Carbonate Cements and Veins as a Proxy for Paleotemperature and Fluid Evolution in the Deep Accretionary Prism, IODP Expedition 348, Nankai Trough, Japan

Abstract:

Carbonate cements in accretionary prims record the fluid history of the subduction system during deformation, and may provide a record of faulting and related fluid flow. Site C0002 is the deepest hole ever drilled for scientific exploration at a depth of 3056 meters below sea floor as part of the IODP NanTroSEIZE project. Interstitial fluids were not successfully collected in the deep prism, but carbonates serve as proxy for fluid chemistry. Stable isotope analysis was used to determine if carbonates in the Nankai accretionary prism formed from 1) locally derived fluids, or 2) fluids migrated from depth and are related to deeper consolidation and deformation in the subduction zone. Carbonates are dominantly calcite and occur both as pore-filling cements and veins; the latter commonly exhibit slickenlines or slickenfibers. δ¹⁸O values range from 0.34 to -11.81 ‰ PDB; δ¹³C values range from -0.82 to -11.74 ‰ PDB. Carbonate cements typically have higher δ¹⁸O values (mean = -5.50 ‰ PDB), whereas veins have more ¹⁸O-depleted values (mean = -9.29 ‰ PDB). δ¹⁸O values of the isotopically most ¹⁸O-depleted veins show a trend of decreasing δ¹⁸O values with depth. The isotopic data can be explained if the cements formed earlier (or shallower) than veined samples, which appear to record a different fluid source. The lowest δ¹⁸O value in carbonate occurs just below the cored fault interval suggesting a conduit for deeper, warmer fluids is possible. Oxygen isotope paleothermometry results from carbonate isotopes parallel temperature extrapolations from thermal gradients calculated in geophysical studies, but the carbonate results are consistent with a gradient roughly 20°C cooler. Alternatively, fluids present during vein formation might have been ¹⁸O-enriched. Temperature is a key parameter controlling seismogenic behavior and will be further evaluated in this study with carbonate clumped isotope analysis, which will provide temperature control and initial δ¹⁸O values for interstitial waters.