¹Hans-Peter Gail, ^2,3Mario Trieloff
Astronomy & Astrophysics 615, A147 Link to Article [https://doi.org/10.1051/0004-6361/201732456]
¹Zentrum für Astronomie, Institut für Theoretische Astrophysik, Heidelberg University, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
e-mail: gail@uni-heidelberg.de
²Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
e-mail: Mario.Trieloff@geow.uni-heidelberg.de
³Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany

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

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

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

Devin L. Schrader^a, Roger R. Fu^b, Steven J. Desch^c, Jemma Davidson^a
Earth and Planetary Science Letters 504, 30-37 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.030]
^aCenter for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, United States of America
^bDepartment of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States of America
^cSchool of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287, United States of America
Copyright Elsevier

The background temperature of the protoplanetary disk is a fundamental but poorly constrained parameter that strongly influences a wide range of conditions and processes in the early Solar System, including the widespread process(es) by which chondrules originate. Chondrules, mm-scale objects composed primarily of silicate minerals, were formed in the protoplanetary disk almost entirely during the first four million years of Solar System history but their formation mechanism(s) are poorly understood. Here we present new constraints on the sub-silicate solidus cooling rates of chondrules at <873 K (600 °C) using the compositions of sulfide minerals. We show that chondrule cooling rates remained relatively rapid (∼10⁰ to 10¹ K/hr) between 873 and 503 K, which implies a protoplanetary disk background temperature of <503 K (230 °C) and is consistent with many models of chondrule formation by shocks in the solar nebula, potentially driven by the formation of Jupiter and/or planetary embryos, as the chondrule formation mechanism. This protoplanetary disk background temperature rules out current sheets and resulting short-circuit instabilities as the chondrule formation mechanism. More detailed modeling of chondrule cooling histories in impacts is required to fully evaluate impacts as a chondrule formation model. These results motivate further theoretical work to understand the expected thermal evolution of chondrules at ≤873 K under a variety of chondrule formation scenarios.

Iffat JABEEN^1,2, Minoru KUSAKABE^1,3, Keisuke NAGAO^1,4, and Arshad ALI^5,1
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13219]
¹Institute for Study of the Earth’s Interior (ISEI), Okayama University, Misasa, Tottori 682-0193, Japan
²Laboratory for Stable Isotope Science (LSIS), Earth Science Department, Western University, London, OntarioN6A 5B7, Canada
³University of Toyama, 3190 Gofuku, Toyama-shi 930-0855, Japan
⁴Division of Polar Earth-System Sciences, Korea Polar Research Institute (KOPRI), Incheon, Korea
⁵Earth Sciences Research Centre (ESRC), Sultan Qaboos University (SQU), Al-Khoudh, Muscat 123, Sultanate of Oman
Published by arrangement with John Wiley & Sons

Precise triple oxygen isotope compositions of 32 Allende bulk chondrules (ABCs) are determined using laser‐assisted fluorination mass spectrometry. Various chemically characterized chondrule types show ranges in δ¹⁸O that vary from −4.80‰ to +1.10‰ (porphyritic olivine; PO, N = 15), −3.10‰ to +1.50‰ (porphyritic olivine pyroxene; POP, N = 9), −3.40‰ to +2.60‰ (barred olivine; BO, N = 4), and −3.60‰ to +1.30‰ (porphyritic pyroxene; PP, N = 3). Oxygen isotope data of these chondrules yield a regression line referred to as the Allende bulk chondrule line (ABC line, slope = 0.86 ± 0.02). Most of our data fall closer to the primitive chondrule minerals line (PCM line, slope = 0.987 ± 0.013) and the carbonaceous chondrite anhydrous mineral line (CCAM line, slope = 0.94 ± 0.02) than the Allende anhydrous mineral line (AAML, slope = 1.00 ± 0.01) with a maximum δ¹⁸O value (+2.60‰) observed in a BO chondrule and a minimum δ¹⁸O value (−4.80‰) shown by a PO chondrule. Similarly, these chondrules depict variable ∆¹⁷O values that range from −5.65‰ to −3.25‰ (PO), −4.60‰ to −2.80‰ (POP), −4.95‰ to −3.00‰ (BO), −5.30‰ to −3.20‰ (PP), and −4.90‰ (CC). A simple model is proposed for the Allende CV3 chondrite with reference to the AAML and PCM line to illustrate the isotopic variations occurred due to the aqueous alteration processes. The estimated temperature ranging from 10 to 130 °C (mean ~60 °C) implies that the secondary mineralization in Allende happened in a warmer and relatively dry environment compared to Murchison. We further propose that thermal metamorphism could have dehydrated the Allende matrix at temperatures between >150 °C and <600 °C.

Tom G. Sharp^a, Erin L. Walton^b,c, Jinping Hu^d, Carl Agee^e
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.014]
^aArizona State University, School of Earth & Space Exploration, Tempe, AZ, 85287-1404, United States
^bMacEwan University, Department of Physical Sciences, Edmonton, AB, T5J 4S2, Canada
^cUniversity of Alberta, Department of Earth & Atmospheric Sciences, Edmonton, AB, T6G 2E3, Canada
^dCalifornia Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91106, United States
^eUniversity of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, 87121-0001, United States
Copyright Elsevier

NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO₂, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An_50-65). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesite formed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet.