BORON AND BORON ISOTOPES IN PORE WATERS FROM ODP-LEG 127, SEA OF JAPAN

Interstitial waters that have been recovered from the Sea of Japan down to a depth of 662 m below seafloor were analysed for B and deltaB-11. B concentrations cover the range 0.12-3.37 mM (0.3-8 x seawater) and vary isotopically from +11.5 to +51.3 parts per thousand (relative to SRM 951 boric acid). Consistent concentration depth profiles demonstrate that the behaviour of this element is non-conservative and much more complicated during early diagenesis than previously thought. Small departures from seawater B concentration and isotopic composition in the core tops from sites in the Yamato Basin result from the desorption of an isotopically light B fraction. In the Japan Basin a large positive deltaB-11 anomaly seems to be related to the preferential adsorption of B-10 in the depth range of more frequent ash layers. A large negative B anomaly seems to result from the alteration of volcaniclastic sands in a recently uplifted area. It seems that the thermal regime, in particular high heat flow, leads to B and Li maxima, deltaB-11 values lighter than seawater, and rather low Sr-87/Sr-86 ratios. B seems to be removed from pore waters at greater depth. It is finally incorporated into the sediment itself or altered basement rocks. Adsorption/desorption processes and alteration reactions within the sedimentary column and in the basement rocks govern the distribution of B in the pore fluids and should also influence the B content and isotopic composition of the alteration products, mainly clay minerals.