New isotopic temperature estimates for early silica diagenesis in bedded cherts Available to Purchase

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 δ¹⁸O values that contribute to mounting evidence that silica diagenesis in bedded chert can occur at much lower temperatures than previously supposed. Monterey opal-CT δ¹⁸O values of 37.5‰ and 36.3‰ point to diagenetic temperatures at least as low as 17-21 °C, rather than the 35-50 °C values normally inferred. Our results suggest that hydration waters can significantly reduce the bulk δ¹⁸O of opal-CT and that special analytical techniques and consideration of water/rock ratios are warranted for isotope thermometry off diagenetic silica transformations.