¹P.G.Conrad et al. (>10*)
Earth and Planetary Science Letters 454, 1-9 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.08.028]
¹Goddard Space Flight Center, Greenbelt, MD 20771, USA
Copyright Elsevier
*Find the extensive, full author and affiliation list on the publishers website

Mars Science Laboratory’s Sample Analysis at Mars (SAM) investigation has measured all of the stable isotopes of the heavy noble gases krypton and xenon in the martian atmosphere, in situ, from the Curiosity Rover at Gale Crater, Mars. Previous knowledge of martian atmospheric krypton and xenon isotope ratios has been based upon a combination of the Viking mission’s krypton and xenon detections and measurements of noble gas isotope ratios in martian meteorites. However, the meteorite measurements reveal an impure mixture of atmospheric, mantle, and spallation contributions. The xenon and krypton isotopic measurements reported here include the complete set of stable isotopes, unmeasured by Viking. The new results generally agree with Mars meteorite measurements but also provide a unique opportunity to identify various non-atmospheric heavy noble gas components in the meteorites. Kr isotopic measurements define a solar-like atmospheric composition, but deviating from the solar wind pattern at 80Kr and 82Kr in a manner consistent with contributions originating from neutron capture in Br. The Xe measurements suggest an intriguing possibility that isotopes lighter than 132Xe have been enriched to varying degrees by spallation and neutron capture products degassed to the atmosphere from the regolith, and a model is constructed to explore this possibility. Such a spallation component, however, is not apparent in atmospheric Xe trapped in the glassy phases of martian meteorites.

¹David C. Rubie, ¹Vera Laurenz, ^1,2Seth A. Jacobson, ²Alessandro Morbidelli, ³Herbert Palme, ¹Antje K. Vogel, ¹Daniel J. Frost
Science 353, 6304, 1141-1144 Link to Article [DOI: 10.1126/science.aaf6919]
¹Bayerisches Geoinstitut, Bayreuth, Germany.
²Observatoire de la Cote d’Azur, Nice, France.
³Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt, Germany
Reprinted with permission from AAAS

Highly siderophile elements (HSEs) are strongly depleted in the bulk silicate Earth (BSE) but are present in near-chondritic relative abundances. The conventional explanation is that the HSEs were stripped from the mantle by the segregation of metal during core formation but were added back in near-chondritic proportions by late accretion, after core formation had ceased. Here we show that metal-silicate equilibration and segregation during Earth’s core formation actually increased HSE mantle concentrations because HSE partition coefficients are relatively low at the high pressures of core formation within Earth. The pervasive exsolution and segregation of iron sulfide liquid from silicate liquid (the “Hadean matte”) stripped magma oceans of HSEs during cooling and crystallization, before late accretion, and resulted in slightly suprachondritic palladium/iridium and ruthenium/iridium ratios.

^1,2J.H. Hooijschuur, ¹M.F.C. Verkaaik, ²G.R. Davies, ¹F. Ariese
Netherlands Journal of Geosciences – Geologie en Mijnbouw 95, 141-151 Link to Article [DOI: http://dx.doi.org/10.1017/njg.2015.3]
¹LaserLaB, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, the Netherlands
²Deep Earth and Planetary Science, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands

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^1,2Iuliia Myrgorodska, ¹Thomas Javelle, ¹Cornelia Meinert, ¹Uwe J. Meierhenrich
Israel Journal of Chemistry (in Press) Link to Article [DOI: 10.1002/ijch.201600067]
¹Institut de Chimie de Nice ICN, UMR CNRS 7272, Université Nice Sophia Antipolis, Faculté des Sciences, Nice, France
²Synchrotron SOLEIL, L’Orme des Merisiers, Gif-sur-Yvette, France

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¹M.H. Steiner, ¹E.M. Hausrath, ²M.E. Elwood Madden, ¹O. Tschauner, ³B.L. Ehlmann, ⁴A.A. Olsen, ¹S.R. Gainey, ⁵J.S. Smith
Geochimica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.035]
¹Department of Geoscience, University of Nevada, Las Vegas 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010
²School of Geology and Geophysics, University of Oklahoma, 100 E Boyd, Suite 710, Norman, OK 73019
³Division of Planetary Science, California Institute of Technology, 1200 East California Boulevard Pasadena, CA 91125
⁴Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, ME 04469
⁵HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439
Copyright Elsevier

Increasing evidence suggests the presence of recent liquid water, including brines, on Mars. Brines have therefore likely impacted clay minerals such as the Fe-rich mineral nontronite found in martian ancient terrains. To interpret these interactions, we conducted batch experiments to measure the apparent dissolution rate constant of nontronite at 25.0 °C at activities of water (aH2O) of 1.00 (0.01 M CaCl2 or NaCl), 0.75 (saturated NaCl or 3.00 mol kg-1 CaCl2), and 0.50 (5.00 mol kg-1 CaCl2). Experiments at aH2O = 1 (0.01 M CaCl2) were also conducted at 4.0 °C, 25.0 °C, and 45.0 °C to measure an apparent activation energy for the dissolution of nontronite.

Apparent dissolution rate constants at 25.0 °C in CaCl2-containing solutions decrease with decreasing activity of water as follows: 1.18×10-12 ± 9 x 10-14 moles mineral m-2 s-1(aH2O = 1)> 2.36 x 10-13 ± 3.1 x 10-14 moles mineral m-2 s-1(aH2O = 0.75)> 2.05 x 10-14 ± 2.9 x 10-15 moles mineral m-2 s-1 (aH2O = 0.50). Similar results were observed at 25.0 °C in NaCl-containing solutions : 1.89 x 10-12 ± 1 x 10-13 moles mineral m-2 s-1 (aH2O = 1)> 1.98 x 10-13 ± 2.3 x 10-14 moles mineral m-2 s-1(aH2O = 0.75). This decrease in apparent dissolution rate constants with decreasing activity of water follows a relationship of the form: log kdiss = 3.70 ± 0.20 x aH2O – 15.49, where kdiss is the apparent dissolution rate constant, and aH2O is the activity of water. The slope of this relationship (3.70 ± 0.20) is within uncertainty of that of other minerals where the relationship between dissolution rates and activity of water has been tested, including forsteritic olivine (log R = 3.27 ± 0.91 x aH2O – 11.00) ( Olsen et al., 2015)and jarosite (log R = 3.85 ± 0.43 x aH2O – 12.84) ( Dixon et al., 2015), where R is the mineral dissolution rate. This result allows prediction of mineral dissolution as a function of activity of water and suggests that with decreasing activity of water, mineral dissolution will decrease due to the role of water as a ligand in the reaction.

Apparent dissolution rate constants in the dilute NaCl solution (1.89 x 10-12 ± 1 x 10-13 moles mineral m-2 s-1) are slightly greater than those in the dilute CaCl2 solutions (1.18 x 10-12 ± 9 x 10-14 moles mineral m-2 s-1). We attribute this effect to the exchange of Na with Ca in the nontronite interlayer. An apparent activation energy of 54.6 ± 1.0 kJ/mol was calculated from apparent dissolution rate constants in dilute CaCl2- containing solutions at temperatures of 4.0 °C, 25.0 °C, and 45.0 °C: 2.33×10-13 ± 1.3 x 10-14 moles mineral m-2 s-1(4.0 °C), 1.18 x 10-12 ± 9 x 10-14 moles mineral m-2 s-1(25.0 °C), and 4.98 x 10-12 ± 3.8 x 10-13 moles mineral m-2 s-1(45.0 °C).

The greatly decreased dissolution of nontronite in brines and at low temperatures suggests that any martian nontronite found to be perceptibly weathered may have experienced very long periods of water-rock interaction with brines at the low temperatures prevalent on Mars, with important implications for the paleoclimate and long-term potential habitability of Mars.

^1,2Keisuke Nagao, ^2,3Makiko K. Haba, ¹Jong Ik Lee,¹Taehoon Kim, ¹Mi Jung Lee, ¹Changkun Park, ⁴Yong Joo Jwa, ⁵Byeon-Gak Choi
Geochemical Journal 50, 315-325 Link to Article [http://doi.org/10.2343/geochemj.2.0418]
¹Korea Polar Research Institute
²Geochemical Research Center, Graduate School of Science, The University of Tokyo
³Institute of Geochemistry and Petrology, ETH Zürich
⁴Department of Geology, Gyeongsang National University
⁵Department of Earth Science Education, Seoul National University

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¹Kasey A. Todd, ²Heather C. Watson, ³Tony Yu, ³Yanbin Wang
American Mineralogist 101, 1996-2004 Link to Article [http://dx.doi.org/10.2138/am-2016-5474]
¹Geology and Environmental Geosciences, Northern Illinois University, Davis Hall, Normal Road, Dekalb, Illinois 60115, U.S.A.
²Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, Troy, New York 12180, U.S.A.
³Center for Advanced Radiation Sources, University of Chicago, 9700 South Cass Avenue, Argonne, Illinois 60439, U.S.A.
Copyright: The Mineralogical Society of America

It is well accepted that the Earth formed by the accretion and collision of small (10–100 km), rocky bodies called planetesimals. W-Hf isotopic evidence from meteorites suggest that the cores of many planetesimals formed within a relatively short time frame of ~3 My. While a very hot, deep magma ocean is generally thought to have been the driving mechanism for core formation in large planetary bodies, it inadequately explains differentiation and core formation in small planetesimals due to temperatures potentially being insufficient for wide-scale silicate melting to occur. In order for these planetesimals to differentiate within such a relatively short time without a magma ocean, a critical melt volume of the metallic (core-forming) phase and sufficient melt connectivity and grain size must have existed to attain the required permeability and lead to efficient core formation. Shear deformation may increase the connectedness of melt and the permeability, and thus could have been a major contributing factor in the formation of planetesimal cores. This deformation may have been caused by large impacts and collisions experienced by the planetesimals in the early solar system. The purpose of this work is to test the hypothesis that shear deformation enhances the connectivity and permeability of Fe-S melt within a solid silicate (olivine) matrix, such that rapid core formation is plausible. A rotational Drickamer apparatus (RDA) was used to heat and torsionally deform a sample of solid olivine + FeS liquid through six steps of large-strain shear deformation. After each deformation step, X-ray microtomographs were collected in the RDA to obtain in situ three-dimensional images of the sample. The resulting digital volumes were processed and permeability simulations utilizing the lattice Boltzmann method were performed to determine the effect of shear deformation on connectivity and permeability within the sample. The resulting permeabilities of the sample at various steps of deformation are the same within uncertainty and do not exhibit a change with increasing deformation. Additionally, the migration velocity calculated from the permeability of the sample is not high enough for segregation to take place within the time frame of ~3 My. In addition to further constraining the mechanism of core formation in planetesimals, the image processing techniques developed in this study will be of great benefit to future studies utilizing similar methods.

^1,2Takayuki Ushikubo, ^1,3Travis J. Tenner, ⁴Hajime Hiyagon, ¹Noriko T. Kita
Geochimica et Cosmochmiica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.032]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706 USA
²Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe-otsu, Nankoku, Kochi 783-8502 Japan
³Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545 USA
⁴Department of Earth and Planetary Science, Graduate school of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033 Japan
Copyright Elsevier

Oxygen isotope ratios and corresponding 26Al-26Mg isotope systematics of refractory inclusions from the least metamorphosed carbonaceous chondrites, Acfer 094 (C-ungrouped 3.00) and Yamato 81020 (CO3.05), were measured with an ion microprobe. Most of the samples are fine-grained refractory inclusions which are considered as condensates from high temperature Solar Nebular gas. The refractory inclusions consistently exhibit 16O-enriched signatures among their interior phases (spinel, melilite, and high-Ca pyroxene), as well as phases within their rim structures (spinel, high-Ca pyroxene, and adjacent anorthite). This observation indicates that aggregated refractory condensates and the formation of rim structures occurred in the same 16O-rich environment. Evidence for mass-dependent isotopic fractionation in oxygen and magnesium, which would indicate a later flash heating process, was not observed in rims. All oxygen isotope data from fine-grained CAIs are distributed between the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line and the Primitive Chondrule Mineral (PCM) regression line based on oxygen isotope data from Acfer 094 chondrules. The inferred initial 26Al/27Al ratios, (26Al/27Al)0, of spinel-melilite-rich CAIs are (4.08±0.75)×10−5 to (5.05±0.18)×10−5 (errors are 2σ), which are slightly lower than the canonical value of 5.25×10−5. As there is no petrologic evidence for re-melting after condensation, the lower (26Al/27Al)0 values of these CAIs indicate either they formed up to ∼0.3 Ma after canonical CAIs or they formed before 26Al was homogeneously distributed in the Solar nebula. A pyroxene-anorthite-rich CAI, G92, has an 16O-rich signature like other CAIs but also has an order-of-magnitude less 26Mg-excess in anorthite, corresponding to a (26Al/27Al)0 of (5.21±0.54)×10−6. As there is no evidence for a later Mg isotopic disturbance, G92 anorthite is interpreted to have formed by interaction with 16O-rich nebular gas at 2 to 3 Ma after CAI formation. With the observation that 16O-rich refractory inclusions, relatively 16O-poor chondrules, and extremely 16O-poor cosmic symplectites within Acfer 094 all plot on the PCM line, it suggests that 16O-rich nebular gas and extremely 16O-poor primordial volatiles represent mass-independent fractionated endmembers in the early Solar system and that the PCM line represents a mixing line of these two endmembers.

¹Jean-Alix Barrat,²Albert Jambon,^3,4Akira Yamaguchi,⁵Addi Bischoff,⁶Marie-Laure Rouget,¹Céline Liorzou
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.042]
¹Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané, France
²Sorbonne Universités, UPMC Univ Paris 06, UMR 7193, Institut des Sciences de la Terre Paris (iSTeP), F-75005 Paris, France
³National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁴Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo 190-8518, Japan
⁵Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
⁶Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMS 3113, Place Nicolas Copernic, 29280 Plouzané Cedex, France
Copyright Elsevier

Ureilites are among the most common achondrites and are widely believed to sample the mantle of a single, now-disrupted, C-rich body. We analyzed 17 ureilite samples, mostly Antarctic finds, and determined their incompatible trace element abundances. In order to remove or reduce the terrestrial contamination, which is marked among Antarctic ureilites by light-REE enrichment, we leached the powdered samples with nitric acid. The residues display consistent abundances, which strongly resemble those of the pristine rocks. All the analyzed samples display light-REE depletions, negative Eu anomalies, low (Sr/Eu∗)n, and (Zr/Eu∗)n ratios which are correlated. Two groups of ureilites (groups A and B) are defined. Compared to group A, group B ureilites, which are the less numerous, tend to be richer in heavy REEs, more light-REE depleted, and display among the deepest Eu anomalies. In addition, olivine cores in group B ureilites tend to be more forsteritic (Mg# = 81.9-95.2) than in group A ureilites (Mg# = 74.7-86.1). Incompatible trace element systematics supports the view that ureilites are mantle restites. REE modelling suggests that their precursors were rather REE-rich (ca. 1.8-2 x CI) and contained a phosphate phase, possibly merrillite. The REE abundances in ureilites can be explained if at least two distinct types of magmas were removed successively from their precursors: aluminous and alkali-rich melts as exemplified by the Almahata Sitta trachyandesite (ALM-A), and Al and alkali-poor melts produced after the exhaustion of plagioclase from the source. Partial melting was near fractional (group B ureilites, which are probably among the least residual samples) to dynamic with melt porosities that did not exceed a couple of percent (group A ureilites). The ureilite parent body (UPB) was almost certainly covered by a crust formed chiefly from the extrusion products of the aluminous and alkali-rich magmas. It is currently uncertain whether the Al and alkali-poor melts produced during the second phase of melting reached the surface of the body. The fact that initial silicate melting of ureilitic precursors would have produced relatively low density liquids capable of forming an external crust to the UPB casts doubt on models that invoke chondritic outer layers to achondritic asteroids.

¹Kevin Righter, ²Steve R. Sutton, ³Lisa Danielson, ³Kellye Pando, ²Matt Newville
American Mineralogist 101Link to Article [DOI: 10.2138/am-2016-5638]
¹NASA-JSC, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
²GSECARS University of Chicago, 9700 South Cass Avenue, Building 434A, Argonne, Illinois 60439, U.S.A.
³ESCG, Jacobs Engineering, Houston, Texas 77058, U.S.A.
Copyright: Mineralogical Society of America

Many igneous rocks contain mineral assemblages that are not appropriate for application of common mineral equilibria or oxybarometers to estimate oxygen fugacity. Spinel-structured oxides, common minerals in many igneous rocks, typically contain sufficient V for XANES measurements, allowing use of the correlation between oxygen fugacity and V K pre-edge peak intensity. Here we report V pre-edge peak intensities for a wide range of spinels from source rocks ranging from terrestrial basalt to achondrites to oxidized chondrites. The XANES measurements are used to calculate oxygen fugacity from experimentally produced spinels of known Embedded Image . We obtain values, in order of increasing Embedded Image , from IW-3 for lodranites and acapulcoites, to diogenites, brachinites (near IW), ALH 84001, terrestrial basalt, hornblende-bearing R chondrite LAP 04840 (IW+1.6), and finally ranging up to IW+3.1 for CK chondrites (where the Embedded Image of a sample relative to the Embedded Image of the IW buffer at specific T). To place the significance of these new measurements into context we then review the range of oxygen fugacities recorded in major achondrite groups, chondritic and primitive materials, and planetary materials. This range extends from IW-8 to IW+2. Several chondrite groups associated with aqueous alteration exhibit values that are slightly higher than this range, suggesting that water and oxidation may be linked. The range in planetary materials is even wider than that defined by meteorite groups. Earth and Mars exhibit values higher than IW+2, due to a critical role played by pressure. Pressure allows dissolution of volatiles into magmas, which can later cause oxidation or reduction during fractionation, cooling, and degassing. Fluid mobility, either in the sub-arc mantle and crust, or in regions of metasomatism, can generate values >IW+2, again suggesting an important link between water and oxidation. At the very least, Earth exhibits a higher range of oxidation than other planets and astromaterials due to the presence of an O-rich atmosphere, liquid water, and hydrated interior. New analytical techniques and sample suites will revolutionize our understanding of oxygen fugacity variation in the inner solar system, and the origin of our solar system in general.