AGING: THE FIRST STEP IN DIAGENESIS OF OPAL-A IN HOT SPRING DEPOSITS

Siliceous rocks host the earliest and best-preserved microfossils. Understanding the mechanism by which siliceous deposits become lithified is an important step in unraveling the history of ancient siliceous rocks. Hot spring deposits are of particular interest because of the ancient lineages of the microbial communities. Silica concentrations in hydrothermal systems are controlled by equilibration with quartz at high temperatures. Once the solution reaches the surface, silica can precipitate according to the degree of supersaturation with respect to any given phase. Silica, originally deposited as X-ray amorphous silica (opal-A; that is, silica with crystal size below the resolution of X-ray diffraction or truly amorphous), undergoes subsequent phase transitions to quartz through intermediate phases, e.g., opal-C or opal-CT. Temperature and degree of oversaturation control the rate of these transitions in subaqueous systems. In subaerial systems, the amount of water flowing through the system also controls of the rate of diagenesis. It is during the transition to crystalline forms that textural detail may be lost. Herein we report results of diagenetic studies of three deposits in Yellowstone National Park, WY. All deposits are dominantly in the opal-A form, but show evidence of aging in X-ray diffractograms, as defined by Herdianita et al. (2000) and infrared spectroscopy. These changes are accompanied by differences in rock geochemistry, which could be an influential factor even at this early stage of diagenesis.