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

Boron and lithium contents and isotopic compositions were determined in nucleating and growing nanometer-sized illite crystals (< 0.02 to 0.2 μm) 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 (475 μg/g) and heavy δ¹¹B (+ 7‰). This combination suggests precipitation of the coarse illite crystals older than 10 Ma from saline brines. The younger illite populations yield lower B contents (30–100 μg/g) and lighter δ¹¹B that decreased from − 5 to − 17‰ while B contents increased. The initial saline fluids were apparently not enriched in Li, as the older, coarse illite fractions yield low Li contents (< 20 μg/g) and high δ⁷Li (+ 7‰), reflecting in turn a water δ⁷Li composition of + 18 to + 26‰ similar to evaporite-type oilfield waters. Younger illite crystals contain increasing Li contents (up to 140 μg/g) with decreasing δ⁷Li (from − 7 to − 22‰). Illite younger than 6 million years contains increasing ¹⁰B and ⁶Li during crystal growth. This correlation parallels an increase in the δ¹¹B and δ⁷Li of kerogen from nearby host shales due to temperature increase, suggesting in turn a release of isotopically light B and Li from kerogen into the interacting pore fluids as hydrocarbons were released. It also suggests that hydrocarbons were generated during the recent 6 million years in the East Slovak Basin, while saline brines influenced illite crystallization before 10 Ma ago. A complementary conclusive aspect of this review is that the smaller the size of the illite crystals and the larger the analyzed field area, the more difficult it becomes to fit the illitization process into a uniform model. The mineral crystallo-chemical variations adapt to changing local conditions during burial and, therefore, re-equilibration of entire crystal populations becomes increasingly difficult to interpret. It is likely that direct analyses at the nano-scale will improve interpretations of variable isotopic tracers, and it is clear that B and Li in illite can be robust indicators of environmental changes over time.