Green, Paul; Lee, Pascal; Osinski, Gordon R.; Cockell, Charles S. (The Meteoritical Society, 2005-01-01)
      We have re-evaluated the published age information for the Haughton impact structure, which was believed to have formed ~23 Ma ago during the Miocene age, and report new Ar/Ar laser probe data from shocked basement clasts. This reveals an Eocene age, which is at odds with the published Miocene stratigraphic, apatite fission track and Ar/Ar data; we discuss our new data within this context. We have found that the age of the Haughton impact structure is ~39 Ma, which has implications for both crater recolonization models and post-impact hydrothermal activity. Future work on the relationship between flora and fauna within the crater, and others at high latitude, may resolve this paradox.
    • Re-examining the role of chondrules in producing the elemental fractionations in chondrites

      Alexander, C. M. O'D. (The Meteoritical Society, 2005-01-01)
      The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI-like component. The matrix-normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI-like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe-metal is added to the chondrule composition.Measured matrix compositions are not CI-like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ∼10–30% of refractory-rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI-dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could haveformed from a mixture of CI material and material evaporated from type IA chondrules. The Mg-Si-Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.
    • Recipes for making synthetic CAIs, refractory residues, and minerals for rim-forming experiments

      Wark, David (The Meteoritical Society, 2005-01-01)
      Recipes are presented for synthesizing various type A and type B Ca-Al-rich inclusions (CAIs), refractory volatilization residues, and the minerals forsterite and melilite that are required for experiments. These experiments (described in other works) aim to make two determinations: 1) the conditions under which the surfaces of CAIs were either "flash-heated" or "volatilized subsolidus" to form a temporary ultra-refractory residue, and 2) the conditions under which the residue was then metasomatized to form the mineral layers making up Wark-Lovering (WL) rims on CAIs.
    • Reclassification and thermal history of Trenzano chondrite

      Fioretti, A. M.; Domeneghetti, M. C.; Molin, G.; Cámara, F.; Alvaro, M.; Agostini, L. (The Meteoritical Society, 2007-01-01)
      We present a new single-crystal X-ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low-shocked H6 Trenzano meteorite. The quenched intracrystalline Fe^(2+)-Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature Tc, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene Tc of 522 +/- 13 degrees C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this Tc is about 100 degrees C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two-pyroxene equilibrium temperature of Trenzano (824 +- 24 degrees C), calculated with QUILF95, is similar to the two-pyroxene temperature of 750-840 degrees C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).
    • Recovering the elemental composition of comet Wild 2 dust in five Stardust impact tracks and terminal particles in aerogel

      Ishii, H. A.; Brennan, S.; Bradley, J. P.; Luening, K.; Ignatyev, K.; Pianetta, P. (The Meteoritical Society, 2008-01-01)
      The elemental (non-volatile) composition of five Stardust impact tracks and terminal particles left from capture of comet 81P/Wild 2 dust were mapped in a synchrotron X-ray scanning microprobe with full fluorescence spectra at each pixel. Because aerogel includes background levels of several elements of interest, we employ a novel "dual threshold" approach to discriminate against background contaminants: an upper threshold, above which a spectrum contains cometary material plus aerogel and a lower threshold below which it contains only aerogel. The difference between normalized cometary-plus-background and background-only spectra is attributable to cometary material. The few spectra in-between are discarded since misallocation is detrimental: cometary material incorrectly placed in the background spectrum is later subtracted from the cometary spectrum, doubling the loss of reportable cometary material. This approach improves accuracy of composition quantification. We present the refined whole impact track and terminal particle elemental abundances for the five impact tracks. One track shows mass increases in Cr and Mn (1.4x), Cu, As and K (2x), Zn (4x), and total mass (13%) by dual thresholds compared to a single threshold. Major elements Fe and Ni are not significantly affected. The additional Cr arises from cometary material containing little Fe. We exclude Au intermixed with cometary material because it is found to be a localized surface contaminant carried by comet dust into an impact track. The dual threshold technique can be used in other situations where elements of interest in a small sample embedded in a matrix are also present in the matrix itself.
    • Redistribution of elements in the heavily shocked Yanzhuang chondrite

      Kong, P.; Xie, X. (The Meteoritical Society, 2003-01-01)
      Compositions of metal, sulfide, olivine, pyroxene, and plagioclase/plagioclase glass were studied for the melted and unmelted parts of the heavily shocked H6(S6) chondrite-Yanzhuang. We found that the partitioning of some trace elements significantly changed between the 2 parts; compared with the corresponding minerals in the unmelted part, Ga is enriched in the metal, Co, Cr, and Zn are enriched in the sulfide, Cr is enriched in olivine and pyroxene, and Ti is enriched in the plagioclase glass of the melt pocket. These detailed studies of the mineral phases put constraints on 3 important parameters (temperature, pressure, and duration) associated with the post-shock melting process. The coexistence of melted and unmelted olivine in the melt pocket of Yanzhuang implies a peak temperature after shock that approaches the melting point of olivine. The lack of Ni in the olivine crystallized from a melt suggests crystallization of olivine at pressures below 10 kbar. The resetting of Ga partitioning between metal and silicate in the melt pocket indicates that the interval from the peak temperature after shock to the crystallization of metal-sulfide and plagioclase glass in the melted part of Yanzhuang is longer than 500 sec.
    • REE abundances in the matrix of the Allende (CV) meteorite: Implications for matrix origin

      Inoue, M.; Kimura, M.; Nakamura, N. (The Meteoritical Society, 2004-01-01)
      The trace element distributions in the matrix of primitive chondrites were examined using four least-contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (~1 x CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1-0.5 x CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI-normalized La/Yb ratio = 1.32-1.65; Ce/Ce* = 1.16-1.28; Eu/Eu* = 0.98-1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium-aluminum-rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.
    • Reevaluating the impact cratering kill curve

      Kring, D. A. (The Meteoritical Society, 2002-01-01)
    • Refractory inclusions and aluminum-rich chondrules in Sayh al Uhaymir 290 CH chondrite: Petrography and mineralogy

      Zhang, A.; Hsu, W. (The Meteoritical Society, 2009-01-01)
      Here we report the petrography, mineralogy, and bulk compositions of Ca,Al-rich inclusions(CAIs), amoeboid olivine aggregate (AOA), and Al-rich chondrules (ARCs) in Sayh al Uhaymir (SaU) 290 CH chondrite. Eighty-two CAIs (0.1% of the section surface area) were found. They are hibonite-rich (9%), grossite-rich (18%), melilite +/- spinel-rich (48%), fassaite +/- spinel-rich (15%), and fassaite-anorthite-rich (10%) refractory inclusions. Most CAIs are rounded in shape and small in size (average = 40 micrometers). They are more refractory than those of other groups of chondrites. CAIs in SaU 290 might have experienced higher peak heating temperatures, which could be due to the formation region closer to the center of protoplanetary disk or have formed earlier than those of other groups of chondrites. In SaU 290, refractory inclusions with a layered texture could have formed by gas-solid condensation from the solar nebula and those with an igneous texture could have crystallized from melt droplets or experienced subsequent melting of pre-existing condensates from the solar nebula. One refractory inclusion represents an evaporation product of pre-existing refractory solid on the basis of its layered texture and melting temperature of constituting minerals. Only one AOA is observed (75 micrometers across). It consists of olivine, Al-diopside, anorthite, and minor spinel with a layered texture. CAIs and AOA show no significant low-temperature aqueous alteration. ARCs in SaU 290 consist of diopside, forsterite, anorthite, Al-enstatite, spinel, and mesostasis or glass. They can be divided into diopsiderich, Al-enstatite-rich, glass-rich, and anorthite-rich chondrules. Bulk compositions of most ARCs are consistent with a mixture origin of CAIs and ferromagnesian chondrules. Anorthite and Al-enstatite do not coexist in a given ARC, implying a kinetic effect on their formation.
    • Refractory inclusions in the CH/CB-like carbonaceous chondrite Isheyevo: I. Mineralogy and petrography

      Krot, A. N.; Ulyanov, A. A.; Ivanova, M. A. (The Meteoritical Society, 2008-01-01)
      The CH/CB-like chondrite Isheyevo consists of metal-rich (7090 vol% Fe,Ni-metal) and metal-poor (720 vol% Fe,Ni-metal) lithologies which differ in size and relative abundance of Fe,Nimetal and chondrules, as well as proportions of porphyritic versus non-porphyritic chondrules. Here, we describe the mineralogy and petrography of Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in these lithologies. Based on mineralogy, refractory inclusions can be divided into hibonite-rich (39%), grossite-rich (16%), melilite-rich (19%), spinel-rich (14%), pyroxeneanorthite- rich (8%), fine-grained spinel-rich CAIs (1%), and AOAs (4%). There are no systematic differences in the inclusion types or their relative abundances between the lithologies. About 55% of the Isheyevo CAIs are very refractory (hibonite-rich and grossite-rich) objects, 20-240 micrometers in size, which appear to have crystallized from rapidly cooling melts. These inclusions are texturally and mineralogically similar to the majority of CAIs in CH and CB chondrites. They are distinctly different from CAIs in other carbonaceous chondrite groups dominated by the spinel-pyroxene +/- melilite CAIs and AOAs. The remaining 45% of inclusions are less refractory objects (melilite-, spinel- and pyroxene-rich CAIs and AOAs), 40-300 micrometers in size, which are texturally and mineralogically similar to those in other chondrite groups. Both types of CAIs are found as relict objects inside porphyritic chondrules indicating recycling during chondrule formation. We infer that there are at least two populations of CAIs in Isheyevo which appear to have experienced different thermal histories. All of the Isheyevo CAIs apparently formed at an early stage, prior to chondrule formation and prior to a hypothesized planetary impact that produced magnesian cryptocrystalline and skeletal chondrules and metal grains in CB, and possibly CH chondrites. However, some of the CAIs appear to have undergone melting during chondrule formation and possibly during a major impact event. We suggest that Isheyevo, as well as CH and CB chondrites, consist of variable proportions of materials produced by different processes in different settings: 1) by evaporation, condensation, and melting of dust in the protoplanetary disk (porphyritic chondrules and refractory inclusions), 2) by melting, evaporation and condensation in an impact generated plume (magnesian cryptocrystalline and skeletal chondrules and metal grains; some igneous CAIs could have been melted during this event), and 3) by aqueous alteration of pre-existing planetesimals (heavily hydrated lithic clasts). The Isheyevo lithologies formed by size sorting of similar components during accretion in the Isheyevo parent body; they do not represent fragments of CH and CB chondrites.
    • Regolith history of lunar meteorites

      Lorenzetti, S.; Busemann, H.; Eugster, O. (The Meteoritical Society, 2005-01-01)
      The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y-) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y-981031 indicates an exposure at the lunar surface. A neon three-isotope diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 +/- 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y-981031 was obtained. The shielding depth of these samples was between 10.5 g/cm2 and >500 g/cm^2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm^2. Assuming a mean density of the lunar regolith of 1.8 g/cm^3, 10.5 g/cm^2 corresponds to a depth of 5.8 cm and 500 g/cm^2 to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.