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

• Published in: Clays and clay minerals Vol. 47; no. 1; pp. 84 - 95
• Main Authors: COMPTON, J. S, CONRAD, M. E, VENNEMANN, T. W
• Format: Journal Article
• Language: English
• Published: Boulder, CO Clay Minerals Society 01.02.1999
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Isotope geochemistry; Isotope geochemistry. Geochronology; United States; California; illite; Neogene; chemical weathering; O-18; textures; X-ray diffraction; upper Tertiary; SEM data; bentonite; North America; deuterium; volcanic glass; sheet silicates; silicates; Cenozoic; clastic rocks; volcanic ash; scanning electron microscopy; stable isotopes; volcanic rocks; Tertiary; igneous rocks; chemical analysis; burial diagenesis; smectite; sedimentary rocks; X-ray fluorescence; hydration; clay minerals; Miocene; Phanerozoic; D/H
• ISSN: 0009-8604; 1552-8367
• DOI: 10.1346/CCMN.1999.0470109

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 δ l8 O and δD values that are significantly higher than fresh volcanic glasses. The positive shift in δ 18 O 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 δ 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 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 δ 18 O 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 δ 18 O and higher δD values than their precursor. The δ 18 O 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.