Bridget Y. Lynne, Kathleen A. Campbell, Bryony J. James, Patrick R. L. Browne, and Joseph Moore
Tracking crystallinity in siliceous hot-spring deposits
American Journal of Science (March 2007), 307(3):612-641

Abstract:
Siliceous hot spring deposits (sinters) entrap paleoenvironmentally significant components and are used as extreme-environment analogs in the search for early Earth and extraterrestrial life. However, sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals. For ancient hydrothermal settings including those close to the dawn of life, these transformations have long since occurred, so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change. Three young sinters preserve the entire diagenetic sequence of silica phases, from opal-A to quartz. The 6000 to approximately 11,500 years BP+ or -70 years sinter at Steamboat Springs, Nevada, the approximately 1600-1900+ or -160 years BP Opal Mound sinter at Roosevelt Hot Springs, Utah, and the approximately 456+ or -35 years BP deposit at Sinter Island, Taupo Volcanic Zone, New Zealand, provide an opportunity to track crystallographic, mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches. Worldwide, sinter forms from cooling, alkali chloride waters as noncrystalline opal-A, transforming first into noncrystalline opal-A/CT, then paracrystalline opal-CT+ or -moganite, paracrystalline opal-C, and eventually to microcrystalline quartz. In this study, these changes were identified by the novel and combined application of electron backscatter diffraction, X-ray powder diffraction, and scanning electron and optical microscopy techniques. We show that mineralogical changes precede morphological and accompanied crystallographic transformations. During this modification, silica particles grow and shrink several times from the micron- to nano-meter scales via dissolution, reprecipitation and recrystallization, and diagenesis follows the Ostwald Step rule. All deposits followed nearly identical diagenetic pathways, with time as the only variable in the march toward physicochemically stable quartz crystals. Diagenesis alters original environmental signatures trapped within sinters. After five silica phase changes, filamentous microfossils are modified but still remain recognizable within sinter from the Opal Mound and Steamboat Springs deposits, and during the opal-A to opal-CT silica phase transformations at Sinter Island. Therefore, delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot-spring deposits.