Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Hydrothermally-altered dacite terrains in the Methana peninsula Greece: Relevance to Mars Edward A. Cloutis a, ⁎ , Victoria Jonatanson a , Joshua L. Bandfield b , Elena S. Amador c , Frances Rivera-Hernández d , P. Mann a , Stanley A. Mertzman e a Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9 b Space Science Institute, Boulder, CO 80301, USA c Department of Earth and Space Sciences, University of Washington, 4000 15th Avenue NE, Seattle, WA 98195-1310, USA d Department of Earth and Planetary Sciences, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA e Department of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-2615, USA ARTICLE INFO Keywords: Mars Spectroscopy Igneous Mineralogy ABSTRACT Dacitic rocks, often indicative of crustal recycling on Earth, have been identified in some regions on Mars, as have possible hydrothermally/aqueously-altered dacites. To enable more robust identification of unaltered and altered dacites on Mars and other planetary bodies, we undertook a spectroscopic-structural-compositional study of altered and unaltered dacites from a dacitic volcanic region in Methana, Greece. Dacites erupted in this region range from fresh to pervasively hydrothermally altered, resulting in friable, Si-enriched products, as well as fumarolic deposition of Si and S-rich precipitates. Spectrally, fresh dacites are unremarkable in the 0.35–2.5 μm region with low, generally flat, reflectance and few, if any, absorption bands. Dacite infrared spectra exhibit Si-O absorption features in the 8–10 μm region (which are characteristic of Si-bearing rocks, in general). With increasing alteration, reflectance over the 0.35–2.5 μm range increases, absorption bands in the 1.4 and 1.9 μm region, associated with H 2 O/OH, and in the 2.2–2.3 μm region, associated with SiOH, become deeper, Fe 3+ - associated absorption bands in the 0.43 and 0.9 μm region appear, and the Christiansen feature near 8 μm moves to shorter wavelengths. Silica-rich coatings appear to be spectrally indistinguishable from Si-rich alteration. Alteration-formed sulfates may be detectable by the presence of diagnostic absorption features in the 0.35– 2.5 μm region. Spectral similarities between different poorly crystalline high-Si phases make it difficult to uniquely determine the processes that formed high-Si surfaces that have been identified on Mars. However, the samples described here show a variety of spectral features that correspond to variable amounts of alteration. We find a similar range of spectral features, likely due to similar phases, on Mars, perhaps indicating a similar range of alteration environments. Comparison of laboratory spectra to Mars observational data also suggests that the major Si-rich regions likely consist of assemblages that more mineralogically complex than those included in this study. 1. Introduction Dacite is defined by the International Union of Geological Sciences (IUGS) as an intermediate igneous rock containing between 63% and 77% SiO 2 and up to 7% total alkalis (Na 2 O+K 2 O) (Le Bas and Streckeisen, 1991). Petrologically, they are defined as being plagioclase feldspar-rich and quartz- and alkali feldspar-poor (Le Bas and Streckeisen, 1991). Dacitic-type compositions can form as primary igneous products, or as a result of alteration of preexisting rocks through a variety of processes. Therefore, the presence of dacitic rocks can be used to infer or determine the presence of more evolved magmatic processes and/or the operation of hydrothermal alteration that can result in an increase in bulk silica of more mafic precursors. The spectroscopic properties of dacites have not been comprehen- sively studied. Hunt et al. (1973) measured reflectance spectra (0.35– 2.5 μm) of three dacites. They found that the low wavelength region was dominated by Fe 3+ -associated absorption bands, while the longer wavelength region spectra were flat to slightly red-sloped and showed absorption bands near 1.4, 1.9, and 2.5 μm, all attributable to water of hydration. They noted that the bands were surprisingly weak given the http://dx.doi.org/10.1016/j.pss.2017.01.013 Received 3 August 2016; Received in revised form 24 January 2017; Accepted 26 January 2017 ⁎ Corresponding author. E-mail addresses: e.cloutis@uwinnipeg.ca (E.A. Cloutis), tjonatanson@gmail.com (V. Jonatanson), paul.mann347@gmail.com (J.L. Bandfield), jbandfield@spacescience.org (E.S. Amador), esamador@uw.edu (F. Rivera-Hernández), friverah@ucdavis.edu (P. Mann), stan.mertzman@fandm.edu (S.A. Mertzman). Planetary and Space Science 138 (2017) 55–74 Available online 09 February 2017 0032-0633/ © 2017 Elsevier Ltd. All rights reserved. MARK