^1,2Alexander N. Krot, ¹Kazuhide Nagashima, ²Elishevah M.M. van Kooten, ²Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.001]
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Centre for Star and Planet Formation, Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark
Copyright Elsevier

¹Rhiannon G. Mayne,¹Samantha E. Smith,²C. M. Corrigan
Meteoritics & Planetary Sciences (in Press) Link to Article [DOI: 10.1111/maps.12730]
¹Monnig Meteorite Collection and Gallery, School of Geology, Energy and the Environment, Texas Christian University, Fort Worth, Texas, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution NW, Washington, DC, USA
Published by arrangement with John Wiley & Sons

204 howardites in the National Meteorite Collection at the Smithsonian were examined for the presence of fine-grained eucrite clasts, with the goal of better understanding the formation of the uppermost crust of asteroid 4Vesta. Eight clasts were identified and characterized in terms of their textures and mineral chemistry, and their degree of thermal metamorphism was assessed. The paucity of fine-grained eucrites, both within the unbrecciated eucrites and as clasts within the howardites, suggests that they originate from small-scale units on the surface of Vesta, most likely derived from partial melting. Six of the eight clasts described were found to be unequilibrated, meaning that they preserve their original crystallization trends. The vast majority of eucrites are at least partially equilibrated, making these samples quite rare and important for deciphering the petrogenesis of the vestan crust. Biomodal grain populations suggest that eucrite melts often began crystallizing pyroxene and plagioclase during their ascent to the surface, where they were subject to more rapid cooling, crystallization, and later metasomatism. Pyroxene compositions from this study and prior work indicate that the products of both primitive and evolved melts were present at the vestan surface after its formation. Two howardite thin sections contained multiple eucrite composition clasts with different crystallization and thermal histories; this mm-scale diversity reflects the complexity of the current day vestan surface that has been observed by Dawn.

^1,2M. Kimura,^2,3A. Yamaguchi,⁴M. Miyahara
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12738]
¹Faculty of Science, Ibaraki University, Mito, Japan
²National Institute of Polar Research, Tokyo, Japan
³Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, Japan
⁴Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Japan
Published by arrangement with John Wiley & Sons

Shock-induced features are abundantly observed in meteorites. Especially, shock veins, including high-pressure minerals, characterize many kinds of heavily shocked meteorite. On the other hand, no high-pressure phases have been yet reported from enstatite chondrites. We studied a heavily shocked EH3 chondrite, Asuka 10164, containing a vein, which comprises fragments of fine-grained silicate and opaque minerals, and chondrules. In this vein, we found a silica polymorph, coesite. This is the first discovery of a high-pressure phase in enstatite chondrites. Other high-pressure polymorphs were not observed in the vein. The assemblages and chemical compositions of minerals, and the occurrence of coesite indicate that the vein was subjected to the high-pressure and temperature condition at about 3–10 GPa and 1000 °C. The host also experienced heating for a short time under lower temperature conditions, from ~700 to ~1000 °C, based on the opaque minerals typical of EH chondrites and textural features. Although the pressure condition of the vein in this chondrite is much lower than those in the other meteorites, our results suggest that all major meteorite groups contain high-pressure polymorphs. Heavy shock events commonly took place in the solar system.

^1,2,3Ekaterina V. Korochantseva,^1,2,3Alexei I. Buikin,^1,3Jens Hopp,²Cyrill A. Lorenz,²Alexander V. Korochantsev,^4,5Ulrich Ott,^1,4Mario Trieloff
Meteoritics & Planetary Society (in Press) Link to Article [10.1111/maps.12732]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Vernadsky Institute of Geochemistry, Moscow, Russia
³Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany
⁴University of West Hungary, Szombathely, Hungary
⁵Max-Planck-Institut für Chemie, Mainz, Germany
Published by arrangement with John Wiley & Sons

Dhofar 280 recorded a complex history on the Moon revealed by high-resolution 40Ar-39Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic 38Ar, resulting in a (low-temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low-temperature) 40Ar-39Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP-rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low 40Ar/36Ar ratio. It is apparently a mixture of two components, one endmember with 40Ar/36Ar = 17.5 ± 0.2 and a second less well-constrained endmember with 40Ar/36Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (40Ar/36Ar)trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.

^1,2Christine E. Jilly-Rehak, ²Gary R. Huss, ²Kazuhide Nagashima
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.033]
¹Department of Geology and Geophysics, University of Hawai‘i at Mānoa, 1680 East-West Road, POST 701, Honolulu, HI 96822, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East-West Road, POST 602, Honolulu, HI 96822, USA
Copyright Elsevier

We present 53Mn-53Cr ages of secondary carbonates in Renazzo-like (CR) chondrites, determined by secondary ion mass spectrometry. The timing of aqueous alteration in CR chondrites has been unconstrained in the literature. We measured 53Mn-53Cr isotope systematics in carbonates from three different CR-chondrite lithologies. Calcite in the interchondrule matrix of Renazzo, calcite in the matrix of GRO 95577, and dolomite in a dark inclusion of Renazzo all show excesses in 53Cr, interpreted as the daughter product from the decay of 53Mn. The Renazzo calcite yields an initial ratio of (53Mn/55Mn)0 = (3.6 ± 2.7) × 10-6, and the Renazzo dark inclusion dolomite ranges from (53Mn/55Mn)0 = (3.1 ± 1.4) × 10-6 (corrected to the RSF of a calcite standard) to (3.7 ± 1.7) × 10-6 (corrected to an inferred dolomite RSF). When anchored to the D’Orbigny angrite, the Renazzo carbonates yield ages between 4563.6 to 4562.6 Ma, or ∼ 4.3 to 5.3 Myr after the formation of CV CAIs. Calcite measured in the heavily altered specimen GRO 95577 yields a shallower slope of (53Mn/55Mn)0 = (7.9 ± 2.8) × 10-7, corresponding to a much younger age of 4555.4 Ma, or ∼ 12.6 Myr after CAI formation. The two Renazzo ages are contemporaneous with Mn-Cr ages of carbonates in Tagish Lake, CI, and CM chondrites, but the GRO 95577 age is uniquely young. These findings suggest that early aqueous alteration on chondritic parent bodies was a common occurrence, likely driven by internal heating from 26Al decay after accretion. The young carbonate ages of GRO 95577 suggest that either the CR parent body was sufficiently large to sustain heating from 26Al for ∼ 8 Myr, or that late-stage impact events supplied heat to the region where GRO 95577 originated.

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.