¹E. A. MacLagan,^1,2E. L. Walton,¹C. D. K. Herd,³M. Dence
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13122]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Department of Physical Sciences, MacEwan UniversityEdmonton, Alberta, Canada
³2602 – 38 Metropole Pvt.Ottawa, ON, Canada
Published by arrangement with John Wiley & Sons

The Steen River impact structure (SRIS) formed in mixed target rocks, with Devonian carbonates, shales, and evaporites overlying granitic basement rocks of the Canadian Shield. A detailed study of impact melt phases within a continuous sequence of polymict impact breccia, as intersected by drill core, evaluated the relationship of impact melt to the breccia, identified the target rocks that contributed to the melt, and calculated the amount of melt within the breccia. Impact melt in the SRIS breccia occurs in three main forms (1) as individual matrix‐supported clasts, (2) as rims enveloping granitic clasts, and (3) as layers of agglomerated melt. Major and minor element abundances of large impact melt clasts (>1 mm) are similar to granitic basement, aside from elevated CaO and K2O wt% oxides in these melt clasts from incorporation of carbonates and calcareous shales. In contrast, submillimeter‐sized impact melt clasts have a composition derived almost exclusively from melting of shales. The small size of the shale‐derived melt clasts is attributed to increased fragmentation and a wider dispersion due to the volatile‐rich nature of the shale protolith. The wide compositional range of impact‐melted target lithologies documented at the SRIS contradicts breccia clast formation by impact melts that merged into larger bodies but were subsequently disrupted. Our observations are consistent with disruption of impact melt early in its formation history, and the volatile‐rich nature of the target materials likely contributed to this disruption. Bimodal thin section scans provide an estimate of the proportion of impact melt phases in the SRIS breccias (~19 vol%). When compared to similarly sized, mixed‐target impact structures, our results are consistent with the estimated volume of impact melt clasts from Ries, Germany (21 vol%), but are roughly twice that observed at Haughton, Canada (<10 vol%).

^1,2,3Steven B. Simon,^1,4Stephen R. Sutton
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13123]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
²The Field Museum of Natural History, Chicago, Illinois, USA
³Institute of Meteoritics, University of New Mexico Albuquerque, New Mexico, USA
⁴Center for Advanced Radiation Sources (CARS), The University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks via XANES (X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti3+, averaging 15–20% Ti3+. Assuming Ti3+ is more compatible in pyroxene than Ti4+, then even lower Ti3+ proportions are indicated for the parental melts. The VLT pyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced in VLT pyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr in VLT pyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti3+, with no Ti3+ and higher proportions of Ti in tetrahedral coordination in the VLTs than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr2+ than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, the VLTs, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristic fO2s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H2) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.

¹Tasha L. Dunn,¹Oriana K. Battifarano,^2,3Juliane Gross,¹Emma J. O’Hara
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13118]
¹Department of Geology, Colby College Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History New York, New York, USA
Published by arrangement with John Wiley & Sons

Based on the chemical heterogeneity of chondrule and matrix olivine, Northwest Africa (NWA) 5343 is the least metamorphosed CK chondrite reported so far. To better constrain the lower limit of metamorphism in the CK chondrites, we performed a detailed analysis of matrix material in NWA 5343, including characterization of the texture and bulk composition and analyses of individual silicate minerals. Results suggest that NWA 5343 is petrologic type 3.6 or 3.7. Although silicate minerals in the matrix seem to be equilibrated to roughly the same extent throughout the sample, there are recognizable differences in grain size and shape. These textural differences may be the result of transient heating events during impacts, which would be likely on the CK chondrite parent body. The difference between the extent of chemical equilibration and texture may also suggest that grain size and shape are still sensitive to metamorphism at petrologic subtypes where silicate mineral equilibration is nearly complete (e.g., >3.7). Carbonate material present in NWA 5343 is a product of terrestrial weathering; however, infiltration of a Ca‐bearing fluid did not influence the composition of silicate minerals in the matrix. To evaluate the possibility of a continuous metamorphic sequence between the CV and CK chondrites, the bulk matrix composition of NWA 5343 is compared to the CVred chondrite, Vigarano. Although the matrix composition of NWA 5343 could be derived by secondary processing of a Vigarano‐like precursor, porosity and texture of matrix olivine in NWA 5343 are hard to reconcile with a continuous metamorphic sequence.

¹J. Leitner, ²C. Vollmer, ³T. Henkel, ^1,4U. Ott, ¹P. Hoppe
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.05.025]
¹Max Planck Institute for Chemistry, Particle Chemistry Department, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
²Institute for Mineralogy, Westfälische Wilhelms-Universität, Correnstrasse 24, 48149 Münster, Germany
³The University of Manchester, School of Earth and Environmental Sciences, Williamson Building, Oxford Road, Manchester, M13 9PL, UK
⁴MTA Atomki, Bem tér 18/c, 4826 Debrecen, Hungary
Copyright Elsevier

We report an in-situ investigation of silicon nitride (Si3N4) grains from several enstatite chondrites of low petrologic types (EL3, EH3, and EH4). The grains occurred in various host phases, including Fe,Ni metal, schreibersite, sulfides, and also in the silicate fraction, and are of Solar System origin. Energy-dispersive X-ray spectroscopy (EDS) showed that carbon and oxygen are present in all investigated grains. Carbon- and N-isotopic compositions of 288 grains were measured by NanoSIMS. Nitrogen is isotopically light compared to terrestrial air, with an average δ15N = –60±1 ‰. The average carbon isotopic composition does not deviate significantly from the terrestrial PDB standard. TOF-SIMS investigation of one particularly large Si3N4 grain (10 µm ×2 µm) showed that the O is located within the grain and not in adjacent particles, and also revealed the presence of chromium. Transmission electron microscopy (TEM) analysis showed that the Cr is present as carlsbergite (CrN) inclusions. TEM investigation of three Si3N4 grains showed them to be polycrystalline, with no consistent crystallographic relationship with the host material. Estimated Si3N4 abundances for four metal-sulfide assemblages demonstrate that the amount of nitrogen bound in the nitrides exceeds the maximum concentration of N that can be stored in Fe,Ni metal in solid solution. Thus, even if all of the N in the metal would have been exsolved, it would not have been enough to form the observed amounts of Si3N4 grains, giving further evidence against formation by exsolution. This clearly shows that they did not form by exsolution from the host materials, as has been suggested in earlier studies. Instead, the Si3N4 must have formed prior to incorporation into the enstatite chondrite parent bodies, either by shock wave-induced condensation processes, or by precipitation from the host phases in the presence of NH3.

¹Jean-Pierre Lorand, ²Sylvain Pont, ³Vincent Chevrier, ⁴Ambre Luguet, ²Brigitte Zanda, ²Roger Hewins
American Mineralogist 103, 872-885 Link to Article [DOI: https://doi.org/10.2138/am-2018-6334]
¹Laboratoire de Planétologie et Géodynamique à Nantes, CNRS UMR 6112, Université de Nantes, 2 Rue de la Houssinère, BP 92208, 44322 Nantes Cédex 3, France
²Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC)—Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, 75005 Paris, France
³W.M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, MUSE 202, University of Arkansas, Fayetteville, Arkansas 72701, U.S.A.
⁴Rheinische Friedrich-Wilhelms-Universität Bonn, Steinmann Institut für Geologie, Mineralogie und Paläontologie, Poppelsdorfer Schloss, 53115 Bonn, Germany
Copyright: The Mineralogical Society of America

The Chassigny meteorite, a martian dunite, contains trace amounts (0.005 vol%) of Fe-Ni sulfides, which were studied from two polished mounts in reflected light microscopy, scanning electron microscope (SEM), and electron microprobe (EMP). The sulfide phases are, by decreasing order of abundance, nickeliferous (0–3 wt% Ni) pyrrhotite with an average composition M0.88±0.01S (M = Fe+Ni+Co+Cu+Mn), nickeliferous pyrite (0–2.5 wt% Ni), pentlandite, millerite, and unidentified Cu sulfides. Pyrrhotite is enclosed inside silicate melt inclusions in olivine and disseminated as polyhedral or near spherical blebs in intergranular spaces between cumulus and postcumulus silicates and oxides. This sulfide is considered to be a solidification product of magmatic sulfide melt. The pyrrhotite Ni/Fe ratios lie within the range expected for equilibration with the coexisting olivine at igneous temperatures. Pyrite occurs only as intergranular grains, heterogeneously distributed between the different pieces of the Chassigny meteorite. Pyrite is interpreted as a by-product of the low-T (200 °C) hydrothermal alteration events on Mars that deposited Ca sulfates + carbonates well after complete cooling. The shock that ejected the meteorite from Mars generated post-shock temperatures high (300 °C) enough to anneal and rehomogenize Ni inside pyrrhotite while pyrite blebs were fractured and disrupted into subgrains by shock metamorphism. The negligible amount of intergranular sulfides and the lack of solitary sulfide inclusions in cumulus phases (olivine, chromite) indicate that, like other martian basalts so far studied for sulfur, the parental melt of Chassigny achieved sulfide-saturation at a late stage of its crystallization history. Once segregated, the pyrrhotite experienced a late-magmatic oxidation event that reequilibrated its metal-to-sulfur ratios.

¹Shaunna M. Morrison et al. (>10)
American Mineralogist 103, 857-871 Link to Article [DOI: https://doi.org/10.2138/am-2018-6124]
¹University of Arizona, 1040 E 4th St, Tucson, Arizona 85721, U.S.A.
²Geophysical Laboratory, Carnegie Institution, 5251 Broad Branch Rd NW, Washington, D.C. 20015, U.S.A.
Copyright: The Mineralogical Society of America

Crystal chemical algorithms were used to estimate the chemical composition of selected mineral phases observed with the CheMin X-ray diffractometer onboard the NASA Curiosity rover in Gale crater, Mars. The sampled materials include two wind-blown soils, Rocknest and Gobabeb, six mudstones in the Yellowknife Bay formation (John Klein and Cumberland) and the Murray formation (Confidence Hills, Mojave2, and Telegraph Peak), as well as five sandstones, Windjana and the samples of the unaltered Stimson formation (Big Sky and Okoruso) and the altered Stimson formation (Greenhorn and Lubango). The major mineral phases observed with the CheMin instrument in the Gale crater include plagioclase, sanidine, P21/c and C2/c clinopyroxene, orthopyroxene, olivine, spinel, and alunite-jarosite group minerals. The plagioclase analyzed with CheMin has an overall estimated average of An40(11) with a range of An30(8) to An63(6). The soil samples, Rocknest and Gobabeb, have an average of An56(8) while the Murray, Yellowknife Bay, unaltered Stimson, and altered Stimson formations have averages of An38(2), An37(5), An45(7), and An35(6), respectively. Alkali feldspar, specifically sanidine, average composition is Or74(17) with fully disordered Al/Si. Sanidine is most abundant in the Wind-jana sample (∼26 wt% of the crystalline material) and is fully disordered with a composition of Or87(5). The P21/c clinopyroxene pigeonite observed in Gale crater has a broad compositional range {[Mg0.95(12)–1.54(17)Fe0.18(17)–1.03(9)Ca0.00–0.28(6)]∑2Si2O6}with an overall average of Mg1.18(19)Fe0.72(7)Ca0.10(9)Si2O6. The soils have the lowest Mg and highest Fe compositions [Mg0.95(5)Fe1.02(7)Ca0.03(4)Si2O6] of all of the Gale samples. Of the remaining samples, those of the Stimson formation exhibit the highest Mg and lowest Fe [average = Mg1.45(7)Fe0.35(13)Ca0.19(6)Si2O6]. Augite, C2/c clinopyroxene, is detected in just three samples, the soil samples [average = Mg0.92(5)Ca0.72(2)Fe0.36(5)Si2O6] and Windjana (Mg1.03(7)Ca0.75(4)Fe0.21(9)Si2O6). Orthopyroxene was not detected in the soil samples and has an overall average composition of Mg0.79(6)Fe1.20(6)Ca0.01(2)Si2O6 and a range of [Mg0.69(7)–0.86(20)Fe1.14(20)–1.31(7)Ca0.00–0.04(4)]∑2Si2O6, with Big Sky exhibiting the lowest Mg content [Mg0.69(7)Fe1.31(7)Si2O6] and Okoruso exhibiting the highest [Mg0.86(20)Fe1.14(20)Si2O6]. Appreciable olivine was observed in only three of the Gale crater samples, the soils and Windjana. Assuming no Mn or Ca, the olivine has an average composition of Mg1.19(12)Fe0.81(12)SiO4 with a range of 1.08(3) to 1.45(7) Mg apfu. The soil samples [average = Mg1.11(4)Fe0.89SiO4] are significantly less magnesian than Windjana [Mg1.35(7)Fe0.65(7)SiO4]. We assume magnetite (Fe3O4) is cation-deficient (Fe3–x☐xO4) in Gale crater samples [average = Fe2.83(5)☐0.14O4; range 2.75(5) to 2.90(5) Fe apfu], but we also report other plausible cation substitutions such as Al, Mg, and Cr that would yield equivalent unit-cell parameters. Assuming cation-deficient magnetite, the Murray formation [average = Fe2.77(2)☐0.23O4] is noticeably more cation-deficient than the other Gale samples analyzed by CheMin. Note that despite the presence of Ti-rich magnetite in martian meteorites, the unit-cell parameters of Gale magnetite do not permit significant Ti substitution. Abundant jarosite is found in only one sample, Mojave2; its estimated composition is (K0.51(12)Na0.49)(Fe2.68(7)Al0.32)(SO4)2(OH)6. In addition to providing composition and abundances of the crystalline phases, we calculate the lower limit of the abundance of X-ray amorphous material and the composition thereof for each of the samples analyzed with CheMin. Each of the CheMin samples had a significant proportion of amorphous SiO2, except Windjana that has 3.6 wt% SiO2. Excluding Windjana, the amorphous materials have an SiO2 range of 24.1 to 75.9 wt% and an average of 47.6 wt%. Windjana has the highest FeOT (total Fe content calculated as FeO) at 43.1 wt%, but most of the CheMin samples also contain appreciable Fe, with an average of 16.8 wt%. With the exception of the altered Stimson formation samples, Greenhorn and Lubango, the majority of the observed SO3 is concentrated in the amorphous component (average = 11.6 wt%). Furthermore, we provide average amorphous-component compositions for the soils and the Mount Sharp group formations, as well as the limiting element for each CheMin sample.

¹Shaunna M. Morrison et al. (>10)
American Mineralogist 103, 848-856 Link to Article [DOI: https://doi.org/10.2138/am-2018-6123]
¹University of Arizona, 1040 E 4th Street, Tucson, Arizona 85721, U.S.A.
¹Geophysical Laboratory, Carnegie Institution, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
Copyright: The Mineralogical Society of America

Mathematical relationships between unit-cell parameters and chemical composition were developed for selected mineral phases observed with the CheMin X-ray diffractometer onboard the Curiosity rover in Gale crater. This study presents algorithms for estimating the chemical composition of phases based solely on X-ray diffraction data. The mineral systems include plagioclase, alkali feldspar, Mg-Fe-Ca C2/c clinopyroxene, Mg-Fe-Ca P21/c clinopyroxene, Mg-Fe-Ca orthopyroxene, Mg-Fe olivine, magnetite, and other selected spinel oxides, and alunite-jarosite. These methods assume compositions of Na-Ca for plagioclase, K-Na for alkali feldspar, Mg-Fe-Ca for pyroxene, and Mg-Fe for olivine; however, some other minor elements may occur and their impact on measured unit-cell parameters is discussed. These crystal-chemical algorithms can be applied to material of any origin, whether that origin is Earth, Mars, an extraterrestrial body, or a laboratory.

¹Steven J. Jaret,²Jeffrey R. Johnson,¹Melissa Sims,¹Nicholas DiFrancesco,¹Timothy D. Glotch
Journal of Geophysical Research (in Press) Link to Article [https://doi.org/10.1029/2018JE005523]
¹Department of Geosciences, Stony Brook University Stony Brook, NY
²Johns Hopkins University Applied Physics Laboratory Laurel, MD
Published by arrangement with John Wiley & Sons

Plagioclase feldspars are common on the surfaces of planetary objects in the Solar System such as the Moon and Mars, and in meteorites. Understanding their response to shock deformation is important for interpretations of data from remote sensing, returned samples, and naturally shocked samples from impact craters. We used optical petrography, micro‐Raman, and micro‐FTIR spectroscopy to systematically document vibrational spectral differences related to structural changes in experimentally shocked (0‐56 GPa) albite‐, andesine‐, and bytownite‐rich rocks as a function of pressure and composition. Across all techniques, the specific composition of feldspars was confirmed to affect shock deformation, where more Ca‐rich feldspars transform to an amorphous phase at lower shock conditions than more Na‐rich feldspars. Onset of amorphization occurs at ~50 GPa for albite, between 28 and 30 GPa for andesine, and between 25 and 27 GPa for bytownite. Complete amorphization occurred at pressures greater than ~55 GPa for albite, ~47 GPa for andesine, and ~38 GPa for bytownite. Petrographically, these experimentally shocked samples do not exhibit the planar microstructures common in naturally shocked plagioclase, despite showing the expected trends of internal disordering and deformation as seen in the micro‐Raman and infrared spectra. Average spectra of hyperspectral images of these samples mimic macro‐scale measurements acquired previously. However, we see micro‐scale heterogeneity in the shock response, resulting from either variations in composition, crystal orientation, or the inherent heterogeneity of the shock wave topology. This is an important factor to consider when deducing shock pressures in naturally shocked samples.

¹Paul S. Szabo et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.05.028]
¹Institute of Applied Physics, TU Wien, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria
Copyright Elsevier

The sputtering of wollastonite (CaSiO3) by solar wind-relevant ions has been investigated experimentally and the results are compared to the binary collision approximation (BCA) codes SDTrimSP and SRIM-2013. Absolute sputtering yields are presented for Ar projectiles as a function of ion impact energy, charge state and impact angle as well as for solar wind H projectiles as a function of impact angle. Erosion of wollastonite by singly charged Ar ions is dominated by kinetic sputtering. The absolute magnitude of the sputtering yield and its dependence on the projectile impact angle can be well described by SDTrimSP as long as the actual sample composition is used in the simulation. SRIM-2013 largely overestimates the yield especially at grazing impact angles. For higher Ar charge states, the measured yield is strongly enhanced due to potential sputtering. Sputtering yields under solar wind-relevant H+ bombardment are smaller by two orders of magnitude compared to Ar. Our experimental yields also show a less pronounced angular dependence than predicted by both BCA programs, probably due to H implantation in the sample. Based on our experimental findings and extrapolations to other solar wind ions by using SDTrimSP, we present a model for the complete solar wind sputtering of a flat wollastonite surface as a function of projectile ion impact angle, which predicts a sputtering yield of 1.29 atomic mass units per solar wind ion for normal impact. We find that mostly He and some heavier ions increase the sputtering yield by more than a factor of two as compared to bombardment with only H+ ions.