Isotope Geochemistry of Syntectonic Carbonate Cements and Veins from the Oregon Margin: Implications for the Hydrogeologic Evolution of the Accretionary Wedge

Abstract

Isotopic compositions of carbonate cements are used to infer the compositions of historical pore fluids of Sites 891 and 892 (Oregon transect) of Ocean Drilling Program Leg 146 (Cascadia Margin). Site 891 was drilled through the frontal thrust region, and Site 892 penetrated an out-of-sequen ce thrust farther upslope. Carbonate content in the non-concretionary samples analyzed from Site 891 ranges from 0% to 14%. Calcite and magnesian calcite are the dominant diagenetic carbonate phases, and dolomite is present in many samples. Generally carbonate contents and compositions do not correlate well with related dissolved constituents (alkalinity, Mg2+, Ca2+) in modern pore-water chemistry. δ 13 C PDB values of samples from this site range from -4.4‰ to -0.8%c; oxygen isotope values from the same samples range from δ' 8 O PDB = -16.5‰ to -6.2%c, and are substantially out of equilibrium with the oxygen isotopic composition of the pore waters. Both stable isotopes show a relatively smooth variation with depth. Strontium isotopes of six samples range from 87Sr/86Sr = 0.70990 to 0.71155 and are substantially more radiogenic than modern seawater ( 0.7092). Carbonate content in the non-concretionary samples analyzed from Site 892 are higher and range from 0% to 25%. Calcite and magnesian calcite are the dominant diagenetic carbonate, but dolomite is common in the upper part of the section. The amount of carbonate correlates better with pore-water alkalinity profiles, and dolomite tends to occur where Mg/Ca ratios in pore water are highest, but there is not a strict correlation between carbonate occurrence and pore water profiles. Twenty-seven samples have a large range of carbon isotope values (δ 13 C PDB = -30%c to +26%c); oxygen isotope values from the same samples range from δ' 8 O PDB = -14%o to +1 \%c. Most of the oxygen isotopes are substantially out of equilibrium with oxygen isotope composition of the pore waters; on the other hand, carbon isotopes measured on dissolved total CO2 are high and may explain some of the high δ13C values of the diagenetic carbonates. Both stable isotopes show large fluctuations in their profiles with depth. Strontium isotopes of eight samples range from 87Sr/86Sr = 0.70576 and 0.70885 and are substantially less radiogenic than modern seawater; one sample has a ratio of 0.71140. Most of these values are not in the range of the pore water strontium isotope composition. The oxygen, carbon, and strontium isotope characteristics of Site 891 diagenetic carbonates suggest that deep fluids have migrated from the decollement and other fluid conduits up through most of the stratigraphic section. Isotope-depth patterns of Site 892 diagenetic cements are more complex, reflecting greater deformation, a longer history of fluid flow, and a greater influence of shallow fluid reservoirs involving organic-rich sediments.