Boron isotope geochemistry during burial diagenesis

This research was conducted to test the hypothesis that boron isotopes in deep sedimentary basins can be used to monitor fluid/rock/organic interactions during thermal maturation. The thermal maturation of hydrocarbons occurs at temperatures corresponding to the major diagenetic silicate reaction of smectite to illite. If B is mobilized from a source rock along with the generated hydrocarbons, its interaction with authigenic illite in reservoirs could provide a useful tracer of migrating hydrocarbon-related fluids.; Experiments were conducted to measure the B-isotope fractionation between mineral and water during the illitization of smectite. The temperature dependence of the isotope fractionation was tested at 300° and 350°C. Isotopic equilibrium was approached after 4–5 months when the illite/smectite (I/S) displayed long-range structural ordering (∼70% illitization). Results were used to construct a new B-isotope fractionation curve.; Methods were developed and tested for analyzing B and O-isotopes in clay minerals by secondary ion mass spectrometry (SIMS). A method for analyzing B in organic matter by thermal ionization mass spectrometry was also tested, but analyses by SIMS are simpler. A comparison of results of the two analytical methods shows consistency.; The fractionation curve was tested in a variety of natural geologic settings, including hydrocarbon reservoirs in the U.S. Gulf of Mexico basin, and the Alberta basin. Samples from a contact metamorphic aureole in the Cretaceous Pierre shale (Colorado) provided a test of the temperature dependence of the curve, outside the realm of diagenetic temperatures. The results presented demonstrate that the B-fractionation curve can be applied over a large range of temperatures (diagenetic to metamorphic).; The importance of this research is that it provides new isotopic data that describes B-isotope systematics in sedimentary basins undergoing diagenesis. The magnitude of B-isotope fractionation is large compared to other stable isotopic systems, and the fractionation applies to a variety of diagenetic silicates. The substitution of B in authigenic illite, with an isotopic composition reflecting the fluid chemistry at the temperature of precipitation, may provide a useful diagenetic monitor for the migration of hydrocarbon-related fluids in reservoirs.