Exploring the Mineralogical Landscape of Lava Tubes: Implications for Signatures of Habitability and Life Detection

Abstract:

Terrestrial lava tubes are subsurface environments that are initially formed by volcanic activity but also shaped by biogeochemical processes and mineralogical variations depending on exposure to ground water, volcanic gases, microbial activity, and sunlight. Some alteration minerals and physical features (e.g. corraloids and rinds) are likely the result of chemical weathering of the host basalts, while the development of other mineral phases may involve microbial interactions. Deviations in mineral structure can be attributed to the incorporation of cations used in metabolic processes or from the influence of decaying organic matter on local chemistry. These compositional variations can be used as indicators of habitability or signatures of life in terrestrial lava tubes and applied to the exploration of similar subterranean features on other planetary bodies. In order to establish useful signatures for planetary surface exploration, it is necessary to acquire a comprehensive characterization of the microbial communities, minerals, host rocks, and surroundings in terrestrial environments. Our team conducted field work and collected solid and liquid samples from a basaltic lava tube on the Mauna Loa volcano in Hawaii. We characterized the host basalts, secondary mineral coatings/veneers, powdery precipitates, and seeping groundwater in sections of the lava tube exposed to sunlight and sections that remain dark using a suite of field-portable in situ and laboratory analytical techniques: Raman spectroscopy, X-ray fluorescence (XRF), laser induced breakdown spectroscopy (LIBS), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and metagenomic analysis. The powdery precipitates throughout the lava tube are composed of predominantly metastable sulfate minerals. Many of the precipitated sulfate minerals are associated with microbial communities as well as localized concentrations of cation-rich water seeps. We will discuss the implications of these and other biogeochemical associations for life detection in similar subterranean systems in the solar system.