¹Bérengère Mougel, ^1,2Frédéric Moynier, ¹Christa Göpel, ^3,4Christian Koeberl
Earth and Planetary Science Letters 460, 105-111 Link to Article [https://doi.org/10.1016/j.epsl.2016.12.008]
¹Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR 7154, Paris, France
²Institut Universitaire de France and Université Paris Diderot, Paris, France
³Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
⁴Natural History Museum, Burgring 7, A-1010 Vienna, Austria
Copyright Elsevier

Non-mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra-terrestrial materials in terrestrial impact rocks. Paleoproterozoic spherule layers from Greenland (Grænsesø) and Russia (Zaonega), as well as some distal ejecta deposits (Lake Superior region) from the Sudbury impact (1849±0.3 Ma) event, have been analyzed for their Cr isotope compositions. Our results suggest that 1) these distal ejecta deposits are all of impact origin, 2) the Grænsesø and Zaonega spherule layers contain a distinct carbonaceous chondrite component, and are possibly related to the same impact event, which could be Vredefort (2023±4 Ma) or another not yet identified large impact event from that of similar age, and 3) the Sudbury ejecta record a complex meteoritic signature, which is different from the Grænsesø and Zaonega spherule layers, and could indicate the impact of a heterogeneous chondritic body.

¹Rachel L. Klima, ²Noah E. Petro
Philosophical Transactions of the Royal Astronomical Society A 375, 2094 Link to Article [https://doi.org/10.1098/rsta.2015.0391]
¹Space Exploration Sector, Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
²NASA Goddard Space Flight Center, Greenbelt, MD, USA

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¹Peng Ni, ¹Youxue Zhang, ¹Adam Simon, ²Joel Gagnon
American Mineralogist 102, 1287-1301 Link to Article [https://doi.org/10.2138/am-2017-5885]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.
²Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Copyright: The Mineralogical Society of America

Copper diffusion plays an important role in natural processes, such as metal transport during the formation of magmatic-hydrothermal porphyry-type ore deposits and Cu isotope fractionation during tektite formation. Copper diffusion data in natural silicate melts, however, are limited. In this study, chalcocite (Cu2S) “dissolution” experiments were carried out using chalcocite-rhyolite diffusion “couples” to study Cu (and S) diffusion in rhyolitic melts. Instead of chalcocite dissolution as initially expected, our experiments show that Cu is transferred from the chalcocite crystal to the rhyolitic melt, and Fe is transferred from the rhyolitic melt to chalcocite, whereas the S concentration profile in the rhyolitic melt is essentially flat. From the Cu and Fe exchange profiles in the rhyolitic melts, Cu diffusivities and Fe diffusivities are obtained and reported. Copper diffusivity in rhyolitic melts containing 0.10 to 5.95 wt% H2O at temperatures of 750 to 1391 °C and pressures of 0.5 to 1.0 GPa can be described as: Embedded Image

which allows the estimation of an activation energy for diffusion in dry rhyolitic melts to be 96.8 ± 4.1 kJ/mol. In the above equation, diffusivity (D) is in m2/s, T is the temperature in K, w is the H2O concentration in the rhyolitic melts in wt% and all errors reported are at 1σ level. Combining Cu diffusion data from this study and previous data in basaltic melt gives a general equation for Cu diffusivity in natural silicate melts: Embedded Image

where Si+Al-H is the cation mole fraction of Si plus Al minus H in the silicate melt on a wet basis. Iron diffusivities obtained in this study, in anhydrous to 6 wt% H2O rhyolite, are combined with previous data to get a general equation for Fe diffusion in rhyolitic melts: Embedded Image

Our data demonstrate that Cu diffusion is faster than H2O or Cl in rhyolitic melts containing 6 wt% water, which indicates that the scavenging and transport of Cu by a magmatic volatile phase during formation of porphyry-type ore deposits is not limited by diffusion of Cu. Based on our experimental data, Cu diffusivity is almost four orders of magnitude higher than Zn in anhydrous rhyolitic melts, which supports the explanation of more diffusive loss of Cu leading to more fractionated Cu isotopes than Zn isotopes in tektites.

¹D. J. Joswiak, ¹D. E. Brownlee, ²A. N. Nguyen, ²S. Messenger
Meteoritics&Planetary Sciences (in Press) Link to Article [DOI: 10.1111/maps.12877]
¹Department of Astronomy, University of Washington, Seattle, Washington, USA
²Robert M. Walker Laboratory for Space Science, ARES, NASA JSC, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Transmission electron microscope examination of more than 250 fragments, >1 μm from comet Wild 2 and a giant cluster interplanetary dust particle (GCP) of probable cometary origin has revealed four new calcium-aluminum-rich inclusions (CAIs), an amoeboid olivine aggregate (AOA), and an additional AOA or Al-rich chondrule (ARC) object. All of the CAIs have concentric mineral structures and are composed of spinel + anorthite cores surrounded by Al,Ti clinopyroxenes and are similar to two previous CAIs discovered in Wild 2. All of the cometary refractory objects are of moderate refractory character. The mineral assemblages, textures, and bulk compositions of the comet CAIs are similar to nodules in fine-grained, spinel-rich inclusions (FGIs) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid–gas reactions in the solar nebula. Oxygen isotopes collected on one Wild 2 CAI also match FGIs. The lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory CAI nodules that were produced in the nebula and the small sample sizes collected. In the comet samples, approximately 2–3% of all fragments larger than 1 μm, by number, are CAIs and nearly 50% of all bulbous Stardust tracks contain at least one CAI. We estimate that ~0.5 volume % of Wild 2 material and ~1 volume % of GCP is in the form of CAIs. ARCs and AOAs account for <1% of the Wild 2 and GCP grains by number.

^1,2Marco Pignatari, ³Peter Hoppe, ^4,2Reto Trappitsch, ^5,2Chris Fryer, ^6,7,2F.X. Timmes, ^8,9,2Falk Herwig, ^9,10,2Raphael Hirschi
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.005]
¹E. A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, UK
²The NuGrid Collaboration (http://www.nugridstars.org)
³Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
⁴Department of the Geophysical Sciences, The University of Chicago, and Chicago Center for Cosmochemistry, Chicago, IL 60637, USA
⁵Computational Physics and Methods (CCS-2), LANL, Los Alamos, NM, 87545, USA
⁶Arizona State University (ASU), PO Box 871404, Tempe, AZ, 85287-1404, USA
⁷The Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556, USA
⁸Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P5C2 Canada
⁹Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
¹⁰Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8583, Japan
Copyright Elsevier

Carbon-rich presolar grains are found in primitive meteorites, with isotopic measurements to date suggesting a core-collapse supernovae origin site for some of them. This holds for about 1-2 % of presolar silicon carbide (SiC) grains, so-called Type X and C grains, and about 30 % of presolar graphite grains. Presolar SiC grains of Type X show anomalous isotopic signatures for several elements heavier than iron compared to the solar abundances: most notably for strontium, zirconium, molybdenum, ruthenium and barium. We study the nucleosynthesis of zirconium and molybdenum isotopes in the He-shell of three core-collapse supernovae models of 15, 20 and 25 M☉ with solar metallicity, and compare the results to measurements of presolar grains. We find the stellar models show a large scatter of isotopic abundances for zirconium and molybdenum, but the mass averaged abundances are qualitatively similar to the measurements. We find all models show an excess of 96Zr relative to the measurements, but the model abundances are affected by the fractionation between Sr and Zr since a large contribution to 90Zr is due to the radiogenic decay of 90Sr. Some supernova models show excesses of 95,97Mo and depletion of 96Mo relative to solar. The mass averaged distribution from these models shows an excess of 100Mo, but this may be alleviated by very recent neutron-capture cross section measurements. We encourage future explorations to assess the impact of the uncertainties in key neutron-capture reaction rates that lie along the n-process path.

^1,2,3Adam R. Sarafian, ⁴Erik H. Hauri, ⁵Francis M. McCubbin, ⁶Thomas J. Lapen, ⁷Eve L. Berger, ^2,3Sune G. Nielsen, ^2,8Horst R. Marschall, ²Glenn A. Gaetani, ⁵Kevin Righter, ^1,2Emily Sarafian
Philosophical Transactions of the Royal Society A 375, 2094 Link to Article [https://doi.org/10.1098/rsta.2016.0209]
¹Massachusetts Institute of Technology – Woods Hole Oceanographic Institution Joint Program in Oceanography/Applied Ocean Science and Engineering, Cambridge, MA 02139, USA
²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
³NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
⁵NASA JSC, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USA
⁶Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
⁷GeoControl Systems Inc., Jacobs JETS Contract, NASA JSC, Houston, TX, USA
⁸Goethe Universität Frankfurt, Institut für Geowissenschaften, Altenhöferallee 1, 60438 Frankfurt am Main, Germany

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¹Ivan S. Bazhan, ^1,2Konstantin D. Litasov, ^1,3Eiji Ohtani, ³Shin Ozawa3
American Mineralogist 102, 1279-1286 Link to Article [https://doi.org/10.2138/am-2017-5892]
¹V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, 630090, Russia
²Novosibirsk State University, Novosibirsk, 630090, Russia
³Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Copyright: The Mineralogical Society of America

High-pressure minerals—majorite-pyrope garnet and jadeite—were found in the Pervomaisky L6 ordinary chondrite. Majorite-pyrope (79 mol% majorite) was observed within the fine-grained silicate matrix of a shock-melt vein (SMV), coexisting with olivine and high-Ca pyroxene. This is the first report of a garnet–olivine–high-Ca pyroxene assemblage that crystallized from the melt in the SMV matrix of meteorite. P-T conditions of the formation of the SMV matrix with olivine fragments are 13.5–15.0 GPa and 1750–2150 °C, the lowest parameters among all known majorite-bearing (H,L)-chondrites. The estimated conditions include the olivine/(olivine + ringwoodite) phase boundary and there is a possibility that observed olivine is the result of wadsleyite/ringwoodite back-transformation during a cooling and decompression stage. In the framework of this hypothesis, we discuss the problem of survival of the high-pressure phases at the post-shock stage in the meteorites and propose two possible P-T paths: (1) the high-pressure mineral is transformed to a low-pressure one during adiabatic decompression above the critical temperature of direct transformation; and (2) quenching below the critical temperature of direct transformation within the stability field of the high-pressure phase and further decompression. The aggregates with plagioclase composition (Ab81.1An14.9Or4.1) occur in host-rock fragments near (or inside) of the SMV, and have a radial, concentric “spherulite-like” microstructure previously described in the Novosibirsk meteorite, and that is very similar to the texture of tissintite in the Tissint martian meteorite. It is likely that jadeite is related to crystallization of the SMV and could have formed from albitic feldspar (plagioclase) melt at 13.5–15.0 GPa and ~2000 °C.

¹Hiroyuki Sato, ¹Mark S. Robinson, ²Samuel J. Lawrence, ³Brett W. Denevi, ⁴Bruce Hapke, ⁵Bradley L. Jolliff, ⁶Harald Hiesinger
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.06.013]
¹School of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA
²Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, USA
³Applied Physics Laboratory, Johns Hopkins University, 11100 John Hopkins Rd, Laurel, MD 20723-6005, USA
⁴Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, PA 15260, USA
⁵Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, St Louis, Missouri 63130, USA
⁶Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

The visible (VIS; 400-700 nm) and near-infrared (NIR; 700-2800 nm) reflectance of the lunar regolith is dominantly controlled by variations in the abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g., ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced rims on soil grains, and Fe-metal). The same materials control reflectance into the near-UV (250-400 nm) with varying degrees of importance. A key difference is that while ilmenite is spectrally neutral in the VIS and NIR, it exhibits a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) filters were specifically designed to take advantage of this spectral feature to enable more accurate mapping of ilmenite within mare soils than previously possible. Using the reflectance measured at 321 and 415 nm during 62 months of repeated near-global WAC observations, first we found a linear correlation between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of each sample return site. We then used the coefficients from the linear regression and the near-global WAC multispectral mosaic to derive a new TiO2 map. The average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 values were found in Mare Tranquillitatis (∼ 12.6 wt%) and Oceanus Procellarum (∼ 11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the low TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium) exhibit very low TiO2 values (<2 wt%). We find that the Clementine visible to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically higher values relative to the WAC estimates. The Lunar Prospector Gamma-Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map, although there are local offsets possibly due to the different depth sensitivities and large pixel scale of the GRS relative to the WAC. We find a wide variation of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga), whereas only medium- to high-TiO2 values (average = 6.8 wt%, minimum = 4.5 wt%) are found for younger mare units (<2.6 Ga).

^1,2Phillip Gopon,²Michael J. Spicuzza,³Thomas F. Kelly,³David Reinhard,³Ty J. Prosa,²John Fournelle
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12899]
¹Department of Earth Science, University of Oxford, Oxford, OX1 3AN, UK
²Department of Geoscience, University of Wisconsin, Madison, Wisconsin, USA
³CAMECA Instruments Inc., Madison, Wisconsin, USA
Published by arrangement with John Wiley & Sons

The lunar regolith contains a variety of chemically reduced phases of interest to planetary scientists and the most common, metallic iron, is generally ascribed to space weathering processes (Lucey et al. 2006). Reports of silicon metal and iron silicides, phases indicative of extremely reducing conditions, in lunar samples are rare (Anand et al. 2004; Spicuzza et al. 2011). Additional examples of Fe-silicides have been identified in a survey of particles from Apollo 16 sample 61501,22. Herein is demonstrated the utility of low keV electron probe microanalysis (EPMA), using the Fe Ll X-ray line, to analyze these submicron phases, and the necessity of accounting for carbon contamination. We document four Fe-Si and Si0 minerals in lunar regolith return material. The new Fe-Si samples have a composition close to (Fe,Ni)3Si, whereas those associated with Si0 are close to FeSi2 and Fe3Si7. Atom probe tomography of (Fe,Ni)3Si shows trace levels of C (60 ppma and nanodomains enriched in C, Ni, P, Cr, and Sr). These reduced minerals require orders of magnitude lower oxygen fugacity and more reducing conditions than required to form Fe0. Documenting the similarities and differences in these samples is important to constrain their formation processes. These phases potentially formed at high temperatures resulting from a meteorite impact. Whether carbon played a role in achieving the lower oxygen fugacities—and there is evidence of nearby carbonaceous chondritic material—it remains to be proven that carbon was the necessary component for the unique existence of these Si0 and iron silicide minerals.

^1,2Myriam Telus, ²Gary R. Huss, ²Kazuhide Nagashima, ²Ryan C. Ogliore, ³Shogo Tachibana
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.013]
¹Geology and Geophysics, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Department of Natural History Sciences, Hokkaido University, N10 W8, Sapporo 060-0810, Japan
Copyright Elsevier

The initial 60Fe/56Fe ratio of chondrules from unequilibrated ordinary chondrites (UOCs) can potentially help constrain the stellar source of short-lived radionuclides and develop the 60Fe-60Ni (t1/2=2.6 Ma) system for early solar system chronology. However, progress with the 60Fe-60Ni system has been hindered by discrepancies between initial ratios inferred from bulk and in situ Fe-Ni analyses. Telus et al. (2016) show that discrepancies between these different techniques stem from late-stage open-system Fe-Ni mobilization. Here, we report in situ analyses of the Fe-Ni isotopic composition of ferromagnesian silicates in chondrules from UOCs using the ion microprobe. Of the 24 chondrules analyzed for this study, a few chondrules have clearly resolved excesses in 60Ni of up to 70‰; however, the correlations with the Fe/Ni ratios are weak. Although complications from Fe-Ni redistribution make it difficult to interpret the data, we show that the initial 60Fe/56Fe ratio for UOC chondrules is between 5×10-8 and 3.0×10-7. This is consistent with a late supernova source for 60Fe, but self-enrichment of the molecular cloud is another possible mechanism for incorporating 60Fe in the solar system. Discrepancies between bulk and in situ analyses remain, but likely stem from late-stage open-system Fe-Ni mobilization.