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doi:10.2204/iodp.proc.338.203.2015

Introduction

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.

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