Acceleration of sinter diagenesis in an active fumarole, Taupo volcanic zone, New Zealand
Acceleration of sinter diagenesis in an active fumarole, Taupo volcanic zone, New Zealand
Geology (Boulder) (September 2006) 34 (9): 749-752
- Australasia
- biogenic processes
- biomineralization
- chemically precipitated rocks
- crystal growth
- diagenesis
- experimental studies
- field studies
- framework silicates
- fumaroles
- geochemistry
- ground water
- hot springs
- microorganisms
- New Zealand
- North Island
- opal
- opal-A
- pH
- precipitation
- quartz
- rates
- sedimentary rocks
- SEM data
- silica minerals
- silicates
- siliceous sinter
- springs
- Taupo volcanic zone
- TGA data
- thermal waters
- transformations
- Orakei Korako Field
Siliceous sinters form where nearly neutral pH, alkali chloride waters discharge at the surface (< or =100 degrees C). They may preserve biogenic and abiogenic material and therefore archive paleoenvironmental settings. Freshly precipitated sinters undergo diagenesis through a five-step series of silica mineral phase changes, from opal-A to opal-A/CT to opal-CT to opal-C to quartz. Transformation rates vary among sinters because postdepositional conditions can accelerate or retard diagenesis, meanwhile preserving or destroying biosignals. We monitored alteration and diagenesis of newly precipitated, filamentous microbe-rich sinter during a two-year field experiment, where sinter was suspended inside a fumarole at Orakei Korako, Taupo volcanic zone, New Zealand. Patchy and complex diagenesis resulted from changes in environmental conditions, including variations in temperature, pH, and the intermittent deposition of sulfur. Throughout the experiment, opal-A was dissolved by acidic steam condensate, and reprecipitated locally. Quartz crystals grew on the sinter surface within 21 weeks. Previous reports of transformation rates from opal to quartz are on the order of thousands of years in duration. Thus, our results show that fumarolic overprinting accelerates diagenesis. Microbial preservation was not favorable because primary filamentous fabrics were obscured by deposition of opal-A microspheres, smooth silica infill, and sulfur. If ancient hydrothermal systems were among the likely places where early life flourished, it is necessary to distinguish between depositional features and those inherited during diagenesis. This near-real time experiment enabled observations on environmental controls of diagenetic change in silica minerals and illustrated the variability of conditions that can occur in nature during this complex process.