The gallium isotopic composition of the Moon

1Josh Wimpenny,1Lars Borg,1,2Corliss Kin I Sio
Earth and Planetary Science Letters 578, 117318 Link to Article []
1Lawrence Livermore National Laboratory, Livermore, CA 94550, United States of America
2Department of Earth Sciences, University of Toronto, Ontario, Canada
Copyright Elsevier

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The Ga values of these samples range from -0.10 to +0.66‰ (where Ga is the relative difference between the 71Ga/69Ga ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have Ga values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher Ga values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution.

Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Gaplagioclase-melt of -0.3‰, (where Gaplagioclase-melt is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have Ga values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP.

We estimate that the Ga value of the LMO was ∼0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this Ga value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 ± 0.06‰, Kato et al., 2017) and volatile loss drove the LMO to higher Ga values prior to formation of the lunar crust.

Beckettite, Ca2V6Al6O20, a new mineral in a Type A refractory inclusion from Allende and clues to processes in the early solar system

1Chi Ma,2Alexander N. Krot,1Julie Paque,3Oliver Tschauner,1Kazuhide Nagashima
Meteoritics & Planetary Science (in Press) Link to Article []
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 USA
2Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, 96822 USA
3Department of Geoscience, University of Nevada, Las Vegas, Nevada, 89154 USA
Published by arrangement with John Wiley & Sons

Beckettite (Ca2V6Al6O20; IMA 2015-001) is a newly discovered refractory mineral, occurring as micrometer-sized grains intergrown with hibonite and perovskite, and surrounded by secondary grossular, anorthite, coulsonite, hercynite, and corundum. It occurs within highly altered areas in a V-rich, Type A Ca-Al-rich inclusion (CAI), A-WP1, from the Allende CV3 carbonaceous chondrite. The type beckettite has an empirical formula of (Ca1.99Na0.01)(V3+3.47Al1.40Ti4+0.57Mg0.25Sc0.08Fe2+0.04)(Al5.72Si0.28)O20, with a triclinic structure in space group Purn:x-wiley:10869379:media:maps13771:maps13771-math-0001 and cell parameters a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106.0°, β = 96.0°, γ = 124.7°, V = 739.7 Å3, and Z = 2, which leads to a calculated density of 3.67 g cm−3. Beckettite’s general formula is Ca2(V,Al,Ti,Mg)6Al6O20 and the endmember formula is Ca2V6Al6O20. Beckettite is slightly 16O-depleted (Δ17O = −16 ± 2‰) compared to the coexisting hibonite and spinel −24 ± 2‰. Beckettite is a primary high-temperature mineral resulting from igneous crystallization of an 16O-rich V-rich CAI melt together with V-bearing hibonite, perovskite, burnettite, spinel, and paqueite. Subsequently, beckettite experienced an incomplete isotope exchange with an 16O-poor aqueous fluid (Δ17O = −3 ± 2‰) on the Allende parent asteroid.

Hydrothermal alteration at the basalt-hosted Vista Alegre impact structure, Brazil

1Jitse Alsemgeest,1Fraukje M. Brouwer,2Luis F. Auqué,3Natalia Hauser,3Wolf Uwe Reimold
Meteoritics & Planetary Science (in Press) Link to Article []
1Geology and Geochemistry Cluster, Faculty of Science, Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081HV The Netherlands
2Department of Geosciences, University of Zaragoza, Calle Pedro Cerbuna 12, Zaragoza, 50009 Spain
3Laboratory of Geochronology and Isotope Geochemistry, Geosciences Institute, University of Brasília, Brasília, DF, CEP 70910-900 Brazil
Published by arrangement with John Wiley & Sons

Hydrothermal systems provide a possible habitat for early life and are key targets in the quest for life outside Earth. In impact craters on Mars, hydrous minerals can represent products of impact-generated hydrothermal systems (IGHS) or minerals already present in the crust and exposed during impact-caused excavation. Because of its basaltic target rock, similar in composition to Martian crust, the Vista Alegre impact structure in Brazil is one of the very few analog structures that may reveal the origin of these minerals, if evidence of hydrothermal alteration is established. This work presents the results of a systematic search for evidence of hydrothermal alteration at the Vista Alegre impact structure. Four types of alteration were identified, all within a 2.5–3.0 km radius from the crater center: a zircon-bearing melt veinlet, two sets of hydrothermal veins consisting predominantly of calcite and chabazite, and local alteration comprising saponite. Thermodynamic modeling suggests subsequent heating and cooling for each of the hydrothermal vein sets. Combined thermodynamic and spectrometric evidence indicates that development of a vigorous IGHS is unlikely. If similar processes occur on Mars, hydrous minerals are more likely preimpact phases exposed by excavation, rather than being formed through an IGHS.

Chemical characteristics of iron meteorite parent bodies

1,2Connor D.Hilton,1Richard D.Ash,1Richard J.Walker
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Department of Geology, University of Maryland, College Park, Maryland, 20742, USA
2Present address: Environmental Signatures Team, Pacific Northwest National Laboratory, Richland, Washington, 99354, USA
Copyright Elsevier

The projected relative abundances of the highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, and Pd) for bulk parent bodies of 10 magmatic iron meteorite groups/grouplet (IC, IIAB, IIC, IID, IIF, IIIAB, IIIF, IVA, IVB, and South Byron Trio) are broadly similar and show no resolvable differences between noncarbonaceous (NC) and carbonaceous (CC) genetic heritage. The processes driving genetic isotopic heterogeneity in the early Solar System, therefore, evidently did not leave discernable chemical fingerprints with respect to HSE relative abundances on the bulk planetesimal scale. By contrast, the absolute abundances of HSE projected for parent body cores, which reflect core size, are more variable and, on average, higher in CC bodies compared to NC bodies. Overall, bulk core chemical compositions, as well as core size, are linked to the distribution of Fe within a parent body, which is controlled by its oxidation state. The CC parent bodies are constrained to have formed under heterogeneous oxidizing conditions which were, on average, more oxidizing than those of the NC environment.

Spectral reflectance properties of minerals exposed to martian surface conditions: Implications for spectroscopy-based mineral detection on Mars

1Nathalie Turenne,1Alexis Parkinson,1Daniel M.Applin,1Paul Mann,1Edward A.Cloutisa,2Stanley A.Mertzman
Planetary and Space Science (in Press) Link to Article []
1Centre for Terrestrial and Planetary Exploration, University of Winnipeg, Winnipeg, Manitob, R3B 2E9, Canada
2Department of Earth and Environment, Franklin and Marshall College, P.O. Box 3003, Lancaster, PA, 17604, 3003, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

The evidence for unusually high hydrogen abundances in the central part of Valles Marineris on Mars

Icarus (in Press) Link to Article []
1Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
2European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, Netherlands
Copyright Elsevier

Studies of hydrogen deposition in the shallow Martian subsurface have been conducted by two neutron and one gamma-ray detectors in the past and provided global hydrogen maps (Boynton et al., 2002; Feldman et al., 2002; Mitrofanov et al., 2002). It is known from these maps that hydrogen is most abundant in the polar permafrost areas compared to the equatorial band where frozen water is not stable on the surface. However, the spatial resolution of hundreds of kilometres typical for these maps does not allow for detection of local hydrogen-rich features that can be associated with geological structures. FREND neutron telescope (Mitrofanov et al., 2018) onboard ExoMars TGO (Vago et al., 2015) is capable of a much better spatial resolution for mapping neutron emission of Mars. In this Report we present the analysis of the most intriguing local area of highly suppressed neutron emission in the vicinity of the Martian equator, which coincides with Candor Chaos in the central area of Valles Marineris, thought to be promising for testing water ice (Gourronc et al., 2014). Provided such suppression would be interpreted as the evidence for very high content of hydrogen in the soil, the mean water equivalent hydrogen value in the local suppression area should be as large as 40.3 wt%. This finding is thought to be uncommon for equatorial regions, but is probably associated with particular geomorphological conditions inside Valles Marineris.

MAGMARS: a Melting Model for the Martian Mantle and FeO-rich Peridotite

1Max Collinet,1Ana-Catalina Plesa,2Timothy L. Grove,1Sabrina Schwinger,3,1Thomas Ruedas,1Doris Breuer
Journal of Geophysical Research (Planets) (In Press) Link to Article []
1German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstraße 2, 12 489 Berlin Germany
2Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Avenue, MA, 02 139 USA
3Museum für Naturkunde Berlin, Impact and Meteorite Research, Invalidenstraße 43, 10 115 Berlin Germany
Published by arrangement with John Wiley & Sons

Martian basalts identified by rover in-situ analyses and the study of meteorites represent a direct link to the melting process in the planet’s interior and can be used to reconstruct the composition of the mantle and estimate its temperature. Experimentally calibrated numerical models are powerful tools to systematically search for the mantle compositions and melting conditions that can produce melts similar to primary basalts. However, currently available models are not suitable for modeling the melting of FeO-rich peridotites. In this study, we present experiments performed at 1.0 and 2.4–2.6 GPa on a primitive Martian mantle with various P2O5 contents. We use the new experiments together with a comprehensive database of previous melting experiments to calibrate a new model called MAGMARS. This model can reproduce the experimental melt compositions more accurately than Gibbs free energy minimization software (e.g. pMELTS) and can simulate near-fractional polybaric melting of various mantle sources. In addition, we provide an updated thermobarometer that can estimate the P–T melting conditions of primary melts and can be used as a prior step to constrain the input parameters of the MAGMARS melting model. We apply MAGMARS to estimate the source composition of the Adirondack-class basalts and find that melting a depleted mantle, at 2.3–1.7 GPa (Tp=1390±40°C) can best explain their bulk composition and K2O/Na2O ratio. MAGMARS represents a fast and accurate alternative to calculate the composition of the Martian primary melts and can be used as a stand-alone package or integrated with geodynamical models or other independent modeling software.