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

Abstract

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 fiJsO and bD values that are significantly higher than fresh volcanic glasses. The positive shift in ~L~O and 3D 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 8~80 values of the most hydrated glass, and clay minerals from bentonite and metabentonite layers are approximately in equilibrium with slightly modified seawater. The ~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 g~80 and gD values similar to the most hydrated glass samples, suggesting similar glass-water and smectite-water fractionation factors. Kaolinite and mixedlayered illite-smectite (I-S) altered from smectite have lower g~80 and higher SD values than their precursor. The g~80 and gD of nonvolcanic siliceous mudstones from the Pt. Arguello oil field show an unusual decrease in gD 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.

Eruptive shearing of tube pumice: pure and simple

“…4c). Here, the OH − species concentration of 0.69-0.78 wt % is used in the water speciation equilibrium law of Zhang (1999) to constrain a total magmatic water content of lor et al (1983); HVGL is hydrated volcanic glass line from Taylor (1968); grey circles are hydrated silicic volcanic glasses from Taylor (1968), Cerling et al (1985), and Compton et al (1999); SMOW is the Standard Mean Ocean Water; GMWL is the Global Meteoric Water Line.…”

Marine silicate weathering in the anoxic sediment of the Ulleung Basin: Evidence and consequences

“…Despite this evidence, Kim et al [2013a] noted the need to establish a significant K 1 source as required for the smectite to illite conversion to the degree observed in the basin. Moreover, to date the K 1 source as for this reaction in marine systems is still being debated [e.g., Awwiller 1993;Compton et al, 1999;Wilkinson et al, 2001;Saffer and McKiernan, 2009;Schleicher et al, 2010].…”

Rapid Shifts in Chemical and Isotopic Compositions of Sediment Pore Waters in the Amami Sankaku Basin in Response to Initial Arc Rifting in the Mid‐Oligocene

“…Therefore, the downcore depletion in δ 18 O must be resulted from alteration. Previous studies have shown that low‐temperature weathering of basalts by seawater could increase the δ 18 O in basalts (Compton et al, ; Muehlenbachs & Clayton, ) but decrease δ 18 O value of pore water. Similarly, alteration of volcaniclastic sediments would preferentially take up 18 O from pore water and incorporate them into hydrous diagenetic minerals such as clay and zeolite minerals, leading to depletion of 18 O in pore fluids (Gieskes & Lawrence, ; Lawrence et al, ; Lawrence & Gieskes, ).…”