BORON CONTENTS AND ISOTOPIC COMPOSITIONS IN PORE WATERS - A NEW APPROACH TO DETERMINE TEMPERATURE-INDUCED ARTIFACTS - GEOCHEMICAL IMPLICATIONS

A comprehensive experimental study, utilizing a rocking autoclave hydr othermal apparatus with isotope tracers, was applied to evaluate the t emperature of squeezing artifacts on B contents and isotopic compositi ons in pore waters. The partition coefficient (K-D) was determined at temperatures from 25 degrees to 350 degrees C, at 800 bars, and this i nformation was applied to reconstruct pore water B and delta(11)B in O DP drill sites, where pH, T, and porosity are known. The partition coe fficient of B is a function of temperature, pH, and sediment mineralog y. The solution pH exerts a dominant control at low temperatures; howe ver, K-D decreases to a value of essentially zero (compared to that of K-D = similar to 3.5 at 25 degrees C) at high temperatures indicating no adsorption. Two empirical equations were derived to represent most of the available experimental results. For pelagic clay rich sediment s, a K-D = -3.84 - 0.020T + 0.88pH (R = 0.84; 1 sigma = 0.25) is estab lished. For sediments that have experienced progressive metamorphism, a K-D = -1.38 - 0.008T + 0.59pH (R = 0.81; 1 sigma = 0.37) call be app lied. Similarly the effect on pore water delta(11)B can be corrected i f the fractionation factors at different temperatures are assumed. The corrected B and delta(11)B in ODP Sites 671, 672, and 808 indicate si gnificant mobilization of bulk B in sediment (exchangeable + lattice b ound) at depth, especially near the decollement zone or other potentia l flow conduits. Tectonically expelled fluids from mud diapirs of Barb ados Ridge Complex, hot springs of Rumsey Hills, California, and mud p ot waters of the Salton Sea geothermal held, are enriched in B (up to 20 mM) with lower delta(11)B, supporting the argument of B mobilizatio n as a result of fluid expulsion in accretionary prisms.