Boron in an onshore mud volcanic environment: Case study from the Kerch Peninsula, the Caucasus continental collision zone

Abstract

Numerous mud volcanoes (MVs) of the Kerch Peninsula emit water and mud with extreme boron enrichment. Boron content correlates with the burial depth of the source Oligo-Miocene Maykop mudrocks yielding the highest boron contents in illite-dominated mud (up to ~900–1500 ppm B) and clay fractions found in the Bulganak MV, which represent the deep endmembers (up to 3.5 km) in the Kerch Peninsula. Smectite-dominated mud and clay fractions from shallow depths (1–1.5 km) at small MVs are poorer in both illite and boron (66–250 ppm). B-enrichment of the parent shale and diagenetically-driven smectite illitization and dewatering may be main prerequisites for boron enrichment in MVs. The majority of both Cl-HCO3/Na and HCO3-Cl/Na highly evolved saline MV waters sampled in large MVs are enriched in 18O (δ18O = +9.8 to +14.5‰ VSMOW) and D (δD = −30 to −4‰ VSMOW) isotopes being also rich in boron (average 650 ppm). Waters of small MVs with shallow roots are poorer in 18О (δ18O = +3.6 to +6.1‰ VSMOW) and B (average 130 ppm). MV waters are mainly related to diagenetically altered basinal water diluted by 18O-enriched fresh water released during smectite illitization. The fluid generation temperatures inferred to be ТMg/Li = 34 to 117 °С correspond to source depths of 1.0 to 3.5 km, at 30–35 °C/km regional geothermal gradients. In the hot season, MV waters reach a salinity of 40–70 g/L total dissolved solids and precipitate halite, ulexite, borax and tincalconite. At the Bulganak MV field, there is a unique accumulation of MV-related non-marine borates. Such borate deposits can form at the following essential conditions: venting of B-rich MV waters; environment akin to playa lake; long dry and hot seasons; evaporation and ensuing increased boron concentration in shallow close MV pools; pH of MV water between 8.5 and 9.5; low permeability of clayey mud cover.