Stable isotope evolution of volcanic ash layers during diagenesis of the Miocene Monterey formation, California

The oxygen and hydrogen isotope compositions of volcanic ash layers from co astal outcrops of the Miocene Monterey Formation elucidate the progressive burial diagenesis of glass to bentonite and metabentonite. Volcanic glass t hat texturally appears unaltered is found to have delta(18)O and delta D va lues that are significantly higher than fresh volcanic glasses. The positiv e shift in delta(18)O and delta D values is proportional to the amount of h ydration and chemical alteration of the glass samples. Initial hydration of the glass results in the exchange of Na for H; later hydration results in the loss of K and Fe in exchange for H and Na. The delta(18)O values of the most hydrated glass, and clay minerals from bentonite and metabentonite la yers are approximately in equilibrium with slightly modified seawater. The delta D values of the hydrated glass, and clay minerals from bentonite and metabentonite layers, are significantly depleted in D relative to seawater and suggest meteoric exchange associated with tectonic uplift and erosion i n the Pliocene and Pleistocene. Smectite from bentonite layers has delta(18 )O and delta D values similar to the most hydrated glass samples, suggestin g similar glass-water and smectite-water fractionation factors. Kaolinite a nd mixed-layered illite-smectite (I-S) altered from smectite have lower del ta(18)O and higher delta D values than their precursor. The delta(18)O and delta D of nonvolcanic siliceous mudstones from the Pt. Arguello oil field show an unusual decrease in delta D with increasing burial depth that proba bly reflects the presence of organic hydrogen in the analyzed samples as we ll as possible D-depleted formation waters from detrital illite-water and h ydrocarbon-water exchange.