| Abstract |
Sinters form where near-neutral-pH, alkali chloride waters discharge at the surface. As these waters cool, silica precipitates, coating all biogenic and abiogenic components. Hydrothermal systems are contenders for having hosted early life on Earth. Therefore sinters can be used as extreme-environment analogs in the search for early biosignals. However, they experience diagenetic changes that can modify original environmental signatures. Sinters undergo diagenesis through a 5-step series of phase changes, from opal-A to opal-A/CT to opal-CT to opal-C to quartz (± moganite). Three sinters that preserve the opal-A to quartz diagenetic sequence are: (1) < 11,500 years BP ± 70 years sinter at Steamboat Springs, Nevada, U.S.A, (2) < 1900 ± 160 years BP Opal Mound sinter at Roosevelt Hot Springs, Utah, U.S.A, and (3) 456 ± 35 years BP deposit at Sinter Island, Lake Ohakuri, Taupo Volcanic Zone, New Zealand. Crystallographic, mineralogic and morphologic transformations during diagenesis were tracked using electron backscatter diffraction, X-ray powder diffraction, and scanning electron microscopy. Morphological transformations involve repetitive growing and shrinking of silica particles from the micron- to nano-meter scales. The alignment of morphological features in the siliceous sinter matrix during diagenesis is predetermined by crystallography and represents future crystal-face growth directions. Crystallographic and morphologic indicators of quartz manifest in all precursor silica phases except opal-A. All deposits followed nearly identical diagenetic pathways, with time being a variable in the progress toward stable quartz. Diagenesis modifies original environmental signatures by obscuring primary biotic and abiotic inclusions. Thus, in order to accurately identify biosignals in ancient hot-spring deposits, it is necessary to recognize the diagenetic effects imprinted on fossil sinter deposits. |