¹C.D. Williams, ²T. Ushikubo, ³E.S. Bullock, ¹P.E. Janney, ¹R.R. Hines, ²N.T. Kita,
¹R.L. Hervig, ³G.J. MacPherson, ⁴R.A. Mendybaev,⁴F.M. Richter,¹M. Wadhwa
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.053]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States
²WiscSIMS, Department of Geosciences, University of Wisconsin, Madison, WI 53706, United States
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, United States
⁴Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, United States
Copyright Elsevier

Detailed petrologic, geochemical and isotopic analyses of a new FUN CAI from the Allende CV3 meteorite (designated CMS-1) indicate that it formed by extensive melting and evaporation of primitive precursor material(s). The precursor material(s) condensed in a 16O-rich region (δ17O and δ18O ∼ -49‰) of the inner solar nebula dominated by gas of solar composition at total pressures of ∼10-3 to 10-6 bar. Subsequent melting of the precursor material(s) was accompanied by evaporative loss of magnesium, silicon and oxygen resulting in large mass-dependent isotope fractionations in these elements (δ25Mg = 30.71 – 39.26‰, δ29Si = 14.98 – 16.65‰, and δ18O = -41.57 – -15.50‰). This evaporative loss resulted in a bulk composition similar to that of compact Type A and Type B CAIs, but very distinct from the composition of the original precursor condensate(s). Kinetic fractionation factors and the measured mass-dependent fractionation of silicon and magnesium in CMS-1 suggest that ∼ 80% of the silicon and ∼85% of the magnesium were lost from its precursor material(s) through evaporative processes. These results suggest that the precursor material(s) of normal and FUN CAIs condensed in similar environments, but subsequently evolved under vastly different conditions such as total gas pressure. The chemical and isotopic differences between normal and FUN CAIs could be explained by sorting of early solar system materials into distinct physical and chemical regimes, in conjunction with discrete heating events, within the protoplanetary disk.

¹Joanna V. Morgan et al. (>10)*
Science 354, 6314, 878-882 Link to Article [DOI: 10.1126/science.aah6561]
¹Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK.
Reprinted with permission from AAAS
*Find the extensive, full author and affiliation list on the publishers website

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

¹G. K. Bhatia,¹S. Sahijpal
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12789]
¹Department of Physics, Panjab University, Chandigarh, India
Published by arrangement with John Wiley&Sons

Numerical models dealing with the planetary scale differentiation of Mercury are presented with the short-lived nuclide, 26Al, as the major heat source along with the impact-induced heating during the accretion of planets. These two heat sources are considered to have caused differentiation of Mars, a planet with size comparable to Mercury. The chronological records and the thermal modeling of Mars indicate an early differentiation during the initial ~1 million years (Ma) of the formation of the solar system. We theorize that in case Mercury also accreted over an identical time scale, the two heat sources could have differentiated the planets. Although unlike Mars there is no chronological record of Mercury’s differentiation, the proposed mechanism is worth investigation. We demonstrate distinct viable scenarios for a wide range of planetary compositions that could have produced the internal structure of Mercury as deduced by the MESSENGER mission, with a metallic iron (Fe-Ni-FeS) core of radius ~2000 km and a silicate mantle thickness of ~400 km. The initial compositions were derived from the enstatite and CB (Bencubbin) chondrites that were formed in the reducing environments of the early solar system. We have also considered distinct planetary accretion scenarios to understand their influence on thermal processing. The majority of our models would require impact-induced mantle stripping of Mercury by hit and run mechanism with a protoplanet subsequent to its differentiation in order to produce the right size of mantle. However, this can be avoided if we increase the Fe-Ni-FeS contents to ~71% by weight. Finally, the models presented here can be used to understand the differentiation of Mercury-like exoplanets and the planetary embryos of Venus and Earth.

¹M. C. McCanta,²M. D. Dyar
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12793]
¹University of Tennessee, Knoxville, Tennessee, USA
²Mount Holyoke College, South Hadley, Massachusetts, USA
Published by arrangement with John Wiley & Sons

Oxidation is observed in Ca-pyroxene subjected to a range of shock pressures (21–59 GPa). Changes in the pyroxene redox ratio as measured by the changes in %Fe3+ ranged from 2–6 times the starting composition. Mössbauer and reflectance spectroscopy record the changing Fe3+ concentration as a preferential oxidation of Fe2+ in the M2 crystallographic site. The oxidation is also accompanied by mechanical changes in the pyroxene crystals including fracturing, linear defects, and twinning. As oxygen fugacity is often calculated using mineral redox ratios and thought to represent the prevailing fO2 during crystallization, it is imperative to recognize that the fO2 values measured in impact-derived materials may represent that of the impact and not the magma source region.

^1,2,3J. Brian Balta, ^4,5Matthew E. Sanborn, ⁶Rhiannon G. Mayne, ⁴Meenakshi Wadhwa, ¹Harry Y. McSween Jr, ⁵Samuel D. Crossley
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12744]
¹Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA
²University of Pittsburgh, Pittsburgh, Pennsylvania, USA
³Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA
⁴Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
⁵Department of Earth and Planetary Sciences, University of California at Davis, Davis, California, USA
⁶Monnig Meteorite Collection, School of Geology, Energy, and the Environment, Texas Christian University, Fort Worth, Texas 76109, USA
Published by arrangement with John Wiley&Sons

We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine-grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible-element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.

¹Camille Soulié,²Guy Libourel,¹Laurent Tissandier
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12792]
¹Centre de Recherches Pétrographiques et Géochimiques (UMR 7358, CNRS-Université de Lorraine), Vandoeuvre-lès-Nancy, France
²Observatoire de la Côte d’Azur, UMR 7293 Lagrange, Bd de l’Observatoire, Nice, France
Published by arrangement with John Wiley & Sons

Mg-rich olivine is a ubiquitous phase in type I porphyritic chondrules in various classes of chondritic meteorites. The anhedral shape of olivine grains, their size distribution, as well as their poikilitic textures within low-Ca pyroxene suggest that olivines suffer dissolution during chondrule formation. Owing to a set of high-temperature experiments (1450–1540 °C) we determined the kinetics of resorption of forsterite in molten silicates, using for the first time X-ray microtomography. Results indicate that forsterite dissolution in chondrule-like melts is a very fast process with rates that range from ~5 μm min−1 to ~22 μm min−1. Forsterite dissolution strongly depends on the melt composition, with rates decreasing with increasing the magnesium and/or the silica content of the melt. An empirical model based on forsterite saturation and viscosity of the starting melt composition successfully reproduces the forsteritic olivine dissolution rates as a function of temperature and composition for both our experiments and those of the literature. Application of our results to chondrules could explain the textures of zoned type I chondrules during their formation by gas-melt interaction. We show that the olivine/liquid ratio on one hand and the silica entrance from the gas phase (SiOg) into the chondrule melt on the other hand, have counteracting effects on the Mg-rich olivine dissolution behavior. Silica entrance would favor dissolution by maintaining disequilibrium between olivine and melt. Hence, this would explain the preferential dissolution of olivine as well as the preferential abundances of pyroxene at the margins of chondrules. Incipient dissolution would also occur in the silica-poorer melt of chondrule core but should be followed by crystallization of new olivine (overgrowth and/or newly grown crystals). While explaining textures and grain size distributions of olivines, as well as the centripetal distribution of low-Ca pyroxene in porphyritic chondrules, this scenario could also be consistent with the diverse chemical, isotopic, and thermal conditions recorded by olivines in a given chondrule.

¹Lucy F. Lim, ^1,2Richard D. Starr, ^1,3Larry G. Evans, ¹Ann M. Parsons, ⁴Michael E. Zolensky, ⁵William V.Boynton
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12786]
¹NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
²Catholic University of America, Washington, District of Columbia, USA
³Computer Sciences Corporation, Lanham-Seabrook, Maryland, USA
⁴ARES, NASA Johnson Space Center, Houston, Texas, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Published by agreement with John Wiley & Sons

To evaluate the feasibility of measuring differences in bulk composition among carbonaceous meteorite parent bodies from an asteroid or comet orbiter, we present the results of a performance simulation of an orbital gamma-ray spectroscopy (GRS) experiment in a Dawn-like orbit around spherical model asteroids with a range of carbonaceous compositions. The orbital altitude was held equal to the asteroid radius for 4.5 months. Both the asteroid gamma-ray spectrum and the spacecraft background flux were calculated using the MCNPX Monte-Carlo code. GRS is sensitive to depths below the optical surface (to ≈20–50 cm depth depending on material density). This technique can therefore measure underlying compositions beneath a sulfur-depleted (e.g., Nittler et al. 2001) or desiccated surface layer. We find that 3σ uncertainties of under 1 wt% are achievable for H, C, O, Si, S, Fe, and Cl for five carbonaceous meteorite compositions using the heritage Mars Odyssey GRS design in a spacecraft-deck-mounted configuration at the Odyssey end-of-mission energy resolution, FWHM = 5.7 keV at 1332 keV. The calculated compositional uncertainties are smaller than the compositional differences between carbonaceous chondrite subclasses.

¹Boris Reznik, ¹Agnes Kontny,² Jörg Fritz
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12787]
¹Division of Structural Geology and Tectonophysics, Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany
²Saalbau Weltraum Projekt, Heppenheim, Germany
Published by arrangement with John Wiley &Sons

This study demonstrates a relationship between changes of magnetic susceptibility and microstructure developing in minerals of a magnetite-bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Shock-induced effects on magnetic properties were quantified by bulk magnetic susceptibility measurements while shock-induced microstructures were studied by high-resolution scanning electron microscopy. Microstructural changes were compared between magnetite, quartz, amphibole, and biotite grains. In the 5 GPa sample, a sharp drop of magnetic susceptibility correlates with distinct fragmentation as well as with formation of shear bands and twins in magnetite. At 10 GPa, shear bands and twins in magnetite are accompanied by droplet-shaped nanograins. In this shock pressure regime, quartz and amphibole still show intensive grain fragmentation. Twins in quartz and foam-shaped, highly porous amphibole are formed at 20 and 30 GPa. The formation of porous minerals suggests that shock heating of these mineral grains resulted in localized temperature spikes. The identified shock-induced features in magnetite strongly advise that variations in the bulk magnetic susceptibility result from cooperative grain fragmentation, plastic deformation and/or localized amorphization, and probably postshock annealing. In particular, the increasing shock heating at high pressures is assumed to be responsible for a partial defect annealing which we suggest to be responsible for the almost constant values of magnetic susceptibility above 10 GPa.

¹Stefan Loehle, ^2,3Peter Jenniskens, ⁴Hannah Böhrk, ⁵Thomas Bauer, ⁴Henning Elsäβer, ³Derek W. Sears, ⁶Michael E. Zolensky, ⁷Muawia H. Shaddad
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12788DOI: 10.1111/maps.12788]
¹High Enthalpy Flow Diagnostics Group, Institute of Space Systems, Stuttgart, Germany
²SETI Institute, Carl Sagan Center, Mountain View, California, USA
³NASA Ames Research Center, Mountain View, California, USA
⁴DLR, Institute of Structures and Design, Stuttgart, Germany
⁵DLR, Institute of Technical Thermodynamics, Cologne, Germany
⁶ARES, NASA Johnson Space Center, Houston, Texas, USA
⁷Physics Department, University of Khartoum, Khartoum, Sudan
Published by arrangement with John Wiley & Sons

Asteroid 2008 TC3 was characterized in a unique manner prior to impacting Earth’s atmosphere, making its October 7, 2008, impact a suitable field test for or validating the application of high-fidelity re-entry modeling to asteroid entry. The accurate modeling of the behavior of 2008 TC3 during its entry in Earth’s atmosphere requires detailed information about the thermophysical properties of the asteroid’s meteoritic materials at temperatures ranging from room temperature up to the point of ablation (T ~ 1400 K). Here, we present measurements of the thermophysical properties up to these temperatures (in a 1 atm. pressure of argon) for two samples of the Almahata Sitta meteorites from asteroid 2008 TC3: a thick flat-faced ureilite suitably shaped for emissivity measurements and a thin flat-faced EL6 enstatite chondrite suitable for diffusivity measurements. Heat capacity was determined from the elemental composition and density from a 3-D laser scan of the sample. We find that the thermal conductivity of the enstatite chondrite material decreases more gradually as a function of temperature than expected, while the emissivity of the ureilitic material decreases at a rate of 9.5 × 10−5 K−1 above 770 K. The entry scenario is the result of the actual flight path being the boundary to the load the meteorite will be affected with when entering. An accurate heat load prediction depends on the thermophysical properties. Finally, based on these data, the breakup can be calculated accurately leading to a risk assessment for ground damage.

^1,2Roger H. Hewins et al. (>10)*
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12740]
¹Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, Paris, France
²Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
Published by arrangement with John Wiley&Sons
*Find the extensive, full author and affiliation list on the publishers website

Northwest Africa 7533, a polymict Martian breccia, consists of fine-grained clast-laden melt particles and microcrystalline matrix. While both melt and matrix contain medium-grained noritic-monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast-laden melt rocks contain clump-like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene-plagioclase clump-aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic-monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned-pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr-bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti-bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni-poor pyroxene clasts which are from pristine rocks. Clast-laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013) and a 147Sm-143Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014). This record of protolith interaction with atmosphere-hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015), and formation of Ni-bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015).