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Boron (B) has two isotopes, 10B and 11B, with natural abundances of 19.9% and 80.1%, respectively (Berglund and Wieser, 2011). The element is a useful geochemical tracer given the large relative difference between its two isotopes, its high mobility, and its wide range of isotope ratios at Earth’s surface (Barth, 1993). The distribution of B concentrations and isotope ratios in interstitial waters of accretionary prisms in subduction zones provides important information about fluid migration along permeable sand layers and décollements (Deyhle et al., 2004; Gieskes et al., 1989; Teichert et al., 2005; You et al., 1993, 1996).

B sources tend to have characteristic isotope ratios. In seawater, B(OH)4 is adsorbed onto the surface of settling particles, and B exists mainly as B(OH)3. Generally, 11B preferentially condenses in B(OH)3 and 10B is incorporated in B(OH)4, leading to isotopic fractionation (Kakihana et al., 1977; Oi et al., 1991; Palmer et al., 1987). The B isotope ratio (δ11B) in seawater averages +39.5‰ (Spivack and Edmond, 1987). At shallow depth in marine sediments, exchangeable B is desorbed from the surface of sediments, increasing B concentrations and decreasing δ11B in interstitial waters (You et al., 1996). This desorption is related to the decomposition of organic matter during early diagenesis (Brumsack et al., 1992; You et al., 1993). Additionally, B concentrations and δ11B in interstitial waters may be slightly altered by sampling artifacts because of temperature and pressure changes, and several studies corrected observed values (You et al., 1996; Kopf et al., 2000; Deyhle and Kopf, 2002). At greater depths in the sediment, adsorbed B from sediment particles is incorporated into an interlayer of a clay mineral during late diagenesis, and Si–O bonds are broken and B substitutes for Si in the tetrahedral structure during smectite–illite conversion, which generally occurs between 60° and 160°C (Williams et al., 2001). During this process, 10B is preferentially incorporated into the clay mineral structure, and the residual fluids in interstitial waters from the Cascadia margin have been observed to be 11B-enriched (Teichert et al., 2005). At still greater depths in marine sediments, the temperature increases, porosity decreases, and B in the clay-mineral crystal lattice is released (You and Gieskes, 2001). In subduction zones, such as the Nankai Trough, the Barbados Ridge complex, and the Costa Rica fore arc, increased B concentrations with low isotope ratios have been observed along décollements and fracture zones (Kopf et al., 2000; You et al., 1993, 1995). These features reflect the upward migration of deep-sourced fluids along décollements and fracture zones (Deyhle and Kopf, 2002). Thus, a systematics of B concentrations and isotopic ratios has been used for studies about B sources in interstitial waters and migration processes of interstitial waters.

In this study of a plumbing system in the Nankai accretionary prism, we measured B concentrations and isotopic compositions in interstitial waters from the surface layer of sediments and clarified the presence of deep-sourced fluids, especially along lithologic boundaries formed by faults and mass transport deposits.