¹E.Nakamura et al. (>10)
Proceedings of the Japan Academy. Series B, Physical and biological sciences 95, 165-177 Link to Article [DOI: 10.2183/pjab.95.013]
¹Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Planetary Materials, Okayama University, Japan

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¹Lidia Pittarello et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.04.033]
¹Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Copyright Elsevier

Melting experiments attempting to reproduce some of the processes affecting asteroidal and cometary material during atmospheric entry have been performed in a high enthalpy facility. For the first time with the proposed experimental setup, the resulting material has been recovered, studied, and compared with natural analogues, focusing on the thermal and redox reactions triggered by interaction between the melt and the atmospheric gases under high temperature and low pressure conditions. Experimental conditions were tested across a range of parameters, such as heat flux, experiment duration, and pressure, using two types of sample holders materials, namely cork and graphite. A basalt served as asteroidal analog and to calibrate the experiments, before melting a H5 ordinary chondrite meteorite. The quenched melt recovered after the experiments has been analyzed by μ-XRF, EDS-SEM, EMPA, LA-ICP-MS, and XANES spectroscopy.
The glass formed from the basalt is fairly homogeneous, depleted in highly volatile elements (e.g., Na, K), relatively enriched in moderately siderophile elements (e.g., Co, Ni), and has reached an equilibrium redox state with a lower Fe3+/Fetot ratio than that in the starting material. Spherical objects, enriched in SiO2, Na2O and K2O, concentrations, were observed, inferring condensation from the vaporized material. Despite instantaneous quenching, the melt formed from the ordinary chondrite shows extensive crystallization of mostly olivine and magnetite, the latter indicative of oxygen fugacity compatible with presence of both Fe2+ and Fe3+. Similar features have been observed in natural meteorite fusion crusts and in micrometeorites, implying that, at least in terms of maximum temperature reached and chemical reactions, the experiments have successfully reproduced the conditions likely encountered by extraterrestrial material following atmospheric entry.

¹Charity M.Phillips-Lander,¹Andrew S.Elwood Madden,²Elisabeth M.Hausrath,¹Megan Elwood Madden
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.006]
¹School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, Norman, OK 73069, USA
²Department of Geoscience, University of Nevada, Las Vegas 4505 S. Maryland Ave., Las Vegas, NV 89154
Copyright Elsevier

Both high and low calcium pyroxene minerals have been detected over large portions of the martian surface in addition to widespread salts in martian soils and dust. Calcium pyroxenes in martian meteorites are associated with secondary evaporite phases, including sulfates, chlorides, and perchlorates, suggesting the pyroxene may have been altered in saline solutions. Therefore, understanding pyroxene mineral weathering in high salinity brines may provide insight into past aqueous alteration on Mars. This study examines both solute-based dissolution rates and qualitative assessments of weathering textures developed during pyroxene-brine alteration experiments to link dissolution rates and textures and aid in interpreting weathering features observed in Mars meteorites and future pyroxene samples returned from Mars. Batch reactor dissolution experiments were conducted at 298 K to compare diopside (a high Ca-pyroxene) dissolution rates in water (18 MΩcm-1 ultrapure water (UPW); activity of water (ɑH2O) =1.0), 0.35 mol kg-1 NaCl (ɑH2O =0.99), 0.35 mol kg-1 Na2SO4 (ɑH2O =0.98), 2 mol kg-1 NaClO4 (ɑH2O =0.90), 2.5 mol kg-1 Na2SO4 (ɑH2O =0.95), 5.7 mol kg-1 NaCl (ɑH2O =0.75), and 9 mol kg-1 CaCl2 (ɑH2O =0.35) brines at pH 5-6.6 to determine how changing solution chemistry and activity of water influence pyroxene dissolution. Aqueous Si release rates and qualitative textural analyses indicate diopside dissolution rates are influenced by both solution chemistry and activity of water, with diopside weathering increasing along a trend from: 9 mol kg-1 CaCl2 < UPW (-9.82± 0.03 log mol m-2 s-1) ≈ 2 mol kg-1 NaClO4 ≈ 0.35 mol kg-1 Na2SO4 (-9.80± 0.07) ≈ 5.7 mol kg-1 NaCl (-9.69 ± 0.04) < 0.35 mol kg-1 NaCl (-9.45± 0.34) < 2.5 mol kg-1 Na2SO4 (-8.99± 0.09). Dissolution rates increase in sodium sulfate brines with increasing salinity. In contrast, Si-based dissolution rates in 0.35 mol kg-1 NaCl are faster than those measured in 5.7 mol kg-1 NaCl and UPW. However, all of the Si-based rates measured in the chloride and sulfate salt solutions are likely affected by precipitation of Si-rich secondary clay minerals, which removed Si from solution. Qualitative textural analyses indicate similar degrees of dissolution occurred in UPW and 2 M NaClO4; however, no aqueous rate determinations could be made in perchlorate brines due to explosion hazards. Aqueous Si was below detection limits in the 9 mol kg-1 CaCl2 experiments, but textural analysis suggests limited diopside dissolution occurred. Therefore, despite low water activity, diopside dissolution proceeds in both dilute to high salinity brines, readily forming clay minerals under a wide range of conditions. This suggests that outcrops on Mars containing pyroxene preserved with sulfate, chloride, perchlorate, and/or clay minerals likely record relatively short periods (<1 million years) of aqueous alteration. Si-rich spherules similar to those observed in SNC meteorites were also observed in the 5.7 mol kg-1 NaCl brine experiments, indicating that silicate mineral alteration in chloride brines may lead to Si-rich alteration products and coatings.