Juliane Gross^a,b, Allan H. Treiman^b, Celestine N. Mercer^a,1

^aAmerican Museum of Natural History, New York, NY 10024, United States
^bLunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, United States
¹USGS Denver Federal Center, Denver, CO 80225

The composition of the lunar crust provides clues about the processes that formed it and hence contains information on the origin and evolution of the Moon. Current understanding of lunar evolution is built on the Lunar Magma Ocean hypothesis that early in its history, the Moon was wholly or mostly molten. This hypothesis is based on analyses of Apollo samples of ferroan anorthosites (>90% plagioclase; molarMg/(Mg+Fe)=Mg#<75) and the assumption that they are globally distributed. However, new results from lunar meteorites, which are random samples of the Moonʼs surface, and remote sensing data, show that ferroan anorthosites are not globally distributed and that the Apollo highland samples, used as a basis for the model, are influenced by ejecta from the Imbrium basin. In this study we evaluate anorthosites from all currently available adequately described lunar highland meteorites, representing a more widespread sampling of the lunar highlands than Apollo samples alone, and find that ~80% of them are significantly more magnesian than Apollo ferroan anorthosites. Interestingly, Luna mission anorthosites, collected outside the continuous Imbrium ejecta, are also highly magnesian. If the lunar highland crust consists dominantly of magnesian anorthosites, as suggested by their abundance in samples sourced outside Imbrium ejecta, a reevaluation of the Lunar Magma Ocean model is a sensible step forward in the endeavor to understand lunar evolution. Our results demonstrate that lunar anorthosites are more similar in their chemical trends and mineral abundance to terrestrial massif anorthosites than to anorthosites predicted in a Lunar Magma Ocean. This analysis does not invalidate the idea of a Lunar Magma Ocean, which seems a necessity under the giant impact hypothesis for the origin of the moon. However, it does indicate that most rocks now seen at the Moonʼs surface are not primary products of a magma ocean alone, but are products of more complex crustal processes.

Reference
Gross J, Treiman AH and Mercer CN (2014) Lunar feldspathic meteorites: Constraints on the geology of the lunar highlands, and the origin of the lunar crust. Earth and Planetary Science Letters 388:318–328.
[doi:10.1016/j.epsl.2013.12.006]
Copyright Elsevier

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Ben D. Stanley, Marc M. Hirschmann and Anthony C. Withers

Department of Earth Sciences, University of Minnesota, Minneapolis, MN, 55455, USA

To determine the speciation and concentrations of dissolved C-O-H volatiles in graphite-saturated martian primitive magmas, we conducted piston-cylinder experiments on graphite-encapsulated synthetic melt of Adirondack-class Humphrey basaltic composition. Experiments were performed over three orders of magnitude in oxygen fugacity (IW+2.3 to IW-0.8), and at pressures (1-3.2 GPa) and temperatures (1340-1617 °C) similar to those of possible martian source regions. Oxygen fugacities were determined from compositions of coexisting silicate melt+FePt alloy, olivine+pyroxene+FePt alloy, or melt+Fe-C liquid. Infrared spectra of quenched glasses all show carbonate absorptions at 1430 and 1520 cm^-1, with CO₂ concentrations diminishing under more reduced conditions, from 0.50 wt% down to 26 ppm. Carbon contents of silicate glasses and Fe-C liquids were measured using secondary ion mass spectrometry (SIMS) yielding 36-716 ppm and 6.71-7.03 wt%, respectively. Fourier transform infrared (FTIR) and SIMS analysis produced similar H₂O contents of 0.26-0.85 and 0.29-0.40 wt%, respectively. Raman spectra of glasses reveal evidence for OH^– ions, but no indication of methane-related species. FTIR-measured concentrations of dissolved carbonate diminish linearly with oxygen fugacity, but more reduced conditions yield greater dissolved carbonate concentrations than would be expected based on oxidized conditions in previous work. C contents of silicate glasses determined by SIMS are consistently higher than C as carbonate determined by FTIR. Their difference, termed non-carbonate C, correlates well with additional IR absorptions found in reduced glasses (f_O2<IW+0.4) at 1615, 2205, and 3370 cm^-1. These absorption bands are not seen in more oxidized glasses, except B441 (IW+1.7), presumably because they represent reduced C-bearing complexes. The 2205 cm^-1 peak is attributed to a C=O complex, possibly an Fe-carbonyl ion. The 1615 cm^-1 peak does not correlate with that at 2205 cm^-1, but does correlate with non-carbonate C and is in a region commonly associated with C=O bonding. The origin of the peak at 3370 cm^-1 is poorly understood and could potentially be owing to a variety of C-O-H species or to N-H bonding. The intensities of the 1615 and 3370 cm^-1 peaks correlate with each other leading us to provisionally attribute both to an unspecified complex with both C=O and N-H bonds. These results suggest that dissolved species such as carbonyl or other C=O-bearing species could be a significant source of C fluxes to the martian atmosphere, with minor additions of CO₂ and negligible methane contributions. By assuming that degassed, reduced C ultimately becomes atmospheric CO₂, reduced C outgassing may be incorporated in models of martian atmospheric evolution. At Humphrey source region conditions (1350±50 °C, 1.2±0.1 GPa) the total C contents are equivalent to 1200 ppm CO₂ at IW+1 and 475 ppm CO₂ at IW, which are 2 and 4 times higher than the CO₂ derived from CO₃^2- alone. For reasonable magmatic fluxes over the last 4.5 Ga of martian history, such graphite-saturated magmas would produce 0.25 and 0.60 bars from sources at IW and IW+1, significantly more than expected solely from consideration of dissolved CO₂. The carbon contents of Fe-C liquids in this study are consistent with graphite-saturated carbide liquids becoming more C-rich with increasing temperature. Experiments with melt and Fe-C liquid have values of  between 1.3×10³ and 2.2×10³, potentially allowing planetary mantles to retain significant C following core formation.

Reference
Stanley BD, Hirschmann MM and Withers AC (in press) Solubility of C-O-H volatiles in graphite-saturated martian basalts. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.013]
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Thomas F. Nägler¹, Ariel D. Anbar^2,3, Corey Archer⁴, Tatiana Goldberg⁵, Gwyneth W. Gordon², Nicolas D. Greber¹, Christopher Siebert⁶, Yoshiki Sohrin⁷, Derek Vance⁴

¹Institut für Geologie, Universität Bern, Bern, Switzerland
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, USA
⁴Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland
⁵Department of Earth Science and Engineering, Imperial College London, London, UK
⁶Helmholtz Centre for Ocean Research, GEOMAR, Kiel, Germany
⁷Institute for Chemical Research, Kyoto University, Uji, Japan

Molybdenum isotopes are increasingly widely applied in Earth Sciences. They are primarily used to investigate the oxygenation of Earth’s ocean and atmosphere. However, more and more fields of application are being developed, such as magmatic and hydrothermal processes, planetary sciences or the tracking of environmental pollution. Here, we present a proposal for a unifying presentation of Mo isotope ratios in the studies of mass-dependent isotope fractionation. We suggest that the δ^98/95Mo of the NIST SRM 3134 be defined as +0.25‰. The rationale is that the vast majority of published data are presented relative to reference materials that are similar, but not identical, and that are all slightly lighter than NIST SRM 3134. Our proposed data presentation allows a direct first-order comparison of almost all old data with future work while referring to an international measurement standard. In particular, canonical δ^98/95Mo values such as +2.3‰ for seawater and −0.7‰ for marine Fe–Mn precipitates can be kept for discussion. As recent publications show that the ocean molybdenum isotope signature is homogeneous, the IAPSO ocean water standard or any other open ocean water sample is suggested as a secondary measurement standard, with a defined δ^98/95Mo value of +2.34 ± 0.10‰ (2s).

Reference
Nägler TF, Anbar AD, Archer C, Goldberg T, Gordon GW, Greber ND, Siebert C, Sohrin Y and Vance D (in press) Proposal for an International Molybdenum Isotope Measurement Standard and Data Representation. Geostandards and Geoanalytical Research
[doi:10.1111/j.1751-908X.2013.00275.x]
Published by arrangement with John Wiley & Sons

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