^1,2Steven M. Chemtob,²Ryan D. Nickerson,³Richard V. Morris,⁴David G. Agresti,²Jeffrey G. Catalano
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005331]
¹Department of Earth and Environmental Sciences, Temple University, Philadelphia, PA, U.S.A.
²Department of Earth and Planetary Sciences, Washington University, St. Louis, MO, U.S.A.
³EIS Directorate, NASA Johnson Space Center, Houston, TX, U.S.A.
⁴Department of Physics, University of Alabama at Birmingham, Birmingham, AL, U.S.A.
Published by arrangement with John Wiley & Sons

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe3+-bearing clay minerals. Rover-based detection of Fe2+-bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24–38% Fe3+/ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2-OH bands at 2.28-2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

^1,2Joana S. Oliveira,³Mark A. Wieczorek,^4,5,6Gunther Kletetschka
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005397]
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Paris, France
²CITEUC, Geophysical and Astronomical Observatory, University of Coimbra, Coimbra, Portugal
³Observatoire de la Côte d’Azur, Laboratoire Lagrange, Nice, France
⁴Charles University in Prague, Faculty of Science, Czech Republic
⁵Institute of Geology of the CAS, Prague, Czech Republic
⁶University of Alaska-Fairbanks, Geophysical Institute, USA
Published by arrangement with John Wiley & Sons

Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and can not account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon’s surface within the impact melt sheet. Here, we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt.%, with an uncertainty of a factor of about three. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.

Pia Friend^a, Dominik C.Hezel^a,b, Herbert Palme^c, Addi Bischoff^d, Marko Gellissen^e
Earth and Planetary Science Letters 482, 105-114 Link to Article [https://doi.org/10.1016/j.epsl.2017.09.049]
^aUniversity of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany
^bNatural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD London, UK
^cForschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany
^dInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
^eDepartment of Geosciences, Christian-Albrecht-Universität zu Kiel, Germany
Copyright Elsevier

The complimentary chemical composition of chondrules and matrix has so far only been studied in carbonaceous chondrites. We have extended these studies to the matrix-rich Rumurutis We have determined the chemical composition of 27 bulk chondrules and 100 matrix spots in unequilibrated fragments of three different Rumuruti (R) chondrites (NWA 2446, NWA 753, Hughes 030). Also, the bulk chemical composition of NWA 753 was determined. Bulk R chondrites have about CI chondritic (= solar) ratios of Fe/Mg (1.89), Al/Ti (18.62), and Al/Ca (0.94), while chondrules and matrix are complementary to each other. Mean Fe/Mg ratios in chondrules are 0.43 (NWA 2446), 0.36 (NWA 753), and 0.34 (Hughes). Chondrules are depleted in Fe, while matrices are enriched in Fe with respective values of 2.59, 2.45, and 2.39. Bulk Fe/Mg ratio of NWA 753, the only sample which is unaffected by terrestrial weathering, is reproduced by careful mass balance calculations using compositions and abundances of chondrules, matrix and sulphides from the same thin section. Refractory element ratios are also complementary: Al/Ti and Al/Ca are sub-chondritic in chondrules (Al/Ti: 10.43 in NWA 753, 12.24 in NWA 2446 and 11.47 in Hughes 030; Al/Ca: 0.66 in NWA 2446, 0.53 in NWA 753, and 0.46 in Hughes 030), while matrices generally have super-chondritic Al/Ca and Al/Ti ratios (Al/Ti: 28.00 in NWA 2446, 23.20 in NWA 753 and 19.00 in Hughes 030; Al/Ca: 2.30 in NWA 2446, 1.90 in NWA 753, and 0.41 in Hughes 030). Calcium is enriched in the Hughes 030 matrix due to terrestrial weathering. These complementary element ratios and the CI chondritic bulk strongly suggest a common reservoir from which all R chondrite components formed, similar to carbonaceous chondrites. Further, super-chondritic Si/Mg, and CI chondritic Fe/Mg ratios in bulk R chondrites require addition of Si to their reservoir, most probably before chondrule formation.