Isotope Geochemistry of Calcite and Clay Minerals in Volcanogenic Rocks, Great Valley Sequence, Northern California: Implications for Organic Diagenesis: ABSTRACT

Abstract

Petrographic and isotopic data from an 8,500-m thick section of the Great Valley sequence indicate that widespread calcite cement in sandstones and mudstones was precipitated during at least two distinct stages that are linked to organic and clay-mineral diagenesis. The range of ^dgr13C values for calcite in mudstone is -11.6 to +1.0 ppt PDB, in sandstones is -9,7 to +4.5 ppt PDB, and in veins is -16.7 to -2.2 ppt PDB. The ^dgr13C values are heavier in progressively older and, presumably, more deeply buried strata. A strong shift to heavier 13C/12C ratios of calcite in mudstones and sandstones corresponds with an abrupt 20 ppt shift to more enriched ^dgrD values of OH-hydrogen in diagenetic smectite and a 10A clay-mineral from mudstones. Clay D/H ratios range between -69 to -49 ppt SMOW. The stratigraphic position of the shift corresponds to a modeled burial temperature estimate of about 80 to 100°C. This is interpreted to be the burial temperature for late-stage dehydration and conversion of smect te to a 10A clay-mineral phase. Theoretical considerations indicate that a shallow burial phase of predominantly pore-filling calcite formed in association with bacterial production of methane ( 80°C) that contained thermogenically produced low C2+ gases. Release of dry gas at increasingly elevated temperatures was characterized by continuous 13C-enrichment in CH4. Depleted calcite carbon in the most basal strata was derived from CH4-rich basement fluids. Estimation of ^dgrD values of formation waters indicates that CH4-H2O equilibrium was not attained. End_of_Article - Last_Page 999------------