¹S. W. Lehner,²P. Németh,^3,4M. I. Petaev,¹P. R. Buseck
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12940]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary
³Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
⁴Solar, Stellar, and Planetary Sciences, Harvard-Smithsonian CfA, Cambridge, Massachusetts, USA
Published by arrangement with John Wiley & Sons

Two new occurrences of porous, S-bearing, amorphous silica are described within metal-sulfide nodules (MSN) and as interchondrule patches in EH3 chondrites SAH 97072 and ALH 84170. This porous amorphous material, which was first reported from sulfide-bearing chondrules, consists of sinewy SiO2-rich areas containing S with minor Na or Ca as well as Fe, Mg, and Al. Some pores contain minerals including pyrite, pyrrhotite, and anhydrite. Most pores appear vacant or contain unidentified material that is unstable under analytical conditions. Niningerite, olivine, enstatite, albite, and kumdykolite occur enclosed within porous silica patches. Porous silica is commonly interfingered with cristobalite suggesting its amorphous structure resulted from high-temperature quenching. We interpret the S-bearing porous silica to be a product of silicate sulfidation, and the Na, Ca, Fe, Mg, and Al detectable within this material are chemical residues of sulfidized silicates and metal. The occurrence of porous silica in the cores of MSN, which are considered to be pre-accretionary objects, suggests the sulfidizing conditions occurred prior to final parent-body solidification. Ubiquitous S-bearing porous silica among sulfide-bearing chondrules, MSN, and in the interchondrule clastic matrix, suggests that similar sulfidizing conditions affected all the constituents of these EH3 chondrites.

¹Jérome Gattacceca,^2,3Agata M. Krzesińska,⁴Yves Marrocchi,⁵Matthias M. M. Meier,⁶Michèle Bourot-Denise,⁷Rob Lenssen
Meteoritics & Planetary Science (in Press) Link top Article [DOI: 10.1111/maps.12942]
¹CNRS, Aix-Marseille Univ, IRD, Coll France, CEREGE, Aix-en-Provence, France
²Department of Earth Sciences, Natural History Museum, London, UK
³Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
⁴CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, France
⁵ETH Zurich, Institute of Geochemistry and Petrology, Zurich, Switzerland
⁶IMPMC, MNHN, UPMC, UMR CNRS 7590, Paris, France
⁷Private meteorite collector, The Netherlands
Published by agreement with John Wiley & Sons

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm3 L4 clast. The L clast was incorporated in the breccia through a nondestructive, low-velocity impact. Identical cosmic-ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th-He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.

¹S. P. Beard, ¹T. D. Swindle
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12939]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

We use cosmic-ray exposure (CRE) ages of ureilites, combined with magnesium numbers of olivine, and oxygen isotopes, to search for evidence of specific source events initiating exposure for groups of ureilites. This technique can also be used to investigate the heterogeneity of the body from which the samples were derived. There are a total of 39 ureilites included in our work, which represents the largest collection of ureilite CRE age data used to date. Although we find some evidence of possible clusters, it is clear that most ureilites did not originate in one or two events on a homogeneous parent body.

¹Tasha L. Dunn, ^2,3,4Juliane Gross
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12946]
¹Department of Geology, Colby College, Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁴Lunar and Planetary Institute, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

The single parent body model for the CV and CK chondrites (Greenwood et al. 2010) was challenged by Dunn et al. (2016a), who argued that magnetite compositions could not be reconciled by a single metamorphic sequence (i.e., CV3 → CK3 → CK4–6). Cr isotopic compositions, which are distinguishable between the CV and CK chondrites, also support two different parent bodies (Yin et al. 2017). Despite this, there are many petrographic and mineralogical similarities between the unequilibrated (petrologic type 3) CK chondrites and the CV chondrites (also type 3), which may result in misclassification of samples. Hart and Northwest Africa 6047 (NWA 6047) are an excellent example of this. In this study, we revisit the classification of Hart and NWA 6047 using magnetite compositions, petrography, and compositions of olivine, the most ubiquitous mineral in both CV and CK chondrites. Not only do our results suggest that NWA 6047 and Hart were misclassified, but our assessment of CV and CK3 chondrites has also led to the development of criteria that can be used to distinguish between CV and CK3 chondrites. These criteria include: abundances of Cr₂O₃, TiO₂, NiO, and Al₂O₃ in magnetite; Fa content and NiO abundance of matrix olivine; FeO content of chondrules; and the chondrule:matrix ratio. Classification as a CV chondrite is also supported by the presence of igneous chondrule rims, calcium-aluminum-rich inclusions, and an elongated petrofabric. However, none of these petrographic characteristics can be used conclusively to distinguish between CV and CK3 chondrites.

H. McSween Jr. et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12947]
¹Department of Earth & Planetary Sciences and Planetary Science Institute, University of Tennessee, Knoxville, Tennessee, USA
Published by arrangement with John Wiley & Sons

The mineralogy and geochemistry of Ceres, as constrained by Dawn’s instruments, are broadly consistent with a carbonaceous chondrite (CM/CI) bulk composition. Differences explainable by Ceres’s more advanced alteration include the formation of Mg-rich serpentine and ammoniated clay; a greater proportion of carbonate and lesser organic matter; amounts of magnetite, sulfide, and carbon that could act as spectral darkening agents; and partial fractionation of water ice and silicates in the interior and regolith. Ceres is not spectrally unique, but is similar to a few other C-class asteroids, which may also have suffered extensive alteration. All these bodies are among the largest carbonaceous chondrite asteroids, and they orbit in the same part of the Main Belt. Thus, the degree of alteration is apparently related to the size of the body. Although the ammonia now incorporated into clay likely condensed in the outer nebula, we cannot presently determine whether Ceres itself formed in the outer solar system and migrated inward or was assembled within the Main Belt, along with other carbonaceous chondrite bodies.

¹Masaaki Miyahara, ²Eiji Ohtani, ^3,4Akira Yamaguchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.034]
¹Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
²Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
³National Institute of Polar Research, Tokyo 190-8518, Japan
⁴Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo 190-8518, Japan
Copyright Elsevier

An impact event recorded in the Northwest Africa (NWA) 8275 LL7 ordinary chondrite was investigated based on high-pressure mineralogy of pervasive shock-melt veins present in the rock. NWA 8275 consists of olivine, low-Ca pyroxene, plagioclase (albite–oligoclase composition), and minor high-Ca pyroxene, K-feldspar, phosphate minerals, metallic Fe–Ni and iron sulfide. Plagioclase and K-feldspar grains near the shock-melt veins have transformed to amorphous, although no high-pressure polymorphs of olivine and pyroxene were identified in or adjacent the shock-melt veins. Raman spectroscopy and focused ion beam (FIB)-assisted transmission electron microscopy (TEM) observations reveal that plagioclase entrained around the center portion of the shock-melt veins has dissociated into a jadeite + coesite assemblage. Alternately stacked jadeite and coesite crystals occurred in the original plagioclase. On approaching the host rock/shock-melt vein, only jadeite is present. Based on the high-pressure polymorph assemblage, the shock pressure and temperature conditions recorded in the shock-melt veins are ∼3–12 GPa and ∼1973–2373 K, respectively. Following a Rankine–Hugoniot relationship, the impact velocity was at least ∼0.45–1.54 km/s. The duration of high-pressure and high-temperature (HPHT) conditions required for the albite dissociation reaction is estimated a maximum of ∼4–5 s using the phase transition rate of albite, implying that a body of up to ∼9–12 km across collided with the parent body of NWA 8275. The coexistence of jadeite and coesite, the latter of which rarely accompanies jadeite in shocked ordinary chondrites, as a dissociation product of albite requires relatively long duration HPHT conditions. Thus, the impact event recorded in NWA 8275 was likely caused by a larger-than-typical projectile.

¹Peter Hoppe, ¹Jan Leitner, ¹János Kodolányi
Nature Astronomy (in Press) Link to Article [doi:10.1038/s41550-017-0215-0]
¹Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128, Mainz, Germany

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

H.B.Franz et al. (>10)
Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo3002]
¹NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

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

^1,2Haiyang Wang, ^1,2,3Charles H. Lineweaver, ^2,3Trevor R. Ireland
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.024]
¹Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
²Planetary Science Institute, The Australian National University, Canberra, ACT 2611, Australia
³Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
Copyright Elsevier

To first order, the Earth as well as other rocky planets in the Solar System and rocky exoplanets orbiting other stars, are refractory pieces of the stellar nebula out of which they formed. To estimate the chemical composition of rocky exoplanets based on their stellar hosts’ elemental abundances, we need a better understanding of the devolatilization that produced the Earth. To quantify the chemical relationships between the Earth, the Sun and other bodies in the Solar System, the elemental abundances of the bulk Earth are required. The key to comparing Earth’s composition with those of other objects is to have a determination of the bulk composition with an appropriate estimate of uncertainties. Here we present concordance estimates (with uncertainties) of the elemental abundances of the bulk Earth, which can be used in such studies. First we compile, combine and renormalize a large set of heterogeneous literature values of the primitive mantle (PM) and of the core. We then integrate standard radial density profiles of the Earth and renormalize them to the current best estimate for the mass of the Earth. Using estimates of the uncertainties in i) the density profiles, ii) the core-mantle boundary and iii) the inner core boundary, we employ standard error propagation to obtain a core mass fraction of 32.5 ± 0.3 wt%. Our bulk Earth abundances are the weighted sum of our concordance core abundances and concordance PM abundances. Unlike previous efforts, the uncertainty on the core mass fraction is propagated to the uncertainties on the bulk Earth elemental abundances. Our concordance estimates for the abundances of Mg, Sn, Br, B, Cd and Be are significantly lower than previous estimates of the bulk Earth. Our concordance estimates for the abundances of Na, K, Cl, Zn, Sr, F, Ga, Rb, Nb, Gd, Ta, He, Ar, and Kr are significantly higher. The uncertainties on our elemental abundances usefully calibrate the unresolved discrepancies between standard Earth models under various geochemical and geophysical assumptions.

¹William C. Tucker, ¹Abrar H. Quadery, ¹Alfons Schulte, ^1,3,4Richard G. Blair, ¹William E. Kaden, ^1,2Patrick K. Schelling, ¹Daniel T. Britt
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.027]
¹Department of Physics, University of Central Florida, Orlando, FL 32816-2385, USA
²Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32804, USA
³Cluster for the Rational Design of Catalysts for Energy Applications and Propulsion, University of Central Florida, Orlando, FL 32816, USA
⁴Center for Advanced Turbomachinery and Energy Research, University of Central Florida, Orlando, FL 32816, USA
Copyright Elsevier

It is demonstrated that olivine powders heated to subsolidus temperatures in reducing conditions can develop significant concentrations of  10-50 nm diameter Fe nanoparticles on grain surfaces and that these display strong catalytic activity not observed in powders without Fe nanoparticles. Reduced surfaces were exposed to NH₃, CO, and H₂, volatiles that may be present on the surfaces of comet and volatile-rich asteroids. In the case of NH₃ exposure, rapid decomposition was observed. When exposed to a mixture of CO and H₂, significant coking of the mineral surfaces occurred. Analysis of the mineral grains after reaction indicated primarily the presence of graphene or graphitic carbon. The results demonstrate that strong chemical activity can be expected at powders that contain nanophase Fe particles. This suggests space-weathered mineral surfaces may play an important role in the synthesis and processing of organic species. This processing may be part of the weathering processes of volatile-rich but atmosphereless solar-system bodies.