Evaluation of in situ smectite dehydration as a pore water freshening mechanism in the Nankai Trough, offshore southwest Japan

Abstract

[1] Pore water freshening has been observed within sediments near the trench at numerous subduction zones. Constraining the relative contributions of long-distance updip flow of freshened fluids and in situ clay dehydration holds important implications for margin-scale fluid flow but remains unresolved because the evolution of pore water chemistry expected due to in situ clay dehydration and the budget of fresh water release have generally not been carefully quantified. Here, we address this problem at the wellstudied Nankai Trough offshore SW Japan, where Deep Sea Drilling Project and Ocean Drilling Program drilling have sampled the sedimentary section along two transects: the Muroto transect and the Ashizuri transect, located $100 km southwest along strike. Broad low-chloride anomalies with minimum concentrations significantly below typical seawater values were documented along the Muroto transect at Sites 1173, 1174, and 808 and indicate progressive freshening with increasing burial. In contrast, freshening at Site 1177 along the Ashizuri transect ($24 km seaward of the trench) is characterized by a series of sharp excursions from seawater values. We use a one-dimensional model to simulate sedimentation, thermal history, clay dehydration, and chemical diffusion, in order to quantify the component of pore water freshening attributable to in situ smectite dehydration. We find that simulated reaction progress is in close agreement with observations. The peak magnitude of pore water freshening is reasonably well predicted within the lowermost $150 m of the section but consistently underpredicted at intermediate depths. The residual pore water freshening profiles (not explained by smectite dehydration) reach peak magnitudes of $50-75 mM and approximately straddle the décollement at Sites 1174 and 808. We suggest that these residuals may reflect either an additional in situ dehydration process or a contribution from updip fluid migration along the fault. Our conclusion differs from that of previous studies, which have suggested that most of the observed geochemical anomaly could be generated by in situ smectite dehydration with as little as 15 wt% initial smectite. This substantial discrepancy is readily explained by the high sensitivity of pore water freshening to assumed porosity at the time of dehydration.

Quartz deposition and its influence on the deformation process of megathrusts in subduction zones

“…The S-I reaction at the Nankai margin, as observed in several drilling holes , was well reproduced by this kinetic expression (Saffer et al 2008;Saffer and McKiernan 2009).…”

Organic matter cracking: A source of fluid overpressure in subducting sediments

“…In hot subduction zone settings such as the Nankai accretionary prism, the reaction is already significantly advanced at depth ~1km below seafloor, in the lower part of drilled section (Saffer and McKiernan, 2009). The temperature window where oil and gas generation occurs is a function of the nature of the organic matter buried as well as of the burial rate (Bordenave, 1993;Ungerer et al, 1986).…”

Pore fluid chemistry from the Second Gas Hydrate Drilling Expedition in the Ulleung Basin (UBGH2): Source, mechanisms and consequences of fluid freshening in the central part of the Ulleung Basin, East Sea

“…Mass balance calculations assuming typical smectite and illite compositions (detailed in the Appendix) indicate that there is not enough dissolved K þ in the pore fluids to account for the full degree of freshening observed here. This is also true for other locations where smectite to illite has been documented to take place, such as in the well-studied diagenetic processes at ODP Site 808 in the Nankai Trough (Brown et al, 2001;Saffer et al, 2008;Saffer and McKiernan, 2009). The smectite-to-illite diagenetic transformation has been long established (Hower et al, 1976), and the K þ needed for this reaction is thought to be sourced by remote or local dissolution of K-feldspars, white mica and a range of mafic minerals that may be present in the host rock (Awwiller, 1993;Schleicher et al, 2010).…”

“…The opal A to opal CT transformation occurs at temperatures ranging from 22 to 68 C (e.g., Murray et al, 1992;Pisciotto et al, 1992), and the onset of smectite-to-illite transformation occurs at 60e90 C (Freed and Peacor, 1989). These reactions have been documented by analyses of sediment and pore fluid samples collected by deep drilling in the Yamato Basin and at the toe of the Nankai subduction zone (Brown et al, 2001;Saffer et al, 2008;Saffer and McKiernan, 2009). The boreholes drilled in the Ulleung Basin did not penetrate to depths where the temperatures are high enough to trigger silica diagenesis or smectite-to-illite transformation (also referred to as illitization).…”