Illitization decrypted by B and Li isotope geochemistry of nanometer-sized illite crystals from bentonite beds, East Slovak Basin

摘要

AbstractBoron and lithium contents and isotopic compositions were determined in nucleating and growing nanometer-sized illite crystals (<0.02 to 0.2μm) separated from mixed-layered illite-smectite particles of bentonite beds from East Slovak Basin. Substituted into tetrahedral and octahedral sites of illite, respectively, they were used to deduce fluid sources and chemical changes during nucleation and crystal growth. The positively correlated contents in most size separates suggest that the substitution of both B and Li is probably coincident and that illitization progressed via direct precipitation from pore fluids and not by solid-state substitution. As B and Li are released from their sources into the pore fluids, their trends are recorded in the isotopic compositions of the authigenic illite crystals. The overall concept is that the B and Li contents of nanometer-sized illite crystal record the abundance variations of the interacting fluids, while the isotopic ratios inform about the temperature and fluid isotopic compositions. Preliminary solid-state Nuclear Magnetic Resonance (NMR) spectroscopic determinations of B in various size fractions of three bentonite beds demonstrate an excellent potential to distinguish its crystallographic location in nucleating illite relative to the initial smectite crystals, followed by growth of illite crystals and probable dissolution of smectite.Generally, bentonite beds collected near salt-bearing sediments contain illite crystals with high B contents (475μg/g) and heavy δ11B (+7‰). This combination suggests precipitation of the coarse illite crystal