¹Nicholas J. DiFrancesco, ¹Hanna Nekvasil, Donald H. Lindsley, and G. Ustunisik
¹Department of Geosciences, Stony Brook University, Earth and Space Science Building, Stony Brook, New York 11794, U.S.A.
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, U.S.A.

The presence of anorthosite in the lunar highlands containing plagioclase that is compositionally less calcic than plagioclase in the ferroan anorthosites cannot be readily explained by the current lunar paradigm in which lunar anorthosite was produced as a floatation cumulate in the lunar magma ocean. Phase-equilibrium experiments were conducted to investigate whether such anorthosite could arise locally from crystallization of aluminous magma at shallow levels within the lunar crust. The experiments were conducted on a synthetic analog of Cl-, F-, and S-bearing aluminous highland basalt 14053 at pressures of approximately 1 bar and fO2 at ~QIF. Pyroxene and plagioclase (An93–89) saturation occurs early, and with continued crystallization, the residual liquid evolves to a silica-poor, halogen-, Fe-, and Ti-rich melt with a computed density of >3.1 g/mL. This liquid remains higher in density than the plagioclase over the crystallization interval, providing the possibility of plagioclase/melt separation by liquid draining.
A model is proposed in which “alkali” anorthosite, consisting of sodic anorthite or bytownite, coupled with underlying pyroxenite (or harzburgite) is produced locally during crystallization of plagioclase from “Al-rich” magmas at or within roughly a kilometer of the lunar surface. In this model, segregation of plagioclase would be attained by settling of ferromagnesian minerals to the bottom of a shallow magma chamber, and draining of low-viscosity, low-silica, Fe-Ti-K-REE-P-enriched residual basaltic melt to deeper regions of the crust, or into topographic lows. Such residual melt may be represented by magma compositions similar to some of the intermediate- to high-Ti mare basalts. This model would provide a mechanism that can account for the more “alkali” anorthosite identified in widespread isolated locales on the Moon and allow for variable ages for such anorthosite that may extend to ages of the mare basalts.

Reference
DiFrancesco NJ, Nekvasil H, Lindsley DH, Ustunisik G (2015) Low-pressure crystallization of a volatile-rich lunar basalt: A means for producing local anorthosites? American Mineralogist 100, 983-990
Link to Article [doi:10.2138/am-2015-4885]

Copyright: The American Mineralogical Society

¹Sarah T. Crites, ¹Paul G. Lucey
¹Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, 1680 East West Road, POST 602, Honolulu, Hawaii 96822, U.S.A.

Mineralogical measurements from spectral remote sensing and remote geochemical measurements from gamma-ray and neutron spectrometers are complementary data sets that have been used together successfully to study the distributions of iron, titanium, and rare earth elements on the Moon. We compare neutron and gamma-ray data sets from Lunar Prospector and find them in good agreement with each other within the errors of previously developed equations that relate neutron flux with geochemistry, but find small adjustments to the nominal values are warranted. We used the neutron-validated LP GRS oxides to improve Clementine-based global mineral maps. The comparison was enabled by converting the minerals of Lucey (2004) to oxides using stoichiometry and assumptions about Mg#, calcium content of clinopyroxenes, and An#. We find that FeO and Al2O3 derived from the maps of Lucey (2004) do not follow the expected negative correlation seen in lunar samples, but can be brought into agreement with samples and with LP GRS oxides by increasing plagioclase in proportion with orthopyroxene abundance, while simultaneously decreasing Mg#. We interpreted this to mean that plagioclase and orthopyroxene exist in rocks together (as in a noritic rock) with the spectrally difficult to detect plagioclase being masked by the strong spectral signature of the orthopyroxene. We generated a revised set of maps of the major lunar minerals and a map of Mg# for the mafic minerals that are consistent with Lunar Prospector neutron and gamma-ray spectrometer results and show greatly improved agreement with lunar soil samples over previous global mineral maps from Clementine.

Reference
Crites ST, Lucey PG (2015) Revised mineral and Mg# maps of the Moon from integrating results from the Lunar Prospector neutron and gamma-ray spectrometers with Clementine spectroscopy. American Mineralogist 100, 973-982
Link to Article [doi:10.2138/am-2015-4874]

Copyright: The American Mineralogical Society

¹Thomas F. Bristow et al. (>10)*
¹Exobiology Branch, NASA Ames Research Center, Moffett Field, California 94035, U.S.A.
*Find the extensive, full author and affiliation list on the publishers website

The Mars Science Laboratory (MSL) rover Curiosity has documented a section of fluvio-lacustrine strata at Yellowknife Bay (YKB), an embayment on the floor of Gale crater, approximately 500 m east of the Bradbury landing site. X-ray diffraction (XRD) data and evolved gas analysis (EGA) data from the CheMin and SAM instruments show that two powdered mudstone samples (named John Klein and Cumberland) drilled from the Sheepbed member of this succession contain up to ~20 wt% clay minerals. A trioctahedral smectite, likely a ferrian saponite, is the only clay mineral phase detected in these samples. Smectites of the two samples exhibit different 001 spacing under the low partial pressures of H2O inside the CheMin instrument (relative humidity <1%). Smectite interlayers in John Klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 Å, but largely remain open in the Cumberland sample with a basal spacing of ~13.2 Å. Partial intercalation of Cumberland smectites by metal-hydroxyl groups, a common process in certain pedogenic and lacustrine settings on Earth, is our favored explanation for these differences.
The relatively low abundances of olivine and enriched levels of magnetite in the Sheepbed mudstone, when compared with regional basalt compositions derived from orbital data, suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine. Mass-balance calculations are permissive of such a reaction. Moreover, the Sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim. Early diagenetic fabrics suggest clay mineral formation prior to lithification. Thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants, allowing circum-neutral pH. The kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years. Mineralogical evidence of the persistence of benign aqueous conditions at YKB for extended periods indicates a potentially habitable environment where life could establish itself. Mediated oxidation of Fe2+ in olivine to Fe3+ in magnetite, and perhaps in smectites provided a potential energy source for organisms.

Reference
Bristow TF et al. (2015) The origin and implications of clay minerals from Yellowknife Bay, Gale crater, Mars.
American Mineralogist 100, 824-836
Link the Article [doi:10.2138/am-2015-5077CCBYNCND]

Copyright: The American Mineralogical Society

¹Janice L. Bishop, ²Enver Murad, ³M. Darby Dyar
¹Carl Sagan Center, SETI Institute, 189 Bernardo Avenue, Mountain View, California 94043, U.S.A.
²Bahnhofstrasse 1, D-95615 Marktredwitz, Germany
³Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

The ferric oxyhydroxide minerals akaganéite and schwertmannite are associated with acidic environments and iron alteration on Earth and may be present on Mars as well. These minerals have a tunnel structure and are crystallographically related. The extended visible region reflectance spectra of these minerals are characterized by a broad Fe3+ electronic transition centered near 0.92 μm, a reflectance maximum near 0.73 μm, and a shoulder near 0.59 μm. The near-infrared (NIR) reflectance spectra of each of these minerals are dominated by broad overtones and combinations of the H2O vibration features. These occur near 1.44–1.48 and 1.98–2.07 μm (~6750–6950 and 4830–5210 cm−1) in akaganéite spectra, while in schwertmannite spectra they occur at 1.44–1.48 and 1.95–2.00 μm (~6750–6950 and 5005–5190 cm−1). Additional bands due to OH vibrational overtones are found near 1.42 μm (~7040 cm−1) in akaganéite and schwertmannite spectra and due to OH combination bands in akaganéite spectra at 2.46 μm (4070 cm−1) with weaker components at 2.23–2.42 μm (4134–4492 cm−1). A strong and broad band is observed near 2.8–3.1 μm (~3300–3600 cm−1) in reflectance and transmittance spectra of akaganéite and schwertmannite due to overlapping OH and H2O stretching vibrations. H2O bending vibrations occur near 1620 cm−1 (~6.17 μm) in akaganéite spectra and near 1630 cm−1 (~6.13 μm) in schwertmannite spectra with additional bands at lower frequencies due to constrained H2O molecules. OH bending vibrations occur near 650 and 850 cm−1 (~15.4 and 11.8 μm) in akaganéite spectra and near 700 cm−1 (~14.3 μm) in schwertmannite spectra. Sulfate vibrations are observed for schwertmannite as a ν3 triplet at 1118, 1057, and 1038 cm−1 (~8.9, 9.5, and 9.6 μm), ν1 at 982 cm−1 (~10.2 μm), ν4 near 690 cm−1 (~14.5 μm), and ν2 at 608 cm−1 (~16.5 μm). Fe-O bonds occur near 410–470 cm−1 (μm) for akaganéite and schwertmannite. Both minerals readily absorb H2O molecules from the environment and adsorb them onto the mineral surfaces and incorporate them into the tunnels. If akaganéite and schwertmannite were present on the surface of Mars they could enable transport of H2O from the near-surface to the atmosphere as the partial pressure of H2O varies diurnally.

Reference
Bishop JL, Murad E, Dyar MD (2015) Akaganéite and schwertmannite: Spectral properties and geochemical implications of their possible presence on Mars. American Mineralogist, 100, 738-746
Link to Article [doi:10.2138/am-2015-5016]

Copyright: The American Mineralogical Society

¹Javier Cuadros
¹Earth Sciences Department, Natural History Museum, Cromwell Road, London SW7 5BD, U.K.

The Curiosity rover on Mars, landed in 2012, is capable of mineralogical investigation using X-ray diffraction, complementing the abundant infrared remote sensing data already available on clay minerals. We can, however, expect that the in situ X-ray diffraction information will convey a more complex picture than that inferred from infrared spectroscopy alone. CheMin has landed.

Reference
Cuadros J (2015) Clays are messy—also on Mars. American Mineralogist, 100, 669-670
Link to Article [doi:10.2138/am-2015-5229]

Copyright: The Mineralogical Society of America