¹William H. Farrand, ²Jeffrey R. Johnson, ³Melissa S. Rice, ⁴Alian Wang, ⁵James F. Bell III
American Mineralogist 101, 2005-2019  Link to Article [DOI: 10.2138/am-2016-5627]
¹Space Science Institute, 4750 Walnut Street, number 205, Boulder, Colorado 80301, U.S.A.
²Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, U.S.A.
³Department of Geology, Western Washington University, 516 High Street, Bellingham, Washington 98225, U.S.A.
⁴Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, Missouri 63130, U.S.A.
⁵School of Earth and Space Exploration, Arizona State University, P.O. Box 87104, Tempe, Arizona 85287, U.S.A.
Copyright: The Mineralogical Society of America

Multispectral visible and near infrared (VNIR) observations from the Mars Exploration Rover Pancam multispectral stereo camera systems are consistent with materials having been subjected to various aqueous processes. Ferric oxides in the form of hematite in the Burns and Grasberg formations of Meridiani Planum have been well characterized by Opportunity on the basis of strong 535 and 864 nm absorptions and positive 754–1009 nm and 934–1009 nm slopes. On the rim of Noachian-aged Endeavour crater, Opportunity has observed light-toned veins with high Ca and S, as determined by the rover’s Alpha Particle X-ray Spectrometer (APXS), and a negative 934–1009 nm slope in VNIR spectra extracted from Pancam data, indicative of a 1000 nm H2O overtone absorption. Together these observations indicate that the veins are composed of gypsum. Rocks overturned by Opportunity on the Murray Ridge portion of the Endeavour crater rim display dark- and light-toned coatings. The dark-toned coatings have a red, featureless slope that is consistent with the slope observed in laboratory spectra of high-valence manganese oxide minerals. Potential Mn oxide coatings may also be associated with some exposures of the Grasberg formation. APXS results for high Mg and S in the light-toned coatings of the Murray Ridge overturned rocks and a negative 934–1009 nm slope are consistent with hydrated Mg-sulfates. Opportunity has also observed spectral features in rocks that are consistent with orbital observations of Fe-smectites, as well as Al-smectites and possible hydrated silica in light-toned fracture-fill materials. The Spirit rover observed sulfate-rich light-toned soils exposed by the rover’s wheels. Several of these soil observations contained spectral features, such as a broad absorption centered near 800 nm, consistent with ferric sulfate minerals, a finding confirmed by the rover’s Mössbauer spectrometer. Spirit also excavated light-toned Si-rich soils. These soils have a flat near-infrared spectrum with a drop in reflectance from 934–1009 nm that is consistent with free water contained in voids or adsorbed onto the surface of the silica.

^1,2P. Mane, ¹R. Hervig, ^1,2M. Wadhwa, ^1,2L. A. J. Garvie, ^3,4J. B. Balta, ³H. Y. McSween Jr
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12717]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Center for Meteorite Studies, Arizona State University, Tempe, Arizona, USA
³Department of Earth and Planetary Sciences and Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee, USA
⁴Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
Published by arrangement with John Wiley & Sons

The hydrogen isotopic composition of planetary reservoirs can provide key constraints on the origin and history of water on planets. The sources of water and the hydrological evolution of Mars may be inferred from the hydrogen isotopic compositions of mineral phases in Martian meteorites, which are currently the only samples of Mars available for Earth-based laboratory investigations. Previous studies have shown that δD values in minerals in the Martian meteorites span a large range of −250 to +6000‰. The highest hydrogen isotope ratios likely represent a Martian atmospheric component: either interaction with a reservoir in equilibrium with the Martian atmosphere (such as crustal water), or direct incorporation of the Martian atmosphere due to shock processes. The lowest δD values may represent those of the Martian mantle, but it has also been suggested that these values may represent terrestrial contamination in Martian meteorites. Here we report the hydrogen isotopic compositions and water contents of a variety of phases (merrillites, maskelynites, olivines, and an olivine-hosted melt inclusion) in Tissint, the latest Martian meteorite fall that was minimally exposed to the terrestrial environment. We compared traditional sample preparation techniques with anhydrous sample preparation methods, to evaluate their effects on hydrogen isotopes, and find that for severely shocked meteorites like Tissint, the traditional sample preparation techniques increase water content and alter the D/H ratios toward more terrestrial-like values. In the anhydrously prepared Tissint sample, we see a large range of δD values, most likely resulting from a combination of processes including magmatic degassing, secondary alteration by crustal fluids, shock-related fractionation, and implantation of Martian atmosphere. Based on these data, our best estimate of the δD value for the Martian depleted mantle is −116 ± 94‰, which is the lowest value measured in a phase in the anhydrously prepared section of Tissint. This value is similar to that of the terrestrial upper mantle, suggesting that water on Mars and Earth was derived from similar sources. The water contents of phases in Tissint are highly variable, and have been affected by secondary processes. Considering the H2O abundances reported here in the driest phases (most likely representing primary igneous compositions) and appropriate partition coefficients, we estimate the H2O content of the Tissint parent magma to be ≤0.2 wt%.

¹Walter S. Kiefer, ^1,2Qingsong Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12720]
¹Lunar and Planetary Institute, Houston, Texas, USA
²BP, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Based on meteorite evidence, the present-day Martian mantle has a combined abundance of up to a few hundred ppm of H2O, Cl, and F, which lowers the solidus and enhances the magma production rate. Adiabatic decompression melting in upwelling mantle plumes is the best explanation for young (last 200 Myr) volcanism on Mars. We explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting. Relative to a dry mantle, the reduction in solidus temperature due to water increases the magma production rate by a factor of 1.3–1.7 at 50 ppm water and by a factor of 1.9–3.2 at 200 ppm water. Mantle water also decreases the viscosity and increases the vigor of convection, which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity. At conditions relevant to Mars, these indirect effects can cause an order of magnitude increase in the magma production rate. Using geologic and geophysical observations of the Late Amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites, present-day Mars is constrained to have a core–mantle boundary temperature of ~1750 to 1800 °C and a volume-averaged thermal Rayleigh number of 2 × 106 to 107, indicating that moderately vigorous mantle convection has persisted to the present day. Melting occurs at depths of 2.5–6 GPa and is controlled by the Rayleigh number at the low pressure end and by the mantle water concentration at high pressure.