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 hydrothermal apparatus with isotope tracers, was applied to evaluate the temperature of squeezing artifacts on B contents and isotopic compositions in pore waters. The partition coefficient (K_D) was determined at temperatures from 25° to 350°C, at 800 bars, and this information was applied to reconstruct pore water B and δ¹¹B in ODP drill sites, where pH, T, and porosity are known. The partition coefficient of B is a function of temperature, pH, and sediment mineralogy. The solution pH exerts a dominant control at low temperatures; however, K_D decreases to a value of essentially zero (compared to that of K_D = ∼3.5 at 25°C) at high temperatures indicating no adsorption. Two empirical equations were derived to represent most of the available experimental results. For pelagic clay rich sediments, a K_D = -3.84-0.020T+0.88pH (R = 0.84; 1σ = 0.25) is established. For sediments that have experienced progressive metamorphism, a K_D = -1.38-0.008T+0.59pH (R = 0.81; 1σ = 0.37) can be applied. Similarly the effect on pore water δ¹¹B can be corrected if the fractionation factors at different temperatures are assumed. The corrected B and δ¹¹B in ODP Sites 671, 672, and 808 indicate significant mobilization of bulk B in sediment (exchangeable + lattice bound) at depth, especially near the décollement zone or other potential flow conduits. Tectonically expelled fluids from mud diapirs of Barbados Ridge Complex, hot springs of Rumsey Hills, California, and mud pot waters of the Salton Sea geothermal field, are enriched in B (up to 20 mM) with lower δ¹¹B, supporting the argument of B mobilization as a result of fluid expulsion in accretionary prisms.