TY - JOUR
T1 - Boron isotope geochemistry during diagenesis. Part II. Applications to organic-rich sediments
AU - Williams, Lynda
AU - Hervig, Richard
AU - Hutcheon, Ian
N1 - Funding Information:
This research was funded by a grant from the U.S. Department of Energy (DE FG04 97ER14414). Samples were collected with Dr. Ray Ferrell from Louisiana State University (LSU). We gratefully acknowledge the technical assistance of Wanda LeBlanc (LSU) and Al Higgs (ASU) who keeps our SIMS in excellent operating condition. Dave Pevear kindly provided the Norphlet Fm sample.
PY - 2001
Y1 - 2001
N2 - The measured clay-water isotope fractionation for boron was applied to natural organic-rich sediments undergoing illitization. Two field areas were chosen that show illitization occurring over a range of temperatures (80-500°C). Samples representing diagenetic temperatures of illitization (80-200°C) are from the Gulf of Mexico sedimentary basin at 4 to 6-km depth in the Eocene Wilcox Fm and Jurassic Norphlet Fm. The higher temperatures of illitization (200-400°C) occur in a contact metamorphic aureole of the Cretaceous Pierre shale near Walsenburg, Colorado. Here the kinetics of the illitization reaction are more rapid than in a slowly subsiding sedimentary basin, but the chemical and mineralogical variations are minimized as complete illitization occurs over a small lateral distance in a single bentonite layer. These studies indicate that B-isotopes provide a more sensitive indicator of fluid variations in sedimentary basins than O-isotopes, and that B-isotope analyses of authigenic illite can be a valuable geochemical tracer of fluid/rock interactions. Boron isotope ratios in authigenic illite (pore filling) and muscovite (stylolites) from reservoir sandstones in the Gulf of Mexico are distinct from adjacent illitic mudstones, whereas the oxygen isotopic ratios show little variation. Fluids in equilibrium with the mudstones cannot precipitate the authigenic clays with higher δ11B values measured in the hydrocarbon reservoirs. This suggests that the reservoir fluids were not in communication with the adjacent mudstone pore fluids but were introduced from another source area, perhaps carrying a B-isotopic label derived from the hydrocarbon source region. Authigenic illite formed in the Pierre shale meta-bentonite shows large isotopic fractionations of boron (20‰) during illitization at high temperatures. Incorporations of 500 ppm B in illite formed at 500°C shows that illite is a host for B even at metamorphic temperatures. By using the experimentally derived B-isotope fractionation data for silicates the measured δ11B values of I/S could be predicted by using a Rayleigh fractionation model. Oxygen isotopic variations were negligible over this temperature range and thus provide little information on the mechanism of illitization.
AB - The measured clay-water isotope fractionation for boron was applied to natural organic-rich sediments undergoing illitization. Two field areas were chosen that show illitization occurring over a range of temperatures (80-500°C). Samples representing diagenetic temperatures of illitization (80-200°C) are from the Gulf of Mexico sedimentary basin at 4 to 6-km depth in the Eocene Wilcox Fm and Jurassic Norphlet Fm. The higher temperatures of illitization (200-400°C) occur in a contact metamorphic aureole of the Cretaceous Pierre shale near Walsenburg, Colorado. Here the kinetics of the illitization reaction are more rapid than in a slowly subsiding sedimentary basin, but the chemical and mineralogical variations are minimized as complete illitization occurs over a small lateral distance in a single bentonite layer. These studies indicate that B-isotopes provide a more sensitive indicator of fluid variations in sedimentary basins than O-isotopes, and that B-isotope analyses of authigenic illite can be a valuable geochemical tracer of fluid/rock interactions. Boron isotope ratios in authigenic illite (pore filling) and muscovite (stylolites) from reservoir sandstones in the Gulf of Mexico are distinct from adjacent illitic mudstones, whereas the oxygen isotopic ratios show little variation. Fluids in equilibrium with the mudstones cannot precipitate the authigenic clays with higher δ11B values measured in the hydrocarbon reservoirs. This suggests that the reservoir fluids were not in communication with the adjacent mudstone pore fluids but were introduced from another source area, perhaps carrying a B-isotopic label derived from the hydrocarbon source region. Authigenic illite formed in the Pierre shale meta-bentonite shows large isotopic fractionations of boron (20‰) during illitization at high temperatures. Incorporations of 500 ppm B in illite formed at 500°C shows that illite is a host for B even at metamorphic temperatures. By using the experimentally derived B-isotope fractionation data for silicates the measured δ11B values of I/S could be predicted by using a Rayleigh fractionation model. Oxygen isotopic variations were negligible over this temperature range and thus provide little information on the mechanism of illitization.
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U2 - 10.1016/S0016-7037(01)00558-0
DO - 10.1016/S0016-7037(01)00558-0
M3 - Article
AN - SCOPUS:0034965608
SN - 0016-7037
VL - 65
SP - 1783
EP - 1794
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 11
ER -