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

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 δ¹³ (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 δ ¹¹B are needed to refine the relationship with temperature, but the existing data indicate the following temperature dependent relationship: T (°C) = (δ¹¹ B_tetrahedral - δ¹¹B_interlayer + 30)/0.05.