S.R. Gainey^a, E.M. Hausrath^a, J.A. Hurowitz^b, R.E. Milliken^c

^aDepartment of Geoscience, University of Nevada, Las Vegas 4505 S. Maryland Parkway Las Vegas, NV 89154-4010
^bDepartment of Geosciences, Stony Brook University, 255 Earth and Space Sciences Building (ESS), Stony Brook University, Stony Brook, NY 11794-2100
^cGeological Sciences, Brown University, 324 Brook Street Box 1846 Providence, RI 02912

The Fe-rich smectite nontronite M⁺_1.05[Si_6.98Al_1.02][Al_0.29Fe_3.68Mg_0.04]O₂₀(OH)₄ has been detected using orbital data at multiple locations in ancient terrains on Mars, including Mawrth Vallis, Nilli Fossae, north of the Syrtis Major volcanic plateau, Terra Meridiani, and the landing site of the Mars Science Laboratory (MSL), Gale Crater. Given the antiquity of these sites (>3.0Ga), it is likely that nontronite has been exposed to the martian environment for long periods of time and therefore provides an integrated record of processes in near surface environments including pedogenesis and diagenesis. In particular, nontronite detected at Mawrth Vallis, is overlain by montmorillonite and kaolinite, and it has been previously suggested that this mineralogical sequence may be the result of surface weathering. In order to better understand clay mineral weathering on Mars, we measured dissolution rates of nontronite in column reactors at solution pH values of 0.9, 1.7, and 3.0, and two flow rates (0.16 ml/hr and 0.32 ml/hr). Solution chemistry indicates stoichiometric dissolution at pH = 0.9 and non-stoichiometric dissolution at pH = 1.7 and 3.0. Mineral dissolution rates based on elemental release rates at pH = 1.7 and 3.0 of Ca, Si and Fe follow the order interlayer> tetrahedral> octahedral sites, respectively. The behavior of all experiments suggest far from equilibrium conditions, with the exception of the experiment performed at pH 3.0 and flow rate 0.16 ml/h. A pH-dependent dissolution rate law was calculated through Si release from experiments that showed no dependence on saturation (far from equilibrium conditions) under both flow rates and is r = 10^{-12.06 (±0.123)} • 10^{-0.297 (±0.058)•pH} where r has the units mol mineral m^-2s^-1. When compared to dissolution rates from the literature, our results indicate that nontronite dissolution is significantly slower than dissolution of the primary phases present in basalt under acidic conditions, suggesting that once nontronite forms it could remain stable at or near the surface of Mars for extended periods of time. Nontronite dissolution rates are faster than dissolution rates of montmorillonite (Rozalén et al., 2008) and kaolinite (Huertas et al., 1999), suggesting that chemical weathering of a mixed clay deposit would enrich the proportions of montmorillonite and kaolinite through the preferential dissolution of nontronite. VIS-NIR analyses of our reacted products and thermodynamic modeling of our experimental conditions both indicate the precipitation of amorphous silica within columns, and amorphous silica has also been observed in association with phyllosilicates on the martian surface ( ,  and ). Therefore, chemical weathering of strata containing mixtures of montmorillonite, nontronite and kaolinite provides a potential formation mechanism for the mineralogic stratigraphy observed at Mawrth Vallis and other locations on Mars.

Reference
Gainey SR, Hausrath EM, Hurowitz JA and Milliken RE (in press) Nontronite Dissolution Rates and Implications for Mars. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.055]
Copyright Elsevier

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Koji Wada¹, Hidekazu Tanaka², Satoshi Okuzumi³, Hiroshi Kobayashi⁴, Toru Suyama⁵, Hiroshi Kimura⁶ and Tetsuo Yamamoto⁷

¹Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, 275-0016 Chiba Japan
²Institute of Low Temperature Science, Hokkaido University, 060-0819 Sapporo, Japan
³Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, 152-8511 Tokyo, Japan
⁴Department of Physics, Nagoya University, Nagoya, 464-8602 Aichi, Japan
⁵Nagano City Museum, Hachimanpara Historical Park Ojimada-machi, 381-2212 Nagano, Japan
⁶Graduate School of Science, Kobe University, c/o CPS (Center for Planetary Science), Chuo-ku Minatojima Minamimachi 7-1-48, 650-0047 Kobe, Japan
⁷CPS (Center for Planetary Science), Kobe University, Chuo-ku Minatojima Minamimachi 7-1-48, 650-0047 Kobe, Japan

Context. Collisional growth of dust aggregates is an essential process in forming planetesimals in protoplanetary disks, but disruption through high-velocity collisions (disruption barrier) could prohibit the dust growth. Mass transfer through very different-sized collisions has been suggested as a way to circumvent the disruption barrier.
Aims. We examine how the collisional growth efficiency of dust aggregates with different impact parameters depends on the size and the mass ratio of colliding aggregates.
Methods. We used an N-body code to numerically simulate the collisions of different-sized aggregates.
Results. Our results show that high values for the impact parameter are important and that the growth efficiency averaged over the impact parameter does not depend on the aggregate size, although the growth efficiency for nearly head-on collisions increases with size. We also find that the averaged growth efficiency tends to increase with increasing mass ratio of colliding aggregates. However, the critical collision velocity, above which the growth efficiency becomes negative, does not strongly depend on the mass ratio. These results indicate that icy dust can grow through high-velocity offset collisions at several tens of m s^-1, the maximum collision velocity experienced in protoplanetary disks, whereas it is still difficult for silicate dust to grow in protoplanetary disks.

Reference
Wada K, Tanaka H, Okuzumi S, Kobayashi H, Suyama T, Kimura H and Yamamoto T (2013) Growth efficiency of dust aggregates through collisions with high mass ratios. Astronomy & Astrophysics 559:A62.
[doi:10.1051/0004-6361/201322259]
Reproduced with permission © ESO

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Gerd Weckwerth

Institut für Geologie und Mineralogie, Universität Köln, Zülpicher Str. 49b, 50674, Köln, Germany

We currently seek a copyright agreement with Springer to display abstracts of their cosmochemistry related publications.

Reference
Weckwerth G (in press) Instrumental neutron activation analyses of the most Earth-like meteorites. Journal of Radioanalytical and Nuclear Chemistry
[doi:10.1007/s10967-013-2817-z]

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