Stable Isotope Evolution of Volcanic Ash Layers during Diagenesis of the Miocene Monterey Formation, California

J. S. Compton1, M. E. Conrad2 and T. W. Vennemann3
1 Department of Geological Sciences, University of Cape Town, Rondebosch 7700 South Africa
2 Center for Isotope Geochemistry, Mailstop 70A-3363, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3 Institute for Geochemistry, University of Tuebingen, Wilhelmstr.56, 72074 Tuebingen, Germany

Abstract: 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 δ18O and δD values that are significantly higher than fresh volcanic glasses. The positive shift in δ18O and δD 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 δ18O values of the most hydrated glass, and clay minerals from bentonite and metabentonite layers are approximately in equilibrium with slightly modified seawater. The δD 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 δ18O and δD 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 δ18O and higher δD values than their precursor. The δ18O and δD of nonvolcanic siliceous mudstones from the Pt. Arguello oil field show an unusual decrease in δD 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.

Key Words: Bentonite • Diagenesis • Monterey Formation • Stable Isotopes • Volcanic Ash

Clays and Clay Minerals; February 1999 v. 47; no. 1; p. 84-95; DOI: 10.1346/CCMN.1999.0470109
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)