Application of Boron Isotopes to the Understanding of Fluid–Rock Interactions in Mud Volcanic Aqueous Fluids of the Kerch Peninsula, the Northern Black Sea

Numerous onshore mud volcanoes of the Kerch Peninsula act as conduits to channel aqueous fluids produced from the 1.0–3.5 km depth in the Caucasus continental collisional zone. Input of porewater excluded by Oligocene–Miocene Maykop shale compaction through burial and deformation, with a composition similar to seawater (SW), is the most noticeable in the waters of shallow-rooted (estimated HMg/Li 1.0–1.8 km) small mud volcanoes. These waters have mixing compositions highly distorted by specific surface processes (evaporation, mineral precipitation, dilution, etc.). In contrast, deeper-rooted (estimated HMg/Li 2.5–3.5 km) large mud volcanoes (MVs) of the area mainly discharge freshened waters with a high contribution (52–74%) of dehydration water released by smectite-to-illite transition and specific isotope-geochemical signatures. They include low dissolved Cl concentration (4600 ppm on average), heavy δ18O (+ 9.0 to +13.5‰ VSMOW), and B (418–907 ppm) enrichment, but low δ11B (+10.7 to + 13.6‰) and δD (−24 to −10‰). The boron (δ11B) and oxygen (δ18O) isotopic compositions of MV water are inversely proportional and show significant correlation with fluid generation temperatures (TMg/Li). δ11B and δ13C(DIC) were compiled from our previous studies, for all 13 mud-volcano and borehole waters characterized in this study, and applied systematically here as an independent screening criterion for the first time. Carbon isotope compositions of dissolved inorganic carbon (δ13C(DIC)) independently corroborate this classification, distinguishing deep-rooted fluids from those modified by surface processes. A two-endmember mixing model (δ11B vs. 1/B, R2 = 0.81) demonstrates that boron enrichment in deep-rooted MVs is primarily driven by the progressive expulsion of isotopically light, lattice-bound B(OH)4 during smectite illitization at temperatures 100–150°C (R1–R3 ordering stages), overriding shallow adsorbed B signatures.