Peter Fawdon^a, Sanjeev Gupta^b, Joel M. Davis^c, Nicholas H. Warner^d, Jacob B. Adler^e, Matthew R. Balme^a, James F. Bell III^e, Peter M. Grindrod^c, Elliot Sefton-Nash^f
Earth and Planetary Science Letters 503, 88-94 Link to Article [https://doi.org/10.1016/j.epsl.2018.07.040]
^aSchool of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
^bDepartment of Earth Sciences and Engineering, Imperial College London, London, SW7 2AZ, UK
^cDepartment of Earth Sciences, Natural History Museum, Cromwell Road, Kensington, London, SW7 5BD, UK
^dDepartment of Geological Sciences, Integrated Science Center, State University of New York at Geneseo, One College Circle, Geneseo, NY 14454, USA
^eSchool of Earth and Space Exploration, Arizona State University, ISTB4 Room 795, 781 Terrace Mall, Tempe, AZ 85287, USA
^fEuropean Space Research and Technology Centre, Keplerlaan 1, 2201 AZ Noordwijk, Netherlands
Copyright Elsevier

One of the most contentious hypotheses in the geological history of Mars is whether the northern lowlands ever contained an oceanic water body. Arguably, the best evidence for an ocean comes from the presence of sedimentary fans around Mars’ dichotomy boundary, which separates the northern lowlands from the southern highlands. Here we describe the palaeogeomorphology of the Hypanis Valles sediment fan, the largest sediment fan complex reported on Mars (area >970 km²). This has an extensive catchment (<span id="MathJax-Element-1-Frame" class="MathJax_SVG" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.399999618530273px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml=""
4.6×105 km2) incorporating Hypanis and Nanedi Valles, that we show was active during the late-Noachian/early-Hesperian period (∼3.7 Ga). The fan comprises a series of lobe-shaped sediment bodies, connected by multiple bifurcating flat-topped ridges. We interpret the latter as former fluvial channel belts now preserved in inverted relief. Meter-scale-thick, sub-horizontal layers that are continuous over tens of kilometres are visible in scarps and the inverted channel margins. The inverted channel branches and lobes are observed to occur up to at least 140 km from the outlet of Hypanis Valles and descend ∼500 m in elevation. The progressive basinward advance of the channellobe transition records deposition and avulsion at the margin of a retreating standing body of water, assuming the elevation of the northern plains basin floor is stable. We interpret the Hypanis sediment fan to represent an ancient delta as opposed to a fluvial fan system. At its location at the dichotomy boundary, the Hypanis Valles fan system is topographically open to Chryse Planitia – an extensive plain that opens in turn into the larger northern lowlands basin. We conclude that the observed progradation of fan bodies was due to basinward shoreline retreat of an ancient body of water which extended across at least Chryse Planitia. Given the open topography, it is plausible that the Hypanis fan system records the existence, last highstand, and retreat of a large sea in Chryse Planitia and perhaps even an ocean that filled the northern plains of Mars.

Ming CHEN^1,2, Jinfu SHU³, Xiande XIE^2,4, and Dayong TAN^2,4
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13222]
¹State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640Guangzhou, China
²Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640Guangzhou, China
³Center for High Pressure Science and Technology Advanced Research, 201203 Shanghai, China
⁴Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China
Published by arrangement with John Wiley & Sons

Maohokite, a post‐spinel polymorph of MgFe₂O₄, was found in shocked gneiss from the Xiuyan crater in China. Maohokite in shocked gneiss coexists with diamond, reidite, TiO₂‐II, as well as diaplectic glasses of quartz and feldspar. Maohokite occurs as nano‐sized crystallites. The empirical formula is (Mg_0.62Fe_0.35Mn_0.03)²⁺Fe³⁺₂O₄. In situ synchrotron X‐ray microdiffraction established maohokite to be orthorhombic with the CaFe₂O₄‐type structure. The cell parameters are a = 8.907 (1) Å, b = 9.937(8) Å, c = 2.981(1) Å; V = 263.8 (3) ų; space group Pnma. The calculated density of maohokite is 5.33 g cm⁻³. Maohokite was formed from subsolidus decomposition of ankerite Ca(Fe²⁺,Mg)(CO₃)₂ via a self‐oxidation‐reduction reaction at impact pressure and temperature of 25–45 GPa and 800–900 °C. The formation of maohokite provides a unique example for decomposition of Fe‐Mg carbonate under shock‐induced high pressure and high temperature. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2017‐047). The mineral was named maohokite after Hokwang Mao, a staff scientist at the Geophysical Laboratory, Carnegie Institution of Washington, for his great contribution to high pressure research.

M. D. SUTTLE^1,2,3, M. J. GENGE^1,2, L. FOLCO³, M. VAN GINNEKEN^4,5, Q. LIN1,S. S. RUSSELL², and J. NAJORKA²
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13220]
¹Department of Earth Science and Engineering, Imperial College London, South Kensington, London SW7 2AZ, UK
²Department of Earth Science, The Natural History Museum, Cromwell Rd, London SW7 5BD, UK
³Dipartimento di Scienze della Terra, Universita di Pisa, 56126 Pisa, Italy
⁴Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Av. F.D. Roosevelt 50,1050 Brussels, Belgium
⁵Laboratoire G-Time, Universite Libre de Bruxelles, Franklin Rooseveltlaan 50, 1050 Brussels, Belgium
Published by arrangement with John Wiley & Sons

The early stages of atmospheric entry are investigated in four large (250–950 μm) unmelted micrometeorites (three fine‐grained and one composite), derived from the Transantarctic Mountain micrometeorite collection. These particles have abundant, interconnected, secondary pore spaces which form branching channels and show evidence of enhanced heating along their channel walls. Additionally, a micrometeorite with a double‐walled igneous rim is described, suggesting that some particles undergo volume expansion during entry. This study provides new textural data which links together entry heating processes known to operate inside micrometeoroids, thereby generating a more comprehensive model of their petrographic evolution. Initially, flash heated micrometeorites develop a melt layer on their exterior; this igneous rim migrates inwards. Meanwhile, the particle core is heated by the decomposition of low‐temperature phases and by volatile gas release. Where the igneous rim acts as a seal, gas pressures rise, resulting in the formation of interconnected voids and higher particle porosities. Eventually, the igneous rim is breached and gas exchange with the atmosphere occurs. This mechanism replaces inefficient conductive rim‐to‐core thermal gradients with more efficient particle‐wide heating, driven by convective gas flow. Interconnected voids also increase the likelihood of particle fragmentation during entry and, may therefore explain the rarity of large fine‐grained micrometeorites among collections.

M. Weyrauch^a, J. Zipfel^b, S. Weyer^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.022]
^aInstitut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, 30167 Hannover, Germany
^bSenckenberg Forschungsinstitut und Naturmuseum Frankfurt, 60325 Frankfurt, Germany
Copyright Elsevier

The formation processes of the unusually metal-rich CB chondrites are a matter of debate. It is widely accepted that metal grains have formed by condensation. However, it is still debated whether they condensed directly from the solar nebula or from an impact-induced vapor plume. In this study, we present high precision Fe and Ni isotope and trace element composition of zoned and unzoned metal grains from the CBb chondrites Hammadah al Hamra 237, QUE 94411, and MAC 02675, and the CH/CBb breccia Isheyevo and unzoned metal from the CBa chondrites Bencubbin, Gujba, and NWA 4025. Data were obtained using femtosecond laser ablation (multicollector) inductively coupled plasma mass spectrometry (fs-LA-(MC)-ICP-MS). Zoned metal grains from CBb meteorites generally display parallel profiles of Ni and Fe isotope compositions with very low δ56Fe and δ60Ni, and elevated concentrations of refractory siderophile elements in their cores. These findings are consistent with dominantly kinetic isotope- and trace element fractionation during condensation from a confined and fast cooling gas reservoir. Tungsten and Mo are frequently depleted relative to other refractory elements, particularly in zoned metal grains, which is suggestive for elevated oxygen fugacities in the gas reservoir. Such conditions are indicative of the formation of these metal grains during an impact event.

Compared to zoned metal, unzoned metal grains are isotopically more homogeneous and more similar to the heavier rims of the zoned metal grains. This indicates that they formed under different conditions than the zoned metals, i.e., in a more slowly cooling environment. However, several unzoned grains still display significantly variable and correlated δ56Fe and δ60Ni, suggesting that their formation was related to that of the zoned metal grains. The kinetic fractionation-dominated isotopic signatures of the zoned metal grains strongly point to their formation during fast cooling, as may be expected for the exterior envelope of an impact plume. In contrast, the more homogenous isotopic signatures of the unzoned metal grains are more consistent with dominantly equilibrium-like isotope fractionation during condensation, as may be expected for the interior of an impact plume. In this scenario, the isotopically heavier rims of the zoned grains are best explained by a depletion of the outer plume gas reservoir in refractory elements and light isotopes. Accordingly, these findings indicate that zoned and unzoned metal grains likely formed during the same event. The compositional differences among individual unzoned metal grains, but also within some of the zoned grains, indicate turbulent gas mixing, also including movement of metals during their formation, between inner and outer regions of the impact plume.

Reggie L. Hudson
Astrophysical Journal 867, 160 Link to Article [DOI: 10.3847/1538-4357/aae584]
Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

As part of our work on nitrogen-rich ices, the IR spectra and band strengths used in a recent paper to identify and quantify radiation-induced changes in an N₂+H₂O ice near 15 K are examined, along with reports of (i) a chemical tracer for N₂+H₂O ices, (ii) a new IR feature of solid N₂, and (iii) a striking ¹⁵N isotopic enrichment. Problems are found for each IR band strength used and for each of the three claims made, to the extent that none are supported by the results presented to date. In contrast, new work presented here, combined with several older investigations, strongly supports the formation of di- and triatomic nitrogen oxides in irradiated N₂-rich ices. Observations and trends in the chemistry of N₂-rich icy solids are described, and conclusions are drawn. A considerable amount of material from previous chemical studies of N₂-rich systems, spanning more than a century, is brought together for the first time and used to examine the chemistry of N₂-rich ices in extraterrestrial environments. Needs are identified and suggestions made for future studies of N₂-rich interstellar and planetary ice analogs.

¹Reiners, P.W., ²Turchyn, A.V.
Geology 46, 863-866 Link to Article [DOI: 10.1130/G45040.1]
¹Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, United States
²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, Cambridgeshire, CB2 3EQ, United Kingdom

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