¹John T. Wasson,
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.043]
¹Department of Earth, Planetary and Space Sciences and Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1567, USA
Copyright Elsevier

Instrumental neutron-activation (INAA) data for metal in 22 nonmagmatic IIE meteorites show narrow ranges in Ir and other refractory siderophiles; the Ir range is a factor of 2.6, a factor of ∼2 smaller than in nonmagmatic IAB-MG, and orders of magnitude smaller than in the large magmatic groups. Siderophile data show no evidence of fractional crystallization. IIE irons can be split into two sets, a larger main-set and a set of 6 Cu- (or FeS) rich irons. Elemental concentrations in metal from veins in H5 chondrite Portales Valley fall within the IIE range with the exceptions of Co (high) and Ga (low).H-group-chondrite and Au-normalized IIE abundances for siderophiles show that IIE irons are ∼30% higher than H in refractory siderophiles Re, Ir and W and are about 30% lower than H chondrites in the volatiles Ga and Sb, inconsistent with proposals that IIE irons formed from H chondrites. The IIE fractionations contrast with those in L chondrites which are about 15% lower than H in the three refractory elements and 40% higher than H in volatiles indicating that IIE irons did not form from H chondrites but from a more reduced and siderophile-rich kind of ordinary chondrite (“HH” chondrites). Most O-isotope data support a close relationship between IIE irons and H or HH chondrites; lower Δ17O in primitive (chondritic) silicates support an HH classification. Literature isotopic data for Ru and Mo also show that IIE metal formed from an ordinary chondrite parent; it appears that the silicates and metal were formed by melting of a single asteroid. There is no evidence for radiogenic (26Al) heating; this, the rapid cooling recorded in the sizes of parental gamma crystal in the metal and the absence of fractional crystallization strongly support the hypothesis that IIE melting was the result of impacts.To summarize, the weight of the evidence favors the conclusion that IIE meteorites were formed by one or more impacts on an HH asteroid. The target probably had a composition like the chondritic materials in Netschaevo, but was unequilibrated and had much higher porosity

^1,2David G. Weisz, ²Benjamin Jacobsen, ²Naomi E. Marks, ²Kim B. Knight, ²Brett H. Isselhardt, ²Jennifer E. Matzel, ²Peter K. Weber, ¹Stan G. Prussin, ²Ian D. Hutcheon
Geochimica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.036]
¹Department of Nuclear Engineering, University of California, Berkeley, 4113 Etcheverry Hall MC 1730, Berkeley, CA 94720, USA
²Glenn T. Seaborg Institute, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
Copyright Elsevier

In a near-surface nuclear explosion where the resultant fireball can interact with materials from the surface, vaporized materials from the nuclear device can be incorporated into molten soil and other carrier materials from that surface. This mixed material becomes a source of glassy fallout upon quenching and is locally deposited. Fallout formation models have been proposed; however, the specific mechanisms and physical conditions by which soil and other carrier materials interact in the fireball, as well as the subsequent incorporation of device materials with carrier materials, are not well constrained. We observe a surface deposition layer preserved at interfaces where two aerodynamic fallout glasses agglomerated and fused, and characterized 11 such boundaries using spatial analyses to better understand the vaporization and condensation behavior of species in the fireball. Using nanoscale secondary ion mass spectrometry (NanoSIMS), we identify higher enrichments of uranium from the device (235U/238U ratio >7.5) in 8 of the interface layers. Major element analysis of the interfaces reveals the deposition layer to be enriched in Fe, Ca, Mg, Mn, and Na-bearing species and depleted in Ti and Al-bearing species. Most notably, the Fe and Ca-bearing species are enriched approximately 50% at the interface layer relative to the average concentrations measured within the fallout glasses, while Ti and Al-bearing species are depleted by approximately 20%. SiO2 is found to be relatively invariable across the samples and interfaces (∼3% standard deviation). The notable depletion of Al, a refractory oxide abundant in the soil, together with the enrichment of 235U and Fe, suggests an anthropogenic source of the enriched species or an unexpected vaporization/condensation behavior. The presence of both refractory (e.g., Ca and U) and volatile (e.g., Na) species approximately co-located in most of the observed layers (within 1.5 μm) suggests a continuous condensation process may also be occurring. These fallout formation processes deviate from historical models of fallout formation, and have not been previously recognized in the literature.

^1,2Kun Wang, ³Stein B. Jacobsen
Nature 538, 487–490 Link to Article [doi:10.1038/nature19341]
¹Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA
²Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA
³Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA

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

¹Steven J. Jaret,¹Brian L. Phillips,²David T. King Jr,¹Tim D. Glotch,³Zia Rahman,⁴Shawn P. Wright
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12745]
¹Department of Geosciences, Stony Brook University, Stony Brook, New York, USA
²Department of Geosciences, Auburn University, Auburn, Alabama, USA
³Jacobs—NASA Johnson Space Center, Houston, Texas, USA
⁴Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

Coesite has been identified within ejected blocks of shocked basalt at Lonar crater, India. This is the first report of coesite from the Lonar crater. Coesite occurs within SiO2 glass as distinct ~30 μm spherical aggregates of “granular coesite” identifiable both with optical petrography and with micro-Raman spectroscopy. The coesite+glass occurs only within former silica amygdules, which is also the first report of high-pressure polymorphs forming from a shocked secondary mineral. Detailed petrography and NMR spectroscopy suggest that the coesite crystallized directly from a localized SiO2 melt, as the result of complex interactions between the shock wave and these vesicle fillings.

¹Ming-Chang Liu
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.011]
¹Department of Earth, Planetary, and Space Sciences, UCLA
Copyright Elsevier

This paper reports new 41Ca-41K isotopic data for two Type A CAIs, NWA 3118 #1Nb (Compact Type A) and Vigarano 3138 F8 (Fluffy Type A), from reduced CV3 chondrites. The NWA CAI is found to have carried live 41Ca at the level of (4.6±1.9)×10-9(4.6±1.9)×10-9, consistent with the proposed Solar System initial 41Ca/40Ca = 4.2×10-94.2×10-9 by Liu et al. (2012). On the other hand, the Vigarano CAI does not have resolvable radiogenic 41K excesses that can be attributed to the decay of 41Ca. Combined with the 26Al data that have been reported for these two CAIs, we infer that the 41Ca distribution was not homogeneous when 26Al was widespread at the canonical level of 26Al/27Al = 5.2×10-55.2×10-5. Such a 41Ca heterogeneity can be understood under two astrophysical contexts: in-situ charged particle irradiation by the protoSun in the solar nebula that had inherited some baseline 10Be abundance from the molecular cloud, and Solar System formation in a molecular cloud enriched in 26Al and 41Ca contaminated by massive star winds. That said, more high quality 41Ca data are still needed to better understand the origin of this radionuclide.

¹B. E. Strauss,¹J. M. Feinberg,^2,3C. L. Johnson
Journal of Geophysical Research, Planets Link to Article [DOI: 10.1002/2016JE005054]
¹Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA
²Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, VancouverBC, Canada
³Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

Mercury and Earth are the only inner solar system planets with active, internally generated dynamo magnetic fields. The MESSENGER mission recently detected magnetic fields on Mercury that are consistent with lithospheric magnetization. We investigate the physical and chemical environment of Mercury’s lithosphere, past and present, to establish the conditions under which magnetization may have been acquired and modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: redox conditions in the planet’s crust and mantle, the iron content of the lithosphere, and, for any remanent magnetization, the temperature profile of the lithosphere and its evolution over time. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is both hot and highly reducing. The long-term thermal history of Mercury’s crust plays an important role in the longevity of any remanent crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework is used both to constrain plausible candidate minerals that could carry magnetic remanence on Mercury and to evaluate their capacity to acquire and retain sufficient magnetization to be detectable from satellite orbit. We propose that iron metal and its alloys are likely to be the dominant contributors to induced and remanent magnetization in Mercury’s lithosphere, with additional contributions from iron silicides, sulfides, and carbides.

¹Jorge A.G. Davalos, ¹Jorge Márcio Carvano, ²Julio Blancob
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.10.022]
¹Observatório Nacional, rua Gal. José Cristino 77, São Cristóvão, 20921-400, Rio de Janeiro, Brazil
²Observatorio Astronómico del Instituto de Profesores Artigas, Avenida del Libertador 2025, Montevideo, Uruguay
Copyright Elsevier

Radiative transfer models in particulate media (Hapke, 1981 and Hapke, 1993; 2012b; Shkuratov et al., 1999) are the most versatile tool that can be used to retrieve both composition and surface physical properties from observation of asteroids and other atmosphereless bodies of the Solar System. One caveat is that these methods require as input a sufficiently comprehensive set of optical constants of suitable template materials. These optical constants are the real and imaginary parts of the refractive indexes of the material as function of wavelength, and have to be derived from laboratory measurements of samples of minerals and meteorites. Optical constants can be calculated from a variety of types of measurements, and each has its problems and limitations. In particular, a problem with the determination of optical constants from measurement of reflectance is that the measurements need to be themselves interpreted using radiative transfer models. This is an issue because the number of parameters used in the most accurate versions of the radiative transfer models is large, and for most of the samples many of these parameters were not measured independently. As a result, attempts in the literature to retrieve optical constants from reflectance measurements tend to assume values for the unknown parameters, which can lead to uncertainties in the retrieved optical constants that can be difficult to quantify. In this work we propose a numerical method that allows the simultaneous inversion of the optical constant and the model parameters. This model is then applied to a set of reflectance spectra of 5 HED meteorites from the RELAB database that were measured with the same setup for samples with several particle size intervals. Our results indicate that our method is able to retrieve optical constants which are able to reproduce the measured reflectance of the samples over a large range (View the MathML source25−500μm) of particle diameters. It is also found that the solutions obtained in this way are non-unique, in the sense that many combination of the model parameters can yield different sets of optical constants that fit equally well the reflectance spectra. Thus, in the absence of the independent determination of at least some of the model parameter the method is unable to decide which solution correspond to the physical optical constants of the materials. Even so, the dispersion of the model parameters (in particular effective particle diameter and porosity) for acceptable solutions (defined as the ones that reproduce the measured reflectance spectra at all size range with residues smaller than 10%) is within a radius of around 30% of the value of the best fit solution for each parameter. Given the ability of the the optical constants derived with this method to reproduce the sample spectra over a large range of particle sizes, they can be used without other restriction to assess if a given meteorite assemblage is contributing to the observed spectra of asteroids. However, quantitative informations that can also be derived using these optical constants, like particle sizes, porosity and volumetric fractions of each end-member in a mixture should be regarded only as rough estimates.

¹Vicki Darlington,^1,2Tom Blenkinsop,¹Paul Dirks,³Jess Salisbury,³Andrew Tomkins
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12734]
¹College of Science and Engineering, James Cook University, Townsville, Queensland, Australia
²School of Earth and Ocean Science, Cardiff University, Cardiff, UK
³School of Geosciences, Monash University, Melbourne, Victoria, Australia
Published by arrangement with John Wiley & Sons

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. 40Ar-39Ar dating of impact-related melt particles provides a time of impact in the Ordovician (472 ± 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low-strength, porous, and water-saturated, with all three properties affecting the crater morphology. The water-saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid-Ordovician (470 ± 6 Ma).

¹E. Chassefière,^2,3J. Lasue,⁴B. Langlais,⁵Y. Quesnel
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12784]
¹GEOPS, Univ. Paris-Sud, CNRS, Universite Paris-Saclay, Rue du Belvedere, Bat. 504-509, 91405 Orsay, France
²Universite de Toulouse, UPS-OMP, IRAP, Toulouse, France
³CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France
⁴LPG-CNRS & Universite de Nantes, Nantes, France
⁵Aix-Marseille Universite, CNRS, IRD, CEREGE UM34, Aix-en-Provence, France
Published by arrangement with John Wiley & Sons

CH4 has been observed on Mars both by remote sensing and in situ during the past 15 yr. It could have been produced by early Mars serpentinization processes that could also explain the observed Martian remanent magnetic field. Assuming a cold early Mars, a cryosphere could trap such CH4 as clathrates in stable form at depth. The maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 × 1019 to 2 × 1020 moles of methane. We estimate how large amounts of serpentinization-derived CH4 stored in the cryosphere have been released into the atmosphere during the Noachian and the early Hesperian. Due to rapid clathrate dissociation and photochemical conversion of CH4 to H2, these episodes of massive CH4 release may have resulted in transient H2-rich atmospheres, at typical levels of 10–20% in a background 1–2 bar CO2 atmosphere. The collision-induced heating effect of H2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point. We show how local and rapid destabilization of the cryosphere can be induced by large events (such as the Hellas Basin or Tharsis bulge formation) and lead to such releases. Our results show that the early Mars cryosphere had a sufficient CH4 storage capacity to have maintained H2-rich transient atmospheres during a total time period up to several million years or tens of million years, having potentially contributed to the formation of valley networks during the Noachian/early Hesperian.

^1,2Ruslan A. Mendybaev, ³Curtis D. Williams, ⁴Michael J. Spicuzza, ^1,2Frank M. Richter, ⁴John W. Valley, ^1,2Alexei V. Fedkin, ³Meenakshi Wadhwa
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.034]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, United States
²Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL 60637, United States
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States
⁴Department of Geoscience, University of Wisconsin, Madison, WI 53706, United States
Copyright Elsevier

We present the results of laboratory experiments in which a forsterite-rich melt estimated to be a potential precursor of Allende CMS-1 FUN CAI was evaporated into vacuum for different lengths of time at 1900°C. The evaporation of this melt resulted in residues that define trajectories in chemical as well as magnesium, silicon and oxygen isotopic composition space and come very close to the measured properties of CMS-1. The isotopic composition of the evaporation residues was also used to determine the kinetic isotopic fractionation factors [α2,1 (vapor-melt) defined as the ratio of isotopes 2 and 1 of a given element in the evaporating gas divided by their ratio in the evaporating source] for evaporation of magnesium (α25,24 for 25Mg/24Mg), silicon (α29,28 for 29Si/28Si) and oxygen (α18,16 for 18O/16O) from the forsterite-rich melt at 1900°C. The values of α25,24 = 0.98383±0.00033 and α29,28 = 0.99010±0.00038 are essentially independent of change in the melt composition as evaporation proceeds. In contrast, α18,16 changes from 0.9815±0.0016 to ∼ 0.9911 when the residual melt composition changes from forsteritic to melilitic. Using the determined values of α25,24 and α29,28 and present-day bulk chemical composition of the CMS-1, the composition of the precursor of the inclusion was estimated to be close to the clinopyroxene+spinel+forsterite assemblage condensed from a solar composition gas. The correspondence between the chemical composition and isotopic fractionation of experimental evaporation residues and the present-day bulk chemical and isotopic compositions of CMS-1 is evidence that evaporation played a major role in the chemical evolution of CMS-1.