New isotopic temperature estimates for early silica diagenesis in bedded cherts
New isotopic temperature estimates for early silica diagenesis in bedded cherts
Geology (Boulder) (June 1993) 21 (6): 519-522
- bedding
- California
- Cenozoic
- chemical analysis
- chemically precipitated rocks
- chert
- diagenesis
- early diagenesis
- fluorine
- framework silicates
- geologic thermometry
- halogens
- hydration
- isotope ratios
- isotopes
- Lompoc California
- Miocene
- Monterey Formation
- Neogene
- O-18/O-16
- opal
- opal-CT
- oxygen
- planar bedding structures
- pore water
- sample preparation
- Santa Barbara County California
- sedimentary rocks
- sedimentary structures
- silica minerals
- silicates
- Southern California
- stable isotopes
- techniques
- temperature
- Tertiary
- United States
- fluorination
When analyzed by a stepwise fluorination technique that reduces undesired oxygen contributions from hydroxyl and water, opal-CT samples from the siliceous member of the Miocene Monterey Formation, California, yield delta (super 18) O values that contribute to mounting evidence that silica diagenesis in bedded chert can occur at much lower temperatures than previously supposed. Monterey opal-CT delta (super 18) O values of 37.5per mil and 36.3per mil point to diagenetic temperatures at least as low as 17 - 21 degrees C, rather than the 35 - 50 degrees C values normally inferred. Our results suggest that hydration waters can significantly reduce the bulk delta (super 18) O of opal-CT and that special analytical techniques and consideration of water/rock ratios are warranted for isotope thermometry of diagenetic silica transformations.
