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Distributions of nitrogen, boron and lithium were studied in illite–smectite from Ordovician–Silurian K-bentonites in the Baltic Basin. These trace elements are linked to thermal maturation of organic matter, creating fluid signatures that are distinct from those in regionally buried sediments. The fluid chemistry is ideally recorded in authigenic illite within the K-bentonite because B substitutes in tetrahedral sites of illite, Li in octahedral sites, and N is fixed in the interlayers as NH 4 + . Three nanometric size fractions of illite ( 10‰) among these size fractions relate mainly to changes in fluid composition during illite growth. Bentonite (volcanic ash) layers mark stratigraphic timelines, while illitization of the bentonite (K-bentonite) varies as a function of burial and tectonic temperature gradients. In the central Baltic Basin, bentonites were deposited across a carbonate platform near Estonia, grading to deep basin shales near Denmark and Poland. Compositional variations in illite reflect fluids from different sources. High N illite (>2000 ppm) in the southwest basin transitions to high B (>250 ppm) and high Li (>100 ppm) illite to the northeast. These trends follow a thermal gradient decreasing from southwest (200 °C) to northeast (90 °C). Isotope ratios of B and Li show opposing trends in samples across this thermal gradient: the Bisotopes decrease from southwest to northeast (+18‰ to −4‰), while Li-isotopes increase from +5‰ to +32‰. The Cambrian Alum shale, a regional hydrocarbon source rock, was studied as a potential source of B and Li, as more than 50% of its B and Li was associated with organic matter. With increasing thermal maturity, illite δ11B decreased from 0‰ to −13‰, while δ7Li increased from −20‰ to −5‰. These opposing trends are similar to those observed in the K-bentonites. This suggests that organic sources of N, B and Li dominate fluids in bentonites from the southwestern Baltic Basin, whereas basement derived fluids dominate in the northeastern Baltic Basin. These trace elements record paleofluid changes during illite crystal growth and can identify pathways of chemically distinct oil and gas related fluids. |
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