Description: Boron contents and boron, carbon and oxygen stable isotopes were determined for authigenic carbonates recovered from Ocean Drilling Program Leg 146, Oregon margin. Carbonate precipitates are the most widespread authigenic phase in the shallow accretionary wedge and carry chemical information about long-term variations in pore fluid origin and flow paths in the Cascadia subduction zone. Drilling the first ridge (toe area including the frontal thrust) and the second ridge (or Hydrate Ridge) of the prism demonstrated different fluid regimes, with higher B contents in the authigenic precipitates at the toe. The delta11B of 18 authigenic precipitates analysed ranges from 13.9 per mil to as high as 39.8 per mil, extending the upper range of previously reported carbonate delta11B values considerably. When related to the delta11B ratio of their parent solutions, these data are characteristic of fluid-related processes in accretionary prisms. Together with delta13C and delta18O, delta11B ratios of the carbonate concretions, nodules and crusts allow one to distinguish between precipitation influenced by (i) seawater, (ii) fluid reservoirs at different depth levels within the accretionary prism and (iii) cage water from dissociated gas hydrates, the latter possibly indicating a fluctuation of the bottom simulating reflector during most recent Earth's history. From this first systematic boron study on authigenic precipitates from an accretionary prism it is suggested that B contents of such carbonate crusts and concretions exceed those reported for other marine carbonates. Given the abundance of such precipitates at convergent margins, they represent a significant B sink in geochemical cycling. Isotopic compositions of the parent fluids to the carbonates mirror B chemistry of modern pore waters from convergent margins. The precipitates carry information of different subduction-related fluid processes over a certain period of time, and hence are a crucial tracer in the investigation of palaeo-fluid flow.

Description: Interstitial pore waters from Ocean Drilling Program (ODP) Sites 1150 and 1151, both penetrating ca. 1200 m of sub-seafloor sediment from the upper Japan Trench forearc, were analyzed for B concentrations and d11B isotopes. B concentrations cover a wide range from 329 to 3920 µM (0.8-9.3* seawater) and vary isotopically from +20 to +46 per mil (relative to SRM 951). In general, B concentrations increase more or less steadily with depth. At both sites, the B enrichment and isotopic changes seem to be related to a deep fluid influence, possibly coupled with alteration of volcanic products. Silica diagenesis does not affect the B geochemistry of the waters, as is evident from intact diatom tests and high opal-A contents throughout the holes. The B enrichment is more pronounced at Site 1150 in the seismic portion of the forearc. Here, a large positive anomaly in d11B values is observed, which corresponds with abundant fracturing and two prominent shear zones. At about the same depth range at Site 1151, the d11B values show a negative shift, attesting to an enrichment in 10B in the fluid as a result of B desorption from clay due to burial and tectonic stress. We conclude that B release from clay minerals is the major contributor to pore fluid anomalies, and that fluid flow through permeable out-of-sequence thrust (like the shear zones at Site 1150) may be an efficient mechanism for B backflux from the forearc into the hydrosphere.