EXPERIMENTAL-STUDY OF BORON GEOCHEMISTRY - IMPLICATIONS FOR FLUID PROCESSES IN SUBDUCTION ZONES

A comprehensive experimental study, utilizing an autoclave hydrotherma l apparatus with a B-10 isotopic tracer, has been conducted to monitor the geochemical behavior of sediment B during early subduction zone p rocesses. The partition coefficient of exchangeable B (K-D) was determ ined over a temperature range of 25-350 degrees C, at 800 bars and a w ater/rock ratio of 3-1.5 w/w. These K-D are shown to be a complex func tion of temperature, pH, and possibly mineralogy. At low temperatures, K-D is significantly high at similar to 4 in contrast to the value of essentially zero at temperatures higher than similar to 100 degrees C . A K-D of zero represents no B adsorption, implying efficient mobiliz ation of exchangeable B at shallow depths during sediment subduction. Our experimental results demonstrate high mobilization of bulk B in se diments (both exchangeable and lattice bound) at elevated temperatures (200-350 degrees C), in good agreement with previous observations of B in metasediments indicating progressive depletion during metamorphis m. In addition, this study emphasizes the importance of a possible wat er/rock ratio dependence of B mobilization. In other words, the degree of sedimentary B mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution. In low geother mal gradient areas, large amounts of porewater are expelled before sig nificant B mobilization has occurred, so that some sedimentary B will survive and get into the deeper parts of the subduction zone. Our resu lts imply that efficient mobilization of B from the subducted slab mus t occur and that are magmatism recycles most of the remaining subducte d B back to surface reservoirs. A reconsideration of the B budget in s ubduction zones provides critical information with respect to B source s and sinks in the ocean.