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.

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