¹Brett W. Denevi et al. (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12729]
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Published by arrangement with John Wiley & Sons
*Find the extensive, full author and affiliation list on the publishers website

We investigate the depth, variability, and history of regolith on asteroid Vesta using data from the Dawn spacecraft. High-resolution (15–20 m pixel−1) Framing Camera images are used to assess the presence of morphologic indicators of a shallow regolith, including the presence of blocks in crater ejecta, spur-and-gully–type features in crater walls, and the retention of small  (<300 m) impact craters. Such features reveal that the broad, regional heterogeneities observed on Vesta in terms of albedo and surface composition extend to the physical properties of the upper ~1 km of the surface. Regions of thin regolith are found within the Rheasilvia basin and at equatorial latitudes from ~0–90°E and ~260–360°E. Craters in these areas that appear to excavate material from beneath the regolith have more diogenitic (Rheasilvia, 0–90°E) and cumulate eucrite (260–360°E) compositions. A region of especially thick regolith, where depths generally exceed 1 km, is found from ~100–240°E and corresponds to heavily cratered, low-albedo surface with a basaltic eucrite composition enriched in carbonaceous chondrite material. The presence of a thick regolith in this area supports the idea that this is an ancient terrain that has accumulated a larger component of exogenic debris. We find evidence for the gardening of crater ejecta toward more howarditic compositions, consistent with regolith mixing being the dominant form of “weathering” on Vesta.

¹Yoko Kebukawa, ²Michael E. Zolensky, ²Queenie H.S. Chan, ³Keisuke Nagao, ⁴A.L. David Kilcoyne, ⁵Robert J. Bodnar, ⁵Charles Farley, ⁶Zia Rahman, ⁶Loan Le, ⁷George D. Cody
Geochmica et Cosmochimca Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.024]
¹Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
²ARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA
³Geochemical Research Center, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
⁴Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
⁵Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA
⁶Jacobs – NASA Johnson Space Center, Houston, TX 77058, USA
⁷Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA
Copyright Elsevier

Primitive xenolithic clasts, often referred to as “dark clasts”, are well known in many regolith breccias. The Sharps H3.4 ordinary chondrite contains unusually large dark clasts up to ∼1 cm across. Poorly-graphitized carbon (PGC), with Fe, Ni metal and described as “carbon-rich aggregates”, has been reported in these clasts (Brearley, 1990). We report detailed analyses of carbonaceous matter in several identical Sharps clasts using FTIR, Raman, C-XANES, and TEM that provide insight on the extent of thermal processing and possible origin of such clasts. We also prepared acid residues of the clasts using the HCl/HF method and conducted mass spectrometric analysis of the entrained noble gases.

Carbonaceous matter is often used to infer thermal history due to its sensitivity to thermal processes. The FTIR spectra of the acid residue from the Sharps clast suggest that carbonaceous matter in the clast contains less hydrogen and oxygen compared to acid residues from typical type 3.4 ordinary chondrites. The metamorphic temperatures obtained by Raman spectroscopy ranges between ∼380 °C to ∼490 °C. TEM observations indicate that the clasts experienced a peak temperature of 300 °C to 400 °C, based on the carbon d002 layer lattice spacing of C-rich aggregates. These estimates are consistent with an earlier estimate of 330 ± 50 °C, that is also estimated by the d002 layer lattice spacing (Brearley, 1990). It should be noted that the lattice spacing thermometer is based on terrestrial metamorphose rocks, and thus temperature was probably underestimated. Meanwhile, the C-XANES spectra of the C-rich aggregates show high exciton intensities, indicative of graphene structures that developed at around 700 °C to 800 °C following an extensive period of time (millions of years), however, the surrounding matrix areas experienced lower temperatures of less than 300 °C to 500 °C. Noble gas analysis of the acid residue from the Sharps clasts shows that the residue is almost identical with some material reported in carbonaceous chondrites, i.e., heavily enriched in the Q-gas component as well as HL-gas from presolar diamonds and Ne-E(H) from presolar SiC.

These results indicate that the C-rich aggregates in the Sharps clasts formed under relatively high temperature conditions, up to 800 °C, and were subsequently mixed with lower temperature matrix, probably in a different parent body, before they were incorporated into the final Sharps lithology by collision.

^1,2Yuan Li, ²Rajdeep Dasgupta, ²Kyusei Tsuno, ³Brian Monteleone, ³Nobumichi Shimizu
Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo2801]
¹Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
²Department of Earth Science, Rice University, 6100 Main Street, MS 126, Houston, Texas 77005, USA
³Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

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¹Giorgio S. Senesi, ²Gioacchino Tempesta, ³Paola Manzari, ²Giovanna Agrosì
Geostandards and Geoanalytical Research (in Press) Link to Article [DOI: 10.1111/ggr.12126]
¹Istituto di Nanotecnologia (NANOTEC) – PLASMI Lab, CNR, Bari, Italy
²Dipartimento di Scienze della Terra e Geoambientali (DiSTeGeo), University of Bari, Bari, Italy
³Istituto Nazionale di Astrofisica, Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), Roma, Italy
Published by arrangement with John Wiley & Sons

An innovative approach of double pulse laser-induced breakdown spectroscopy (DP-LIBS) coupled with optical microscopy was applied to the characterisation and quantitative analysis of the Agoudal iron meteorite in bulk sample and in petrographic thin section. Qualitative analysis identified the elements Ca, Co, Fe, Ga, Li and Ni in the thin section and the whole meteorite. Two different methods, calibration-free LIBS and one-point calibration LIBS, were used as complementary methodologies for quantitative LIBS analysis. The elemental composition data obtained by LIBS were in good agreement with the compositional analyses obtained by traditional methods generally applied for the analysis of meteorites, such as ICP-MS and EDS-SEM. Besides the recognised advantages of LIBS over traditional techniques, including versatility, minimal destructivity, lack of waste production, low operating costs, rapidity of analysis, availability of transportable or portable systems, etc., additional advantages of this technique in the analysis of meteorites are precision and accuracy, sensitivity to low atomic number elements such as Li and the capacity to detect and quantify Co contents that cannot be obtained by EDS-SEM.

^1,2Laurette Piani, ²Yves Marrocchi, ³Guy Libourel, ²Laurent Tissandier
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.010]
¹Department of Natural History Sciences, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
²CRPG, UMR 7358, CNRS – Université de Lorraine, 54500 Vandoeuvre-lès-Nancy, France
³Laboratoire Lagrange, UMR7293, Université de la Côte d’Azur, CNRS, Observatoire de la Côte d’Azur,F-06304 Nice Cedex 4, France
Copyright Elsevier

The nature and distribution of sulfides within 17 porphyritic chondrules of the Sahara 97096 EH3 enstatite chondrite have been studied by backscattered electron microscopy and electron microprobe in order to investigate the role of gas-melt interactions in the chondrule sulfide formation.

Troilite (FeS) is systematically present and is the most abundant sulfide within the EH3 chondrite chondrules. It is found either poikilitically enclosed in low-Ca pyroxenes or scattered within the glassy mesostasis. Oldhamite (CaS) and niningerite [(Mg,Fe,Mn)S] are present in ≈ 60% of the chondrules studied. While oldhamite is preferentially present in the mesostasis, niningerite associated with silica is generally observed in contact with troilite and low-Ca pyroxene. The Sahara 97096 chondrule mesostases contain high abundances of alkali and volatile elements (average Na2O = 8.7 wt.%, K2O = 0.8 wt.%, Cl = 7000 ppm and S = 3700 ppm) as well as silica (average SiO2 = 63.1 wt.%).

Our data suggest that most of the sulfides found in EH3 chondrite chondrules are magmatic minerals that formed after the dissolution of S from a volatile-rich gaseous environment into the molten chondrules. Troilite formation occurred via sulfur solubility within Fe-poor chondrule melts followed by sulfide saturation, which causes an immiscible iron sulfide liquid to separate from the silicate melt. The FeS saturation started at the same time as or prior to the crystallization of low-Ca pyroxene during the high temperature chondrule forming event(s). Protracted gas-melt interactions under high partial pressures of S and SiO led to the formation of niningerite-silica associations via destabilization of the previously formed FeS and low-Ca pyroxene. We also propose that formation of the oldhamite occurred via the sulfide saturation of Fe-poor chondrule melts at moderate S concentration due to the high degree of polymerization and the high Na-content of the chondrule melts, which allowed the activity of CaO in the melt to be enhanced. Gas-melt interactions thus appear to be a key process that may control the mineralogy of chondrules in the different classes of chondrite.

¹Earl O’Bannon III, ¹Quentin Williams
Physics and Chemistry of Minerals 43, 181–208   Link to Article [doi:10.1007/s00269-015-0786-1]
¹Department of Earth and Planetary Sciences University of California Santa Cruz USA

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^1,2Youmo Zhou,¹Tetsuo Irifune,¹Hiroaki Ohfuji,¹Toru Shinmei,^1,2Wei Du
Physics and Chemistry of Minerals (in Press) Link to Article [DOI: 10.1007/s00269-016-0834-5]
¹Geodynamics Research Center Ehime University Matsuyama Japan
²Earth-Life Science Institute Tokyo Institute of Technology Tokyo Japan

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¹A. Rodríguez, ¹M.J. van Bergen
Netherlands Journal of Geosciences 95, 153-169 Link to Article [DOI: http://dx.doi.org/10.1017/njg.2015.12]
¹Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3508 TA, Utrecht, the Netherlands

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¹Shuai Li, ¹Ralph E. Milliken
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005035]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
Published by arrangement with John Wiley & Sons

Radiance measured by the Moon Mineralogy Mapper (M3) at wavelengths beyond ~2 µm commonly includes both solar reflected and thermally emitted contributions from the lunar surface. Insufficient correction (removal) of the thermal contribution can modify and even mask absorptions at these wavelengths in derived surface reflectance spectra, an effect that precludes accurate identification and analysis of OH and/or H2O absorptions. This study characterized thermal effects in M3 data by evaluating surface temperatures measured independently by the Lunar Reconnaissance Orbiter Diviner radiometer, and results confirm that M3 data (Level 2) currently available in the Planetary Data System (PDS) often contain significant thermal contributions. It is impractical to use independent Diviner measurements to correct all M3 images for the Moon because not every M3 pixel has a corresponding Diviner measurement acquired at the same local time of lunar day. Therefore, a new empirical model, constrained by Diviner data, has been developed based on the correlation of reflectance at 1.55 µm and at 2.54 µm observed in laboratory reflectance spectra of Apollo and Luna soil and glass-rich samples. Reflectance values at these wavelengths follow a clear power law, inline image, for a wide range of lunar sample compositions and maturity. A nearly identical power law is observed in M3 reflectance data that have been independently corrected using Diviner-based temperatures, confirming this is a general reflectance property of materials that typify the lunar surface. These results demonstrate that reflectance at a thermally-affected wavelength (2.54 µm) can be predicted within 2% (absolute) based on reflectance values at shorter wavelengths where thermal contributions are negligible and reflectance is dominant. Radiance at 2.54 µm that is in excess of the expected amount is assumed to be due to thermal emission and is removed during conversion of at-sensor radiance to reflectance or I/F. Removal of this thermal contribution using this empirically-based model provides a more accurate view of surface reflectance properties at wavelengths >2 µm, with the benefit that it does not require independent measurements or modeling of surface temperatures at the same local time as M3 data were acquired. It is demonstrated that this model is appropriate for common lunar surface compositions (e.g., mare and highlands soils, pyroclastic deposits), but surface compositions with reflectance properties that deviate strongly from these cases (e.g., pyroxene-, olivine-, or spinel-rich locations with minimal space weathering) may require the use of more sophisticated thermal correction models or overlapping Diviner temperature estimates.

¹Kyoungwon Min, ¹Annette Farah, ²Seung Ryeol Lee, ³Jong Ik Lee
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.009]
¹Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA
²Korea Institute of Geosciences and Mineral Resources, Daejeon, Korea
³Korea Polar Research Institute, Incheon, Korea
Copyright Elsevier

Shock conditions of martian meteorites provide crucial information about ejection dynamics and original features of the martian rocks. To better constrain equilibrium shock temperatures (Tequi-shock) of martian meteorites, we investigated (U-Th)/He systematics of moderately-shocked (Zagami) and intensively shocked (ALHA77005) martian meteorites. Multiple phosphate aggregates from Zagami and ALHA77005 yielded overall (U-Th)/He ages 92.2 ± 4.4 Ma (2σ) and 8.4 ± 1.2 Ma, respectively. These ages correspond to fractional losses of 0.49 ± 0.03 (Zagami) and 0.97 ± 0.01 (ALHA77005), assuming that the ejection-related shock event at ∼3 Ma is solely responsible for diffusive helium loss since crystallization. For He diffusion modeling, the diffusion domain radius is estimated based on detailed examination of fracture patterns in phosphates using a scanning electron microscope. For Zagami, the diffusion domain radius is estimated to be ∼2-9 μm, which is generally consistent with calculations from isothermal heating experiments (1-4 μm). For ALHA77005, the diffusion domain radius of ∼4-20 μm is estimated.

Using the newly constrained (U-Th)/He data, diffusion domain radii, and other previously estimated parameters, the conductive cooling models yield Tequi-shock estimates of 360-410 °C and 460-560 °C for Zagami and ALHA77005, respectively. According to the sensitivity test, the estimated Tequi-shock values are relatively robust to input parameters. The Tequi-shock estimates for Zagami are more robust than those for ALHA77005, primarily because Zagami yielded intermediate fHe value (0.49) compared to ALHA77005 (0.97). For less intensively shocked Zagami, the He diffusion-based Tequi-shock estimates (this study) are significantly higher than expected from previously reported Tpost-shock values. For intensively shocked ALHA77005, the two independent approaches yielded generally consistent results. Using two other examples of previously studied martian meteorites (ALHA84001 and Los Angeles), we compared Tequi-shock and Tpost-shock estimates. For intensively shocked meteorites (ALHA77005, Los Angeles), the He diffusion-based approach yield slightly higher or consistent Tequi-shock with estimations from Tpost-shock, and the discrepancy between the two methods increases as the intensity of shock increases. The reason for the discrepancy between the two methods, particularly for less-intensively shocked meteorites (Zagami, ALHA84001), remains to be resolved, but we prefer the He diffusion-based approach because its Tequi-shock estimates are relatively robust to input parameters.