Boron systematics of authigenic carbonates: a new approach to identify fluid processes in accretionary prisms

Boron contents and boron, carbon and oxygen stable isotopes were determined for authigenic carbonates recovered from Ocean Drilling Program Leg 146, O regon margin. Carbonate precipitates are the most widespread authigenic pha se in the shallow accretionary wedge and carry chemical information about l ong-term variations in pore fluid origin and flow paths in the Cascadia sub duction zone. Drilling the first ridge (toe area including the frontal thru st) and the second ridge (or Hydrate Ridge) of the prism demonstrated diffe rent fluid regimes, with higher B contents in the authigenic precipitates a t the toe. The delta B-11 of 18 authigenic precipitates analysed ranges fro m 13.9 parts per thousand to as high as 39.8 parts per thousand, extending the upper range of previously reported carbonate delta B-11 values consider ably. When related to the delta B-11 ratio of their parent solutions, these data are characteristic of fluid-related processes in accretionary prisms. Together with delta C-13 and delta O-18, delta B-11 ratios of the carbonat e concretions, nodules and crusts allow one to distinguish between precipit ation 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 si mulating reflector during most recent Earth's history. From this first syst ematic boron study on authigenic precipitates from an accretionary prism it is suggested that B contents of such carbonate crusts and concretions exce ed 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 carb onates mirror B chemistry of modern pore waters from convergent margins. Th e precipitates carry information of different subduction-related fluid proc esses over a certain period of time, and hence are a crucial tracer in the investigation of palaeo-fluid flow. (C) 2001 Elsevier Science B.V. All righ ts reserved.