¹Walter S. Kiefer, ^1,2Qingsong Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12720]
¹Lunar and Planetary Institute, Houston, Texas, USA
²BP, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Based on meteorite evidence, the present-day Martian mantle has a combined abundance of up to a few hundred ppm of H2O, Cl, and F, which lowers the solidus and enhances the magma production rate. Adiabatic decompression melting in upwelling mantle plumes is the best explanation for young (last 200 Myr) volcanism on Mars. We explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting. Relative to a dry mantle, the reduction in solidus temperature due to water increases the magma production rate by a factor of 1.3–1.7 at 50 ppm water and by a factor of 1.9–3.2 at 200 ppm water. Mantle water also decreases the viscosity and increases the vigor of convection, which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity. At conditions relevant to Mars, these indirect effects can cause an order of magnitude increase in the magma production rate. Using geologic and geophysical observations of the Late Amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites, present-day Mars is constrained to have a core–mantle boundary temperature of ~1750 to 1800 °C and a volume-averaged thermal Rayleigh number of 2 × 106 to 107, indicating that moderately vigorous mantle convection has persisted to the present day. Melting occurs at depths of 2.5–6 GPa and is controlled by the Rayleigh number at the low pressure end and by the mantle water concentration at high pressure.

^1,2,3Eve L. Berger, ³Dante S. Lauretta, ^3,4Thomas J. Zega, ⁵Lindsay P. Keller
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12721]
¹GeoControl Systems, Inc.—Jacobs JETS contract—NASA Johnson Space Center, Houston, Texas, USA
²NASA Postdoctoral Program, Oak Ridge, Tennessee, USA
³Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
⁴Naval Research Laboratory, Washington, District of Columbia, USA
⁵NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

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.