ABSTRACT

Stable isotope ratios of C, S, and O have been measured on minerals associated with active hydrothermal vents, plus a free gas sample which was first found and collected from Piip submarine volcano, southern Bering Sea. The active vents were discovered during the 1990 expedition of the R/V “Akademik Mstislav Keldysh” using the deep submersible “MIR”. The free gas sample and a few fragments of anhydrite chimney and surrounding gypsum- and pyrite-bearing precipitates were collected from the high-temperature field of the North Peak of the volcano. The gas is methane-dominant (80.6%, C₁/C₂₊ = 260). CH₄ and CO₂ carbon isotope ratios (−48.7‰ and −21.9‰, respectively), as well as the spectrum of C₁–C₄ hydrocarbons indicate a “thermogenic” origin of the gas. The δ³⁴S values of anhydrite and gypsum (about of 22.5‰) from the northern field are typical for many modern seafloor hydrothermal systems. Pyrite-bearing altered rocks with δ³⁴S = −2.4‰ were formed by recent hydrothermal activity of the volcano. Samples of massive calcite and fragments of aragonite chimneys (containing pyrite) were collected from the low-temperature South Peak field. Aragonite chimney material and massive calcite from the southern field with δ¹³C = −36 to −29‰ and δ¹⁸O = 19–21‰ were formed at about 60°C. Barite and pyrite recovered from the southern field were found to be significantly enriched in ³⁴S (up to 39‰ and +9.2‰, respectively). These data indicate that the C and S isotope compositions of vent material from the southern field are controlled by reduction of marine sulfate by organic sediments or hydrocarbons.