¹Nancy N. Elewa, ¹J. M. Cadogan
Hyperfine Interactions 238, 4 Link to Article [DOI https://doi.org/10.1007/s10751-016-1350-1]
¹School of Physical, Environmental and Mathematical Sciences The University of New South Wales at the Australian Defence Force Academy Canberra Australia

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^1,2Poorna Srinivasan, ^1,3Rhian H. Jones, ¹Adrian J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12921]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
³School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, UK
Published by arrangement with John Wiley & Sons

We studied textures and compositions of sulfide inclusions in unzoned Fe,Ni metal particles within CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411 in order to constrain formation conditions and secondary thermal histories on the CB parent body. Unzoned metal particles in all four chondrites have very similar metal and sulfide compositions. Metal particles contain different types of sulfides, which we categorize as: homogeneous low-Cr sulfides composed of troilite, troilite-containing exsolved daubreelite lamellae, arcuate sulfides that occur along metal grain boundaries, and shock-melted sulfides composed of a mixture of troilite and Fe, Ni metal. Our model for formation proposes that the unzoned metal particles were initially metal droplets that formed from splashing by a partially molten impacting body. Sulfide inclusions later formed as a result of precipitation of excess S from solid metal at low temperatures, either during single stage cooling or during a reheating event by impacts. Sulfides containing exsolution lamellae record temperatures of ≪600 °C, and irregular Fe-FeS intergrowth textures suggest localized shock melting, both of which are indicative of heterogeneous heating by impact processes on the CB parent body. Our study shows that CBa and CBb chondrites formed in a similar environment, and also experienced similar secondary impact processing.

¹Thomas Smith,²Beda A. Hofmann,¹Ingo Leya,
³Silke Merchel,³Stefan Pavetich,³Georg Rugel,³Andreas Scharf
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12928]
¹Physic Institute, University of Bern, Bern, Switzerland
²Naturhistorisches Museum der Burgergemeinde Bern, Bern, Switzerland
³Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
Published by arrangement with John Wiley & Sons

The Twannberg iron meteorite is one out of only six members of the group IIG. The combined noble gas and radionuclide data obtained in this new systematic study indicate that Twannberg with its ~570 recently recovered specimens was a large object with a preatmospheric radius in the range of ~2 m, which corresponds to ~250 × 106 kg. The cosmic-ray exposure age for Twannberg is 182 ± 45 Ma. The most surprising result is the long terrestrial age of Tterr = math formula ka, which is unexpected considering the humid conditions in Switzerland. However, this age is in accord with glaciation events, indicating that the less shielded samples from Mt. Sujet were found close to the position of the original strewn field, whereas the samples from Gruebmatt and Twannbach, which are from more shielded positions, were glacially transported to the east–northeast during the second last ice age (185–130 ka ago) from an original position west of Mt. Sujet.

¹Jiangzhi Chen, ¹Chiara Elmi, ¹David Goldsby, ¹Reto Gieré
Geophysical Research Letters (in Press) Link to Article [DOI: 10.1002/2017GL073843]
¹Department of Earth and Environmental Sciences, University of Pennsylvania, Philadelphia, PA

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^1,2,3Andrew M. Davis, ^1,2,4Junjun Zhang,^1,2,4Nicolas D. Greber, ^1,2,4Jingya Hu,^1,4,5François L.H. Tissot, ^1,2,3,4Nicolas Dauphas
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.07.032]
¹Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA
²Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA
³Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA
⁴Origins Laboratory, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA
⁵Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Copyright Elsevier

Titanium isotopic compositions (mass-dependent fractionation and isotopic anomalies) were measured in 46 calcium-, aluminum-rich inclusions (CAIs) from the Allende CV chondrite. After internal normalization to ⁴⁹Ti/⁴⁷Ti, we found that ε⁵⁰Ti values are somewhat variable among CAIs, and that ε⁴⁶Ti is highly correlated with ε⁵⁰Ti, with a best-fit slope of 0.162±0.030 (95% confidence interval). The linear correlation between ε⁴⁶Ti and ε⁵⁰Ti extends the same correlation seen among bulk solar objects (slope 0.184±0.007). This observation provides constraints on dynamic mixing of the solar disk and has implications for the nucleosynthetic origin of titanium isotopes, specifically on the possible contributions from various types of supernovae to the solar system. Titanium isotopic mass fractionation, expressed as δ′⁴⁹Ti, was measured by both sample-standard bracketing and double-spiking. Most CAIs are isotopically unfractionated, within a 95% confidence interval of normal, but a few are significantly fractionated and the range δ′⁴⁹Ti is from ∼–4 to ∼+4. Rare earth element patterns were measured in 37 of the CAIs. All CAIs with significant titanium mass fractionation effects have group II and related REE patterns, implying kinetically controlled volatility fractionation during the formation of CAIs with those REE patterns.

^1,2Devin L. Schrader, ³Jemma Davidson
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.07.031]
¹Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287-6004, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th& Constitution Avenue NW, Washington, D.C. 20560-0119, USA
³Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
Copyright Elsevier

By investigating the petrology and chemical composition of type II (FeO-rich) chondrules in the Mighei-like carbonaceous (CM) chondrites we constrain their thermal histories and relationship to the Ornans-like carbonaceous (CO) chondrites. We identified FeO-rich relict grains in type II chondrules by their Fe/Mn ratios; their presence indicates chondrule recycling among type II chondrules. The majority of relict grains in type II chondrules are FeO-poor olivine grains. Consistent with previous studies, chemical similarities between CM and CO chondrite chondrules indicate that they had similar formation conditions and that their parent bodies probably formed in a common region within the protoplanetary disk. However, important differences such as mean chondrule size and the lower abundance of FeO-poor relicts in CM chondrite type II chondrules than in CO chondrites suggest CM and CO chondrules did not form together and they likely originate from distinct parent asteroids.

Despite being aqueously altered, many CM chondrites contain pre-accretionary anhydrous minerals (i.e., olivine) that are among the least thermally metamorphosed materials in chondrites according to the Cr2O3 content of their ferroan olivine. The presence of these minimally altered pre-accretionary chondrule silicates suggests that samples to be returned from aqueously altered asteroids by the Hayabusa2 and OSIRIS-REx asteroid sample return missions, even highly hydrated, may contain silicates that can provide information about the pre-accretionary histories and conditions of asteroids Ryugu and Bennu, respectively.

^1,2M. D. Suttle, ^1,2M. J. Genge, ²S. S. Russell
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12927]
¹Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, South Kensington, London, UK
²Department of Earth Science, The Natural History Museum, South Kensington, London, UK
Published by arrangement with John Wiley & Sons

The orientations of dehydration cracks and fracture networks in fine-grained, unmelted micrometeorites were analyzed using rose diagrams and entropy calculations. As cracks exploit pre-existing anisotropies, analysis of their orientation provides a mechanism with which to study the subtle petrofabrics preserved within fine-grained and amorphous materials. Both uniaxial and biaxial fabrics are discovered, often with a relatively wide spread in orientations (40°–60°). Brittle deformation cataclasis and rotated olivine grains are reported from a single micrometeorite. This paper provides the first evidence for impact-induced shock deformation in fine-grained micrometeorites. The presence of pervasive, low-grade shock features in CM chondrites and CM-like dust, anomalously low-density measurements for C-type asteroids, and impact experiments which suggest CM chondrites are highly prone to disruption all imply that CM parent bodies are unlikely to have remained intact and instead exist as a collection of loosely aggregated rubble-pile asteroids, composed of primitive shocked clasts.

¹Adrian R. Muxworthy,²Phillip A. Bland,¹Thomas M. Davison,¹James Moore,¹Gareth S. Collins,³Fred J. Ciesla
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12918]
¹Department of Earth Science and Engineering, Imperial College London, London, UK
²Department of Applied Geology, Curtin University of Technology, Perth, Western Australia, Australia
³Department of Geophysical Science, University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

We conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix’s primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix’s remanent magnetization was found to be consistent and low (~6 μT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization was formed during or after an impact event. Recent mesoscale impact modeling, where chondrules and matrix are resolved, has shown that low-velocity collisions can generate significant matrix temperatures, as pore-space compaction attenuates shock energy and dramatically increases the amount of heating. Nonporous chondrules are unaffected, and act as heat-sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1 km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero-porosity chondrules make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.

¹Toshiki Koga, ^1,2Hiroshi Naraoka
Scientific Reports 7, 636 Link to Article [doi:10.1038/s41598-017-00693-9]
Department of Earth and Planetray Sciences, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
Research Center for Planetary Trace Organic Compounds, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan

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¹Chaney Lin, ¹Lincoln S. Hollister, ³Glenn J. MacPherson, ^4,5Luca Bindi, ⁶Chi Ma, ⁷Christopher L. Andronicos, ^1,8Paul J. Steinhardt
Scientific Reports 7, 1637 Link to Article [doi:10.1038/s41598-017-01445-5]
¹Department of Physics, Princeton University, Jadwin Hall, Princeton, NJ, 08544, USA
²Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ, 08544, USA
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC, 20560, USA
⁴Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121, Florence, Italy
⁵C.N.R. – Istituto di Geoscienze e Georisorse, Via La Pira 4, I-50121, Florence, Italy
⁶Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, 91125, USA
⁷Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, 47907, USA
⁸Princeton Center for Theoretical Science, Princeton University, Princeton, NJ, 08544, USA

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