Chemical and isotopic compositions and age of carbonates from the Lost City hydrothermal field, Mid-Atlantic Ridge

General geochemical conditions, under which authigenic carbonate formation occurs within these two fields, were found.

They include: presence of reduced H2S, H2, and CH4 gases at absence of free oxygen; high alkalinity of waters participating in carbonate formation; similarity of textural and structural features of authigenic aragonite, which represents the initial carbonate mineral of the mounds; paragenesis of aragonite with sulfide minerals; close relation of carbonate mounds with communities of sulfate-reducing and methane-oxidizing microorganisms.