Complexities of Sinter Diagenesis

Abstract

Diagenesis of hot spring rocks (sinters) is complex. 14C AMS dating of five sinters revealed that the rates of silica phase changes (opal-A to quartz) are not controlled by time alone. Sinters may reach the diagenetic end-member of quartz in several hundred years, or they may require thousands of years. Other sinters initially formed as opal-A may remain opal-A for many thousands of years. Sinters may record two distinct hot-spring discharge cycles into a single deposit, leading to misinterpretation of the reconstruction and timing of fluid discharge events. Multiple pulses of thermal fluids also complicate evaluation of the paleohydrology and may create a mineralogic-morphologic disjunct within the sinter. Generally, an increase in density and a decrease in porosity accompanies sinter diagenesis and silica phase mineralogical maturation. However, when a sinter is overprinted with acidic steam condensate dissolution commonly occurs increasing secondary porosity. Therefore, the porosity of a sinter can not be used as an indicator of mineralogical maturation. These problems highlight the complexities of interpreting sinter diagenesis and post-depositional events.