The oxygen and hydrogen isotope compositions of volcanic ash layers from
coastal outcrops of the Miocene Monterey Formation elucidate the
progressive burial diagenesis of glass to bentonite and metabentonite.
Volcanic glass that texturally appears unaltered is found to have
delta(18)O and delta D values that are significantly higher than fresh
volcanic glasses. The positive shift in delta(18)O and delta D values is
proportional to the amount of hydration 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 layers 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 in the
Pliocene and Pleistocene. Smectite from bentonite layers has delta(18)O
and delta D values similar to the most hydrated glass samples,
suggesting similar glass-water and smectite-water fractionation factors.
Kaolinite and mixed-layered illite-smectite (I-S) altered from smectite
have lower delta(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 probably reflects the presence of organic
hydrogen in the analyzed samples as well as possible D-depleted
formation waters from detrital illite-water and hydrocarbon-water
exchange.