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Mineralogy and chemistry of San Carlos high-alkali basalts: Analyses of alteration with application for Mars exploration

Hadnott, Bryné A. and Ehlmann, Bethany L. and Jolliff, Bradley L. (2017) Mineralogy and chemistry of San Carlos high-alkali basalts: Analyses of alteration with application for Mars exploration. American Mineralogist, 102 (2). pp. 284-301. ISSN 0003-004X. doi:10.2138/am-2017-5608.

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The discovery of Fe, Mg, and Al phyllosilicates on Mars using visible and short-wave infrared (VSWIR) spectroscopy from orbit indicates aqueous alteration of basaltic rocks. Analyses at Gusev Crater by the Spirit rover and Gale Crater by the Curiosity rover have discovered alkaline basaltic rocks. In this work, multiple methods—VSWIR spectroscopy, X-ray diffraction (XRD), and chemical analyses—were used to study a suite of alkaline basalts from San Carlos, Arizona, which have been altered by water in an oxidative, semi-arid environment. As an analog for the weathering of alkaline basaltic rocks on Mars, a suite of rocks visually identified to have different degrees of alteration were characterized to understand the spectral, mineralogical, and chemical trends in alteration as sensed by multiple techniques. Samples with strong 1.9 μm H_2O-related absorptions in VSWIR commonly exhibited absorption bands at 1.4, 2.2, and/or 2.3 μm, indicating the presence of clay minerals or silica as well as features at 0.5–0.9 μm indicative of ferric iron oxides. Primary mineralogy for all samples, as determined by point analyses with the microprobe and XRD, consisted of olivine, plagioclase, nepheline, augite, and titanomagnetite. Compositional imaging and spot analyses with the microprobe revealed distinct alteration textures and phases, suggesting weathering pathways involving the oxidation of iron in olivine and primary Fe^(2+) oxides to form Fe^(3+) oxides as well as the formation of aluminum phyllosilicates and magnesium phyllosilicates from feldspars and olivines, respectively, while pyroxene remained relatively unaltered. Bivariate plots of major oxides both from bulk-chemical analysis and microprobe measurements also revealed trends in alkali and silica depletion and calcium enrichment, but there was little chemical fractionation in most of the major oxides. The strength of the 1.9 μm H_2O absorption, loss on ignition, and depletion in silica and sodium, correlated with increasing alteration. The data sets provide an analog for understanding possible weathering pathways in martian alkaline basalts and thresholds for the detection of aqueous alteration in multiple data sets.

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Ehlmann, Bethany L.0000-0002-2745-3240
Additional Information:© 2017 Mineralogical Society of America. Open Access, thanks to the authors’ funding. Article available to all readers via GSW( and the MSA web site. Manuscript received October 15, 2015; Manuscript accepted September 12, 2016; First published on February 01, 2017. Manuscript handled by Pablo Sobron. Many thanks to Raymond Arvidson for guidance and Paul Carpenter for invaluable/help using the electron microprobe and X-ray diffractometer. This work was partially funded by an MSL Participating Scientist grant to Bethany Ehlmann and the Caltech Summer Undergraduate Research Fellowship provided programmatic support to Bryné Hadnott. Use of the Bruker d8 Advance X-ray diffractometer in Earth and Planetary Sciences at Washington University in St. Louis was supported by the National Science Foundation, award no. NSF EAR-1161543.
Group:Astronomy Department
Funding AgencyGrant Number
Mars Science Laboratory (MSL)UNSPECIFIED
Caltech Summer Undergraduate Research Fellowship (SURF)UNSPECIFIED
Subject Keywords:VSWIR spectroscopy, Mars analogs, XRD, geochemistry, alkali basalts, San Carlos
Issue or Number:2
Record Number:CaltechAUTHORS:20170203-082803269
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Official Citation:Mineralogy and chemistry of San Carlos high-alkali basalts: Analyses of alteration with application for Mars exploration Bryné A. Hadnott, Bethany L. Ehlmann, Bradley L. Jolliff American Mineralogist Feb 2017, 102 (2) 284-301; DOI: 10.2138/am-2017-5608
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:74004
Deposited By: Tony Diaz
Deposited On:03 Feb 2017 16:51
Last Modified:11 Nov 2021 05:24

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