ISOTOPIC EVIDENCE (HE, B, C) FOR DEEP FLUID AND MUD MOBILIZATION FROM MUD VOLCANOES IN THE CAUCASUS CONTINENTAL COLLISION ZONE

Abstract

The Caucasian orogenic wedge formed as a consequence of the closure of the Tethyan Ocean, and numerous fields of active mud volcanoes pepper the area adjacent to the Black and Caspian Seas. Stable isotope ratios of boron, helium, and carbon have been measured for gas, fluid and sediment samples from active mud volcanoes of Taman Peninsula and Georgia to estimate the sources and mobilization depths of the fluid phase and mud. Boron concentrations in mud volcano fluids were found to be 5–35× higher than seawater. Fluid isotope ratios vary between δ11B=22 and 39‰, while isotope ratios of the smectite- and illite-rich extruded mud are considerably depleted in heavy 11B (δ11B=−8 to +7‰). B contents of these muds are ~8× higher than modern marine sediments. This suggests that liquefaction prior to mud volcanism was accompanied by both B enrichment and isotope fractionation, most likely at an intermediate depth mud reservoir at 2–4 km. The hydrocarbon-generating source beds to the mud volcanoes are located at 7 to >10 km depth in the folded Maikop Formation and are of proposed Oligocene–Miocene age. The most likely mechanism is re-hydration of these shales by both hydrocarbons and a geochemically mature fluid from greater depth within the orogenic wedge. Such a deep fluid source is supported by our results from gas analyses, which imply an admixture of minor amounts (less than 1%vol) of 3He (Georgia), thermogenic 13C in methane as well as "ultraheavy" 13C in CO2 (both Taman and Georgia). The overall results attest active local flow of geochemically different fluids along deep-seated faults penetrating the two study areas in the Caucasian orogenic wedge, with the waters as well as the gases coming from below the Maikop Formation.