2005: Geochemistry of major constituents, boron and boron isotopes in pore waters from ODP Site 1202

Forty-one interstitial water samples recovered from ODP Site 1202, Okinawa Trough, were analyzed for major constituents, B and δ11B. The geochemical results show that major constituents and boron content varied largely in pore fluids and possibly were affected by sulfate reduction, re-crystallization of biogenic carbonate or silica, ash alteration, and organic matter degradation. Mixing of fluids along high-porosity sandy layers or fracture zones also changes the pore water chemical compositions significantly. The down-core distribution of B and δ11B in the pore waters are sensi-tive tracers for assessing fluid migration and water/sediment interaction at various depths. Pore water B content at Site 1202 falls in a range between 0.25 and 1.16 mM compared to that of 0.42 mM in seawater. The δ11B values, however, vary considerably from ~32.7 to 50.9 0 00 relative to the seawater value of 39.5 0 00. The δ11B vs. 1/B plot indicates that sedimentary B released from clays is the most important source to pore waters, resulting in elevated B with low δ11B. Other processes including precipitation of cal-cium carbonate, fluid advection through high-porosity permeable sandy horizons, interaction with terrigenous sediments and/or ash alteration may also modify the B and δ11B distributions. (Key words: B isotopes; Pore waters; Sedimentary B-desorption; Okinawa Trough) 1