SGER: Exploring Intracrystalline Boron Isotope Variations

Arizona State University, Scottsdale AZ

Investigators

Abstract

Williams EAR-0108852 Boron substitutes for Al in clay minerals with a relatively large isotope fractionation. Illite/smectite (I/S) clays incorporate more boron than other clay types and they are also most common in sedimentary basins. We want to test the hypothesis that the boron contents and isotopic ratios of these clays represent valuable monitors of the temperature and composition of the paleofluids from which they grew during burial to depths of hydrocarbon generation. Research on B in sediments has primarily focussed on the behavior of adsorbed-B and its variations as a function of T and pH. As temperatures exceed 100 degrees C, however, the distribution coefficient for adsorbed-B approaches 0. Therefore, under diagenetic conditions it is more important to understand the fractionation of B during substitution in the clay mineral lattice. There is a large increase in the amount of interlayer-B in I/S during R1 ordering, when the layer charges cause partial collapse of smectite to form illite. Using a mannitol wash, followed by alkali cation saturation we were able to separate adsorbed, interlayer, and tetrahedral layer-B. Each population from different crystallographic sites had a different B-isotope ratio. Using this technique we will study the potential of B as a single-mineral geothermometer under low temperature conditions.