¹Andrew W. Beck et al. (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
*Find the extensive, full author and affiliation list on the publishers website

Here, we construct a comprehensive howardite, eucrite, and diogenite (HED) bulk chemistry data set to compare with Dawn data. Using the bulk chemistry data set, we determine four gamma-ray/neutron parameters in the HEDs (1) relative fast neutron counts (fast counts), (2) macroscopic thermal neutron absorption cross section (absorption), (3) a high-energy gamma-ray compositional parameter (Cp), and (4) Fe abundance. These correspond to the four measurements of Vesta made by Dawn’s Gamma Ray and Neutron Detector (GRaND) that can be used to discern HED lithologic variability on the Vestan surface. We investigate covariance between fast counts and average atomic mass in the meteorite data set, where a strong correlation (r2 = 0.99) is observed, and we demonstrate that systematic offsets from the fast counttrend are linked to changes in Fe and Ni concentrations. To compare the meteorite and GRaND data, we investigate and report covariance among fast counts, absorption, Cp, and Fe abundance in the HED meteorite data set. We identify several GRaND measurement spaces where the Yamato type B diogenites are distinct from all other HED lithologies, including polymict mixtures. The type B’s are diogenites that are enriched in Fe + pigeonite + diopside ± plagioclase, relative to typical, orthopyroxenitic diogenites. We then compare these results to GRaND data and demonstrate that regions north of ~70°N latitude on Vesta (including the north pole) are consistent with type B diogenites. We propose two models to explain type B diogenite compositions in the north (1) deposition as Rheasilvia ejecta, or (2) type B plutons that were emplaced at shallow depths in the north polar region and sampled by local impacts. Lastly, using principal component (PC) analysis, we identify unique PC spaces for all HED lithologies, indicating that the corresponding GRaND measurables may be used to produce comprehensive lithologic maps for Vesta.

Reference
Andrew W. Beck et al. (2015) Using HED meteorites to interpret neutron and gamma-ray data from asteroid 4 Vesta. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12467]

Published by arrangement with John Wiley&Sons

¹T. K. Croat, ¹C. Floss, ¹B. A. Haas, ²M. J. Burchell, ^2,3A. T. Kearsley
¹Laboratory for Space Sciences and Department of Physics, Washington University, St. Louis, Missouri, USA
²Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, CT2 7NH, UK
³Department of Earth Sciences, Natural History Museum, London, UK

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s−1 with a light-gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less-refractory Si3N4 grains. In small (5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI-like minerals.

Reference
Croat TK, Floss C, Haas BA, Burchell MJ, Kearsley AT (2015) Survival of refractory presolar grain analogs during Stardust-like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12474]

Published by arrangement with John Wiley&Sons

¹Miki Nakajima, ¹David J. Stevenson
¹Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd., MC 150-21, Pasadena, CA 91125, USA

The Earth’s Moon is thought to have formed by an impact between the Earth and an impactor around 4.5 billion years ago. This impact could have been so energetic that it could have mixed and homogenized the Earth’s mantle. However, this view appears to be inconsistent with geochemical studies that suggest that the Earth’s mantle was not mixed by the impact. Another outcome of the impact is that this energetic impact melted the whole mantle, but the extent of mantle melting is not well understood even though it must have had a significant effect on the subsequent evolution of the Earth’s interior and atmosphere. To understand the initial state of the Earth’s mantle, we perform giant impact simulations using smoothed particle hydrodynamics (SPH) for three different models: (a) standard: a Mars-sized impactor hits the proto-Earth, (b) fast-spinning Earth: a small impactor hits a rapidly rotating proto-Earth, and (c) sub-Earths: two half Earth-sized planets collide. We use two types of equations of state (MgSiO3 liquid and forsterite) to describe the Earth’s mantle. We find that the mantle remains unmixed in (a), but it may be mixed in (b) and (c). The extent of mixing is most extensive in (c). Therefore, (a) is most consistent and (c) may be least consistent with the preservation of the mantle heterogeneity, while (b) may fall between. We determine that the Earth’s mantle becomes mostly molten by the impact in all of the models. The choice of the equation of state does not affect these outcomes. Additionally, our results indicate that entropy gains of the mantle materials by a giant impact cannot be predicted well by the Rankine–Hugoniot equations. Moreover, we show that the mantle can remain unmixed on a Moon-forming timescale if it does not become mixed by the impact.

Reference
Nakajima M, Stevenson DJ (2015) Melting and mixing states of the Earth’s mantle after the Moon-forming impact. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.06.023]
Copyright Elsevier

^1,2Yang Chen, ²Youxue Zhang, ²Yang Liu, ³Yunbin Guan, ³John Eiler, ³Edward M. Stolper
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

The concentrations of volatile elements in the moon have important implications for the formation of the earth–moon system. There is currently a debate regarding the water content of the lunar mantle: Authors studying H2O in lunar pyroclastic glass beads and in olivine-hosted melt inclusions in such pyroclastic samples and in plagioclase crystals in lunar highland anorthosites infer hundreds of ppm H2O in the lunar mantle. In contrast, authors studying Zn/Fe ratios infer that the H2O concentration in the lunar mantle is ≤1 ppm≤1 ppm, and they argue that the glassy lunar basalts are a local anomaly. We contribute to a resolution of the debate by a broader examination of the concentrations of H2O and other volatile components in olivine-hosted melt inclusions in a wider range of lunar mare basalts, including crystalline melt inclusions that are homogenized by melting in the laboratory. We find that F, Cl, and S concentrations in various lunar melt inclusions (including those in glassy lunar basalts) are similar to one another, and previously studied glassy lunar basalts are not a local anomaly in terms of these volatile concentrations. Furthermore, we estimate the pre-degassing H2O/Ce, F/Nd, and S/Dy ratios of mare basaltic magmas to be at least 64, 4.0 and 100 respectively. These ratios are lower than those of primitive earth mantle by a factor of 3, 5, and 4 respectively. The depletion factors of these volatile elements relative to the earth’s primitive mantle do not correlate strongly with volatility or bonding energy, and indeed they are roughly constant and similar to those of other volatile elements such as Li, Cs, Rb and K. This approximate constancy of volatile depletion in the moon relative to the earth can be explained by assuming that both the earth and the moon acquired volatiles from a similar source or by a similar mechanism but the earth was more efficient in acquiring the volatiles. We estimate the H2O, F and S concentrations in the primitive lunar mantle source to be at least 110, 5.3, and 70 ppm, respectively – similar to or slightly lower than those in terrestrial MORB mantle.

Reference
Chena Y, Zhang Y, Liu Y, Guan Y, Eiler J, Stolper EM (2015) Water, fluorine, and sulfur concentrations in the lunar mantle. Earth and Planetary Science Letters 427, 37–46
Link to Article [doi:10.1016/j.epsl.2015.06.046]
Copyright Elsevier

¹P. Beck et al. (>10)
¹Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France
*Find the extensive, full author and affiliation list on the publishers website

The Martian surface is covered by a fine-layer of oxidized dust responsible for its red color in the visible spectral range (Bibring et al., 2006 and Morris et al., 2006). In the near infrared, the strongest spectral feature is located between 2.6 and 3.6 μm and is ubiquitously observed on the planet (Jouglet et al., 2007 and Milliken et al., 2007). Although this absorption has been studied for many decades, its exact attribution and its geological and climatic implications remain debated. We present new lines of evidence from laboratory experiments, orbital and landed missions data, and characterization of the unique Martian meteorite NWA 7533, all converging toward the prominent role of hydroxylated ferric minerals. Martian breccias (so-called “Black Beauty” meteorite NWA7034 and its paired stones NWA7533 and NWA 7455) are unique pieces of the Martian surface that display abundant evidence of aqueous alteration that occurred on their parent planet (Agee et al., 2013). These dark stones are also unique in the fact that they arose from a near surface level in the Noachian southern hemisphere (Humayun et al., 2013). We used IR spectroscopy, Fe-XANES and petrography to identify the mineral hosts of hydrogen in NWA 7533 and compare them with observations of the Martian surface and results of laboratory experiments. The spectrum of NWA 7533 does not show mafic mineral absorptions, making its definite identification difficult through NIR remote sensing mapping. However, its spectra are virtually consistent with a large fraction of the Martian highlands. Abundant NWA 7034/7533 (and paired samples) lithologies might abound on Mars and might play a role in the dust production mechanism.

Reference
Beck P et al. (2015) A Noachian source region for the “Black Beauty” meteorite, and a source lithology for Mars surface hydrated dust? Earth and Planetary Science Letters 427, 104–111
Link to Article [doi:10.1016/j.epsl.2015.06.033]

Copyright Elsevier

^1,2Joseph R. Michalski, ¹Javier Cuadros, ³Janice L. Bishop, ⁴M. Darby Dyar, ⁵Vesselin Dekov, ⁶Saverio Fiore
¹Dept. of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
²Planetary Science Institute, Tucson, AZ, USA
³SETI Institute, Mountain View, CA, USA
⁴Mount Holyoke College, South Hadley, MA, USA
⁵Département Géosciences Marines, IFREMER, Plouzané, France
⁶University of Bari, Bari, Italy

Near-infrared remote sensing data of Mars have revealed thousands of ancient deposits of Fe/Mg-rich smectitic clay minerals within the crust with relevance to past habitability. Diagnostic metal–OH infrared spectroscopic absorptions used to interpret the mineralogy of these phyllosilicates occur at wavelengths of 2.27–2.32 μm, indicating variable Fe/Mg ratios in the clay structures. The objective of this work is to use these near infrared absorptions to constrain the mineralogy of smectites on Mars. Using Fe/Mg-rich seafloor clay minerals as mineralogical and spectroscopic analogs for Martian clay minerals, we show how crystal–chemical substitution and mixed layering affect the position of the diagnostic metal–OH spectral feature in smectitic clay minerals. Crystal-chemistry of smectites detected on Mars were quantitatively constrained with infrared data and categorized into four mineralogical groups. Possible alteration processes are constrained by comparisons of clay chemistry detected by remote sensing techniques to the chemistry of candidate protoliths. Of the four groups identified, three of them indicate significant segregation of Fe from Mg, suggestive of alteration under water-rich and/or oxidizing conditions on Mars. The fourth group (with low Fe/Mg ratios) may result from alteration in reducing or water-limited conditions, potentially in subsurface environments. Some samples are interstratified di–trioctahedral clay minerals that have characteristics of dioctahedral clay minerals but clear chemical evidence for trioctahedral sheets. Approximately 70% of smectite deposits previously detected on Mars are classified as Fe-rich (FeO/MgO > 10). Only 22% of detections are trioctahedral and relatively Mg-rich. An additional ∼8% are difficult to characterize, but might be very Fe-rich. The segregation of Fe from Mg in Martian clay minerals suggests that Mg should be enriched in other contemporaneous deposits such as chlorides and carbonates.

Reference
Michalski JR, Cuadros J, Bishop JL, Dyar MD, Dekov V, Fiore S (2015) Constraints on the crystal-chemistry of Fe/Mg-rich smectitic clays on Mars and links to global alteration trends. Earth and Planetary Science Letters 427, 215–225.
Link to Article [doi:10.1016/j.epsl.2015.06.020]

Copyright Elsevier

¹Nicolas Dauphas, ²Franck Poitrasson, ¹Christoph Burkhardt, ³Hiroshi Kobayashi, ⁴Kosuke Kurosawa
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60615, USA
²Laboratoire Géosciences Environnement Toulouse, CNRS UMR 5563 – UPS – IRD, 14-16, avenue Edouard Belin, 31400 Toulouse, France
³Department of Physics, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
⁴Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth’s mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe–Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at ∼1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of ∼1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth’s core of 3.6 (+6.0/−3.6) wt%(+6.0/−3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

Reference
Dauphas N, Poitrasson F, Burkhardt C, Kobayashi H, Kurosawa K (2015) Planetary and meteoritic Mg/Si and δ30Siδ30Si variations inherited from solar nebula chemistry. Earth and Planetary Science Letters 427, 236–248
Link to Article [doi:10.1016/j.epsl.2015.07.008]
Copyright Elsevier

¹Maximilian Matthes, ¹Mario Fischer-Gödde, ¹Thomas S. Kruijer, ²Ingo Leya, ¹Thorsten Kleine
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Space Research and Planetology, University of Bern, Bern, Switzerland

The short-lived 107Pd-107Ag system is a versatile tool for dating iron meteorites, but neutron capture reactions during cosmic ray-exposure might have modified Ag isotope compositions. These cosmic ray-induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and, therefore, could bias the age information inferred from Pd-Ag isotope systematics. Our new combined Pd-Ag and Pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray-induced downward shifts of 107Ag/109Ag, which preclude the determination of Pd-Ag isochrons based on measured Ag isotope compositions. For the strongly irradiated iron meteorites Ainsworth (IIAB) and Carbo (IID) these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from 107Pd-decay. For the less strongly irradiated IIIAB iron meteorites Boxhole, Grant and Henbury, the cosmic ray-induced shifts are smaller than the radiogenic 107Ag excesses, but are nevertheless significant. We have developed a method to quantify the cosmic ray-induced Ag isotope shifts using a neutron capture model and Pt isotope compositions as the neutron dose monitor. After correction, Pd-Ag isochrons are obtained for all investigated iron meteorites, even for the most strongly irradiated samples. The Pd-Ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites, indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray-induced shifts on measured Ag isotope compositions. The Pd-Ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few Ma after core formation and solar system formation. Such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies. The collisions would have facilitated rapid cooling below Pd-Ag isotopic closure and so in this case the Pd-Ag ages would effectively date the time of the collisions.
Reference
Matthes M, Fischer-Gödde M, Kruijer TS, Leya I, Kleine T (2015) Pd-Ag chronometry of iron meteorites: correction of neutron capture-effects and application to the cooling history of differentiated protoplanets. Geochimica et Cosmochimica Acta (in Press).
Link to Article [doi:10.1016/j.gca.2015.07.027]
Copyright Elsevier

¹A. M. Jellinek, ²M. G. Jackson
¹Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, V6T 1Z4 Canada
²Department of Earth Science, University of California Santa Barbara, Santa Barbara, California 93106-9630, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Jellinek AM, Jackson MG (2015) Connections between the bulk composition, geodynamics and habitability of Earth. Nature Geoscience (in Press)
Link to Article [doi:10.1038/ngeo2488]

^1,2,3Michael K. Weisberg, ^2,3Denton S. Ebel, ^4,5Daisuke Nakashima, ⁴Noriko T. Kita, ⁶Munir Humayun
¹Department of Physical Sciences, Kingsborough Community College, City University New York, Brooklyn, NY 11235
²Department of Earth and Environmental Sciences, Graduate Center, City University New York, New York, NY 10016
³Department of Earth & Planetary Sciences, American Museum of Natural History, NY, NY 10024
⁴WiscSIMS, Department of Geosciences, University of Wisconsin-Madison, WI 53706
⁵Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
⁶Department of Earth, Ocean & Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310

Northwest Africa (NWA) 5492 and Grosvenor Mountains (GRO) 95551 are metal-rich chondrites having silicate (olivine and pyroxene) compositions that are more reduced than those in other metal-rich chondrites, such as the CH and CB chondrites. Additionally, sulfides in NWA 5492 and GRO 95551 are more abundant and not related to the metal, as in the CB chondrites. Average metal compositions in NWA 5492 and GRO 95551 are close to H chondrite metal. Oxygen isotope ratios of NWA 5492 and GRO 95551 components (chondrules and fragments) show a range of compositions with most having Δ17O values > 0 ‰. Since there is no matrix component, their average chondrule + fragment oxygen isotopic compositions are considered to be representative of whole rock and are sandwiched between the values for enstatite (E) and ordinary (O) chondrites. These data argue for a close relationship between NWA 5492 and GRO 95551 and suggest that they are the first examples of a new type of metal-rich chondrite.
Oxygen isotope ratios of chondrules in NWA 5492 and GRO 95551 show considerable overlap with chondrules in O, E and R chondrites, with average compositions indistinguishable from LL3 chondrules, suggesting considerable mixing between these Solar System materials during chondrule formation and/or that their precursors experienced similar formation environments and/or processes. Another characteristic shared between NWA 5492 and GRO 95551 and O, E and R chondrites is that they are all relatively dry (low abundances of hydrated minerals), compared to many C chondrites and have fewer, smaller CAIs than many C chondrites. (No CAIs were found in NWA 5492 or GRO 95551 but they contain rare Al-rich chondrules.) We suggest that O, E, R and the NWA 5492 and GRO 95551 chondrites are closely related Solar System materials.

Reference
Weisberg MK, Ebel DS, Nakashima D, Kita NT, Humayun M (2015) Petrology and geochemistry of chondrules and metal in nwa 5492 and gro 95551: a new type of metal-rich chondrite. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.021]

Copyright Elsevier