Boron isotope composition of coals: a potential tracer of organic contaminated fluids

The B-content of coals from sedimentary basins is a function of the primary depositional environment and secondary enrichments. Boron has been used to infer paleo-environments of coal deposits, and environmental impact of coal burning power plants. To improve our understanding of B in organic matter, this study used secondary ion mass spectrometry to measure the B-isotopic composition of 25 coals and three kerogens (Type I, II and III). Results show that coal is 10B-enriched compared to most terrestrial waters. The δ11B values measured show large variability within a single ‘homogenizedʹ coal sample indicating that B is heterogeneously bound in various organic macerals. Of greater importance is the observation that all coal δ11B values are negative. The lowest δ11B measured was −70‰ representing the lightest B-isotopic ratios ever reported for terrestrial materials. Bulk analyses of coals may not reveal isotope ratios this low because of the averaging of values from different organic macerals having different δ11B values. This discovery may be important for tracing organic contaminants in fluids. When organic matter is heated during burial, the isotopically light B partitions into fluid phases. The very low δ11B of organic products compared to most groundwaters may be useful in tracing fluids derived from organic sources.