¹Kevin M. Cannon, ¹John F. Mustard, ¹Stephen W. Parman, ²Elizabeth C. Sklute, ²M. Darby Dyar, ¹Reid F. Cooper
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005219]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
²Mount Holyoke College, South Hadley, MA, USA
Published by arrangement with John Wiley & Sons

Thirty silicate glasses were synthesized as realistic analogs to those expected to exist on Mars, the Moon and Mercury. Samples were measured using visible/near-infrared and Mössbauer spectroscopy to determine the effects of varying bulk chemistry, oxygen fugacity, and temperature on spectral properties. For martian glasses, the fO2 during fusion strongly affects absorption band intensities in the spectra, while bulk chemistry has noticeable secondary effects on absorption band positions. Titanium and iron content drive spectral changes in lunar glasses, where Fe3+ is effectively absent. Iron-free Mercury-analog glasses have much higher albedos than all other samples, and their spectral shape is a close match to some pyroclastic deposits on Mercury. Synthetic glass spectra were used as inputs into a spectral unmixing model applied to remote orbital datasets to test for the presence of glass. The model is validated against physical laboratory mixture spectra, as well as previous detections of glass-rich pyroclastic deposits on the Moon. Remote data were then used from suspected impact deposits and possible pyroclastic deposits on Mars as a new application of the model: the results reveal spatially coherent glass-rich material, and the strong spectral match of the synthetic glasses to these remotely sensed data gives new insights into the presence and character of glasses on the martian surface. The large library of glass spectra generated here, acquired from consistently synthesized and measured samples, can serve as a resource for further studies of volcanic and impact processes on planetary bodies.

¹Andrew W. Beck, ¹David J. Lawrence, ¹Patrick N. Peplowski, ¹Christina E. Viviano-Beck, ²Thomas H. Prettyman, ³Timothy J. McCoy, ⁴Harry Y. McSween Jr, ²Naoyuki Yamashita
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2017.01.008]
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, 20723, USA
²Planetary Science Institute, Tucson, Arizona, 85719, USA
³Department of Mineral Sciences, Smithsonian Institution, Washington, District of Columbia, 20560, USA
⁴Department of Earth and Planetary Sciences, Knoxville, Tennessee, 37996-1410, USA
Copyright Elsevier

We use data collected by the Dawn Gamma-Ray and Neutron Detector (GRaND) at Vesta to map compositions corresponding to nearly pure igneous lithologies in the howardite, eucrite, diogenite (HED) meteorite clan (samples likely from Vesta). At the ∼300-km spatial scale of GRaND measurements, basaltic eucrite occurs on only 3% of the surface, whereas cumulate eucrite and orthopyroxenitic diogenite are not detected. The basaltic eucrite region is generally coincident with an area of the surface with thick regolith, elevated H, and moderate crater density, and may represent the best compositional sample of primordial vestan crust. We observe an absence of pure orthopyroxenitic diogenite terrains in the Rheasilvia basin and its ejecta, an observation corroborated by VIR (0.1%), which suggests the south-polar crust was a polymict mixture of igneous lithologies (howardite) at the time of the Rheasilvia impact, or was a thick basaltic eucrite crust with heterogeneously distributed orthopyroxenitic diogenite plutons. The most dominant igneous composition detected (11% of the surface) corresponds to one of the least-abundant igneous lithologies in the HED meteorite collection, the Yamato Type B diogenites (plagioclase-bearing pyroxenites). The distribution of Type B diogenite is spatially correlated with post-Rheasilvia craters in the north-polar region that are in close proximity to the Rheasilvia basin antipode. This suggests that north-polar Type B plutonism may have been associated with the Rheasilvia impact event. We propose that this was either through 1) uplift of pre-existing plutons at the antipode through focusing of Rheasilvia impact stress, or 2) Rheasilvia impact antipodal crustal melting, creating magmas that underwent fractionation to produce Type B plutons.

^1,2Jon M. Friedrich, ³Alex Ruzicka, ^5,6Robert J. Macke, ⁴James O. Thostenson, ⁴Rebecca A. Rudolph, ⁵Mark L. Rivers, ^2,7,8Denton S. Ebel
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.039]
¹Department of Chemistry, Fordham University, Bronx, NY 10458, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
³Cascadia Meteorite Laboratory, Portland State University, Department of Geology, Portland, OR 97207-0751, USA
⁴Microscopy and Imaging Facility, American Museum of Natural History, New York, NY 10024, USA
⁵Center for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA
⁶Vatican Observatory, V-00120 Vatican City-State
⁷Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
⁸Graduate Center of the City University of New York, New York 10016, USA
Copyright Elsevier

Collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals, including the parent bodies of chondrites, but the conditions under which these occurred are not well constrained. A simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric. To test this model, we studied sixteen ordinary chondrites of different groups (H, L, LL) using X-ray computed microtomography (μCT) to measure porosity and metal fabric, ideal gas pycnometry and 3D laser scanning to determine porosity, and optical microscopy (OM) to determine shock stage. These included a subsample of six chondrites previously studied using transmission electron microscopy (TEM) to characterize microstructures in olivine. Combining with previous data, results support the simple model in general, but not for chondrites with low shock-porosity-foliation (low-SPF chondrites). These include Kernouvé (H6), Portales Valley (H6/7), Butsura (H6), Park (L6), GRO 85209 (L6), Estacado (H6), MIL 99301 (LL6), Spade (H6), and Queen’s Mercy (H6), among others. The data for these meteorites are best explained by high ambient heat during or after shock. Low-SPF chondrites tend to have older 40Ar/39Ar ages (∼4435-4526 Ma) than other, non-low-SPF type 6 chondrites in this study. We conclude that the H, L, and LL asteroids all were shock-compacted at an early stage while warm, with collisions occurring during metamorphic heating of the parent bodies. Results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile, and on the nature of accretion and lithification processes for planetesimals.

^1,2K. L. Siebach, ²M. B. Baker, ²J. P. Grotzinger, ¹S. M. McLennan, ³R. Gellert, ⁴L. M. Thompson, ¹J. A. Hurowitz
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005195]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Present address: Department of Geosciences, SUNY Stony Brook, Stony Brook, NY, USA
³Department of Physics, University of Guelph, Guelph, Ontario, Canada
⁴Planetary and Space Science Centre, University of New Brunswick, Fredericton, Canada
Published by arrangement with John Wiley & Sons

Sedimentary rocks are composed of detrital grains derived from source rocks, which are altered by chemical weathering, sorted during transport, and cemented during diagenesis. Fluvio-lacustrine sedimentary rocks of the Bradbury group, observed on the floor of Gale crater by the Curiosity rover during its first 860 sols, show trends in bulk chemistry that are consistent with sorting of mineral grains during transport. The Bradbury group rocks are uniquely suited for sedimentary provenance analysis because they appear to have experienced negligible cation-loss (i.e., open-system chemical weathering) at the scale of the Alpha Particle X-ray Spectrometer bulk chemistry analyses based on low Chemical Index of Alteration values and successful modeling of >90% of the (volatile-free) targets as mixtures of primary igneous minerals. Significant compositional variability between targets is instead correlated to grain size and textural characteristics of the rocks; the coarsest-grained targets are enriched in Al2O3, SiO2, and Na2O, whereas the finer-grained targets are enriched in mafic components. This is consistent with geochemical and mineralogical modeling of the segregation of coarse-grained plagioclase from finer-grained mafic minerals (e.g., olivine and pyroxenes), which would be expected from hydrodynamic sorting of the detritus from mechanical breakdown of subalkaline plagioclase-phyric basalts. While the presence of a distinctive K2O-rich stratigraphic interval shows that input from at least one distinctive alkali-feldspar-rich protolith contributed to basin fill, the dominant compositional trends in the Bradbury group are consistent with sorting of detrital minerals during transport from relatively homogeneous plagioclase-phyric basalts.

^1,2Rebecca J. Smith,²Briony H. N. Horgan,³Paul Mann,³Edward A. Cloutis,⁴Philip R. Christensen
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005112]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
³Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
⁴School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Published by arrangement with John Wiley & Sons

Acid alteration has long been proposed for the Martian surface, and so it is important to understand how the resulting alteration textures affect surface spectra. Two basaltic materials of varying crystallinity were altered in two different H2SO4 solutions (pH 1 and pH 3) for 220 days. The unaltered and altered samples were studied in the visible and near infrared (VNIR) and thermal infrared (TIR), and select samples were chosen for Scanning Electron Microscopy (SEM) analysis. Materials altered in pH 3 solutions showed little to no physical alteration, and their spectral signatures changed very little. In contrast, all materials altered in pH 1 acid displayed silica-rich alteration textures, and the morphology differed based on starting material crystallinity. The more crystalline material displayed extensive alteration reaching into the sample interiors and had weaker silica spectral features. The glass sample developed alteration layers tens of microns thick, exhibiting amorphous silica-rich spectral features that completely obscured the substrate. Thus, the strong absorption coefficient of silica effectively decreases the penetration depth of TIR spectral measurements, causing silica abundances to be grossly overestimated in remote sensing data. Additionally, glass samples with silica layers exhibited distinct concave-up blue spectral slopes in the VNIR. Spectra from the northern lowland plains of Mars are modeled with high abundances of amorphous silica and exhibit concave-up blue spectral slopes, and are thus consistent with acid altered basaltic glass. Therefore, we conclude that large regions of the Martian surface may have formed through the interaction of basaltic glass with strongly acidic fluids.

¹Briony H. N. Horgan,²Rebecca J. Smith,³Edward A. Cloutis,³Paul Mann,⁴Philip R. Christensen
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005111]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
⁴School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Published by arrangement with John Wiley & Sons

Acid leached rinds and coatings occur in volcanic environments on Earth and have been identified using orbital spectroscopy on Mars, but their development is poorly understood. We simulated long-term open-system acid weathering in a laboratory by repeatedly rinsing and submerging crystalline and glassy basalts in pH ~ 1 and pH ~ 3 acidic solutions for 213 days, and compared their visible/near-infrared (0.3-2.5 µm) and thermal-infrared (5-50 µm) spectral characteristics to their microscopic physical and chemical properties from scanning electron microscopy (SEM). We find that while alteration at moderately low pH (~3) can produce mineral precipitates from solution, it has very little spectral or physical effect on the underlying parent material. In contrast, alteration at very low pH (~1) results in clear silica spectral signatures for all crystalline samples while glasses exhibit strong blue concave up near-infrared slopes. SEM indicates that these spectral differences correspond to different modes of alteration. In glass, alteration occurs only at the surface and produces a silica-enriched leached rind, while in more crystalline samples, alteration penetrates the interior to cause dissolution and replacement by silica. We confirm that glass is more stable than crystalline basalt under long-term acidic leaching, suggesting that glass could be enriched and common in terrains on Mars that have been exposed to acid weathering. Leached glasses are consistent with both OMEGA and TES spectra of the martian northern lowlands, and may contribute to the high-silica phases detected globally in TES Surface Type 2. Thus, both glass-rich deposits and acidic weathering may have been widespread on Mars.

¹S.L. Simpson, ²A.J. Boyce, ³P. Lambert, ¹P. Lindgren, ¹M.R. Lee
Earth and Planetary Science Letters 460, 192-200 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.12.023]
¹University of Glasgow, School of Geographical and Earth Sciences, Lilybank Gardens, G12 8QQ, Glasgow, UK
²Scottish Universities Environmental Research Centre, Rankine Ave, East Kilbride, G75 0QF, Glasgow, UK
³Sciences et Applications, 218 Boulevard Albert 1er, 33800 Bordeaux, France
Copyright Elsevier

The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralisation. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ34S values vary over a wide range, from −35.8‰ to +0.4‰. The highest values, δ34S −3.7‰ to +0.4‰, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ34S −35.8‰ to −5.2‰, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ34S values of −10.7‰ to −1.2‰ are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120 °C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor.

¹Yang Li, ^1,2Ai-Cheng Zhang, ¹Jia-Ni Chen, ³Li-Xin Gu, ¹Ru-Cheng Wang
American Mineralogist 102, 98-107 Link to Article [https://doi.org/10.2138/am-2017-5881]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
²Lunar and Planetary Science Institute, Nanjing University, Nanjing 210046, China
³Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Copyright: The Mineralogical Society of America

Phosphorus-rich olivine (P2O5 > 1 wt%) is a mineral that has been reported only in a few terrestrial and extraterrestrial occurrences. Previous investigations suggest that P-rich olivine mainly forms through rapid crystallization from high-temperature P-rich melts. Here, we report a new occurrence of P-rich olivine in an ungrouped carbonaceous chondrite Dar al Gani (DaG) 978. The P-rich olivine in DaG 978 occurs as lath-shaped grains surrounding low-Ca pyroxene and olivine grains. The lath-shaped olivine shows a large variation in P2O5 (0–5.5 wt%). The P-rich olivine grains occur in a chondrule fragment and is closely associated with chlorapatite, merrillite, FeNi metal, and troilite. Tiny Cr-rich hercynite is present as inclusions within the P-rich olivine. The lath-shaped texture and the association with Cr-rich hercynite indicates that the P-rich olivine in DaG 978 formed by replacing low-Ca pyroxene precursor by a P-rich fluid during a thermal event, rather than by crystallization from a high-temperature melt. The large variation of P2O5 within olivine grains on micrometer-scale indicates a disequilibrium formation process of the P-rich olivine. The occurrence of P-rich olivine in DaG 978 reveals a new formation mechanism of P-rich olivine.

¹Edgar S. Steenstra, ¹Yanhao Lin, ^2,3Nachiketa Rai, ¹Max Jansen,¹Wim van Westrenen
American Mineralogist 102,  92-97 Link to Article [doi:10.2138/am-2017-5727]
¹Faculty of Earth and Life Sciences, Vrije Universiteit, Amsterdam, The Netherlands
²Centre for Planetary Sciences, Birkbeck–UCL, London, U.K.
³Department of Earth Sciences, Mineral and Planetary Sciences Division, Natural History Museum, London, U.K.
Copyright: the Mineralogical Society of America

Geophysical and geochemical observations point to the presence of a light element in the lunar core, but the exact abundance and type of light element are poorly constrained. Accurate constraints on lunar core composition are vital for models of lunar core dynamo onset and demise, core formation conditions (e.g., depth of the lunar magma ocean or LMO) and therefore formation conditions, as well as the volatile inventory of the Moon. A wide range of previous studies considered S as the dominant light element in the lunar core. Here, we present new constraints on the composition of the lunar core, using mass-balance calculations, combined with previously published models that predict the metal–silicate partitioning behavior of C, S, Ni, and recently proposed new bulk silicate Moon (BSM) abundances of S and C. We also use the bulk Moon abundance of C and S to assess the extent of their devolatilization. We observe that the Ni content of the lunar core becomes unrealistically high if shallow (< 3 GPa) LMO scenarios are considered for S and C. The moderately siderophile metal–silicate partitioning behavior of S during lunar core formation, combined with the low BSM abundance of S, yields only < 0.16 wt% S in the core, virtually independent of the pressure (P) and temperature (T) conditions during core formation. Instead, our analysis suggests that C is the dominant light element in the lunar core. The siderophile behavior of C during lunar core formation results in a core C content of ~0.6–4.8 wt%, with the exact amount depending on the core formation conditions. A C-rich lunar core could explain (1) the existence of a present-day molten outer core, (2) the estimated density of the lunar outer core, and (3) the existence of an early lunar core dynamo driven by compositional buoyancy due to core crystallization. Finally, our calculations suggest the C content of the bulk Moon is close to its estimated abundance in the bulk silicate Earth (BSE), suggesting more limited volatile loss during the Moon-forming event than previously thought.

^1,2Cyrena A. Goodrich, ³Noriko T. Kita, ⁴Qing-Zhu Yin, ⁴Matthew E. Sanborn, ⁴Curtis D. Williams, ³Daisuke Nakashima, ²Melissa D. Lane, ^1,5Shannon Boyle
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.021]
¹Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058, USA
²Planetary Science Institute, 1700 E. Ft. Lowell Drive, Tucson, AZ 85719, USA
³WiscSIMS, University of Wisconsin-Madison, Madison, WI 53706, USA
⁴Department of Earth and Planetary Sciences, University of California at Davis, Davis, CA 95616, USA
⁵Department of Earth and Planetary Sciences, Rutgers University, Busch Campus, 610 Taylor Road, Piscataway, NJ 08854, USA
Copyright Elsevier

Northwest Africa (NWA) 7325 is an ungrouped achondrite that has recently been recognized as a sample of ancient differentiated crust from either Mercury or a previously unknown asteroid. In this work we augment data from previous investigations on petrography and mineral compositions, mid-IR spectroscopy, and oxygen isotope compositions of NWA 7325, and add constraints from Cr and Ti isotope compositions on the provenance of its parent body. In addition, we identify and discuss notable similarities between NWA 7325 and clasts of a rare xenolithic lithology found in polymict ureilites.

NWA 7325 has a medium grained, protogranular to poikilitic texture, and consists of 10-15 vol.% Mg-rich olivine (Fo 98), 25-30 vol.% diopside (Wo 45, Mg# 98), 55-60 vol.% Ca-rich plagioclase (An 90), and trace Cr-rich sulfide and Fe,Ni metal. We interpret this meteorite to be a cumulate that crystallized at ⩾1200 °C and very low oxygen fugacity (similar to the most reduced ureilites) from a refractory, incompatible element-depleted melt. Modeling of trace elements in plagioclase suggests that this melt formed by fractional melting or multi-stage igneous evolution. A subsequent event (likely impact) resulted in plagioclase being substantially remelted, reacting with a small amount of pyroxene, and recrystallizing with a distinctive texture.

The bulk oxygen isotope composition of NWA 7325 plots in the range of ureilites on the CCAM line, and also on a mass-dependent fractionation line extended from acapulcoites. The ε54Cr and ε50Ti values of NWA 7325 exhibit deficits relative to terrestrial composition, as do ordinary chondrites and most achondrites. Its ε54Cr value is distinct from that of any analyzed ureilite, but is not resolved from that of acapulcoites (as represented by Acapulco).

In terms of all these properties, NWA 7325 is unlike any known achondrite. However, a rare population of clasts found in polymict ureilites (“the magnesian anorthitic lithology”) are strikingly similar to NWA 7325 in mineralogy and mineral compositions, oxygen isotope compositions, and internal textures in plagioclase. These clasts are probably xenolithic in polymict ureilites, and could be pieces of NWA 7325-like meteorites.

Using constraints from chromium, titanium and oxygen isotopes, we discuss two possible models for the provenance of the NWA 7325 parent body: 1) accretion in the inner solar system from a reservoir similar to that of acapulcoites in Δ17O, ε54Cr and ε50Ti; or 2) early (< 1 Ma after CAI formation) accretion in the outer solar system (beyond the snow line), before 54Cr and 50Ti anomalies were introduced to this region of the solar system. The mid-IR emission spectrum of NWA 7325 obtained in this work matches its modal mineralogy, and so can be compared with spectra of new meteorites or asteroids/planets to help identify similar materials and/or the parent body of NWA 7325