Lithium and boron isotopes in illite-smectite : The importance of crystal size

作者: Lynda B. Williams , Richard L. Hervig

DOI: 10.1016/J.GCA.2005.08.005

关键词: Isotopes of oxygenClay mineralsMineralogyIsotope fractionationIsotopes of boronIlliteLithiumSilicateChemistryEquilibrium fractionationAnalytical chemistry

摘要: Abstract Clay minerals record chemical data about the past, acting like natural computer memory chips. To retrieve we must understand how they are stored. achieve this have examined isotopic information revealed by two trace elements, lithium and boron, that incorporated into common clay illite-smectite (I-S) during diagenesis. We used hydrothermal experiments at 300°C, 100 MPa, to speed up reaction of smectite illite normally occurs slow (10–100 Ma) sediment burial. During illitization, Li substitutes octahedral sites B enters tetrahedral silicate framework. Both also adsorbed in interlayer smectite, but is preferred over exchange sites. determine equilibrium isotope fractionation elements it important remove these species. By measuring composition framework reaction, can address relative timing element different crystallographic Furthermore, because illitization a crystal growth process (not an isomorphous replacement) effect size on fractionation. The results show approach steady state when R1 ordering occurs, long before oxygen isotopes equilibrate with fluid. (αmineral-water) for (0.989) similar (0.984) 300°C. However, separated 2.0 μm fractions, there significant differences measured ratios as much 9‰. Crystal mechanisms surface energy effects nanoscale crystals may explain observed differences. fact rates (based size) be applied samples reveal changing paleofluid history, provided conditions equilibrium. This has very implications interpretation diagenetic environments, fluid flow, surficial geochemical cycling.

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