Characterization of Hydrothermal Alteration in Costa Rica: Mineralogy, Methodology, and Implications for Mars

Introduction: Many locations on Mars with mineral assemblages consisting of phyllosilicates, sulfates, and hydrated silica have been identified using the Mars Reconnaissance Orbiter’s (MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the Mars Express’s (MEx) Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) data [1-3]. With our current understanding of hydrothermal alteration and Martian mineralogy, there are some areas on Mars that show putative hydrothermal characteristics, such as Gusev Crater’s Home Plate [48], Nili Patera [9], Noctis Labyrinthus [10, 11], and Eastern Coprates Chasma [12]. Proper characterization of mineralogy is an essential part of geologic interpretation. Hydrothermal alteration products often exist in intimate mixtures, and vary widely across a site due to differing pH, temperature, and fluid/gas chemistries. These characteristics require that we develop a detailed understanding regarding the environmental controls on resultant mineral mixtures, and how these various combinations appear in different instrument data sets. Mars rovers carry with them a wide range of analytical tools to aid in the interpretation of Mars’ geologic history. However, many instruments are best suited to detect specific materials, and may miss others. Often, multiple instruments must be utilized to develop a comprehensive understanding of a sample. If we want to search for signs of ancient life on Mars, one likely target is relict hydrothermal systems. Hydrothermal systems are highly variable on the fine scale, and must be assessed in-situ to provide confident interpretations. Therefore, it is vital that we improve our understanding of alteration processes and products, as well as our interpretative capabilities using instrumentation onboard current and future spacecraft. Methodology: An initial site survey field campaign for Poás and Turrialba volcanoes (chosen for their compositional similarities to TES surface measurements and Pathfinder APXS data, respectively) was conducted in November 2013. 130 in-situ VNIR samples and 58 hand samples were gathered on Poás and Turrialba volcanoes, focusing on areas with active fumaroles, areas of prior fumarolic activity, and areas in the current acidic steam cloud. Sample collection included all visible mineralogical variations, based on color and texture. The field-portable VNIR spectrometers utilized in this study are the TerraSpec4 high resolution reflectance spectrometer, and the TerraSpec Halo Gun from Analytical Spectral Devices (ASD), Inc. in Boulder, Colorado. The XRD utilized in this study is the Terra XRD – a field portable analog for the CheMin XRD, developed in 2007. Laboratory Raman analysis will be conducted using the Horiba LabRAM HR Evolution Raman Microscope-Spectrometer via both single point analysis, and DuoScan composition mapping across a field of view. VNIR and XRD data were gathered for each of the returned hand samples and matched to known library samples to determine the mineralogical composition of each specimen. Raman analysis is currently in progress, and will be completed in January 2016. Results: Instrumentation. As expected, phyllosilicates were more easily identified in VNIR spectra than in XRD patterns due to their often poorly crystalline structure. Conversely, elemental sulfur is easily seen in XRD and Raman spectra due to its crystalline nature and high level of Raman reactivity. The strong sulfur signal may often overpower those of other materials within the same sample.