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Intracrystalline boron isotope partitioning in illite-smectite: Testing the geothermometer

Abstract Intracrystalline B-isotope differences of ~40‰, are observed between the interlayer and tetrahedral crystallographic sites of interstratified illite-smectite (I-S). We tested the hypothesis that partitioning of B-isotopes between these sites could provide a low-temperature, single-mineral geothermometer. Samples studied include a metabentonite transected by a dike in the Cretaceous Pierre Shale (200-500 °C), buried mudstones from the Eocene Wilcox Formation (60-125 °C), and I-S products from hydrothermal experiments (300-350 °C). Different reaction kinetics are represented by these different sample sets, therefore results test the equilibrium partitioning of B in the interlayer vs. tetrahedral sites. In all samples, interlayer δ11B values are isotopically heavier than the tetrahedral δ11B. Because 11B prefers trigonal coordination, we infer that B(OH)3 dominates the interlayer sites. Within each sample set, the intracrystalline differences are greatest (20-40‰) in the most expanded I-S (i.e., smectite-rich), and approach 0 as illitization increases. There is good correlation (R = 0.84) between the interlayer δ11B (calculated by mass balance) and water δ11B indicated by the established maximum temperature of each sample. These results suggest that the interlayer sites of I-S preserve the B isotopic composition of water at the temperature that produced the authigenic illite. Direct measurements of interlayer δ11B equilibrated with water of known δ11B are needed to refine the relationship with temperature, but the existing data indicate the following temperature dependent relationship: T (°C) = (δ11B tetrahedral - δ11B interlayer + 30)/0.05.