MAPPING HYDROTHERMAL ALTERATION ON HAWAIIAN SHIELD VOLCANOES USING IMAGING SPECTROSCOPY: APPLICATIONS TO THE SEARCH FOR RELICT HYDROTHERMAL SYSTEMS ON MARS

Studies suggest that hydrothermal systems may have been refuges for thermophilic microorganisms on Mars, thus making hydrothermal deposits high-priority targets in the search for extraterrestrial life. Search strategies can be fine-tuned by examining hydrothermal deposits on Earth where it is possible to check results in the field. The spectrally detectable mineral associations of terrestrial hydrothermal systems are controlled by tectonic setting, rock composition, and volatile content of the underlying magma. The summits of Mauna Kea and Mauna Loa shield volcanos on the island of Hawaii are among the highest and driest readily accessible sites in the world, and provide one of the best terrestrial analogues to Martian shield volcanoes. With this in mind, AVIRIS (Airborne Visible/InfraRed Imaging Spectrometer) data were collected in April, 2000 along a 75 km NE traverse passing over the summits of these mountains. Data were corrected to reflectance and spectrally mapped using Tetracorder, an absorption feature matching algorithm that produces maps of surface mineralogy. Mineral associations characteristic of hydrothermal alteration were identified in five cinder cones at the summit of Mauna Kea and at two locations on the north wall of the Mauna Loa summit caldera. Hydrothermal deposits on both volcanoes contain a central kaolinite/alunite zone partially surrounded by Al-montmorillonite, ferruginous smectite, and Fe-Mg saponite zones. Some of the alteration minerals found in the relict Hawaiian hydrothermal systems also form during weathering of mafic volcanic rocks. Nontronite-like Fe³⁺ saponite, ferruginous smectite, hematite, and goethite can be formed non-hydrothermally via weathering of basaltic rocks, though hydrothermal processes can enhance their concentrations above background levels. Kaolinite is the most reliable indicator of hydrothermal activity in basaltic rocks at the relatively coarse spatial resolution of imaging spectrometer data, because it is present only in hydrothermally altered rocks on the summits of both volcanoes. Kaolinite may be a key mineral for identifying relict Martian hydrothermal systems. Hydrothermal deposits on these shield volcanoes are relatively small compared to those developed in continental tectonic settings and contain more Fe- & Mg-rich clays and less opal & chalcedony.