Influence of the Precipitation Rate on the Isotope (δ18O, δ13C and δ88Sr) Composition of Carbonate Chimneys of the Lost City Hydrothermal Field (30° N, Mid-Atlantic Ridge)

The behavior of four isotope systems (δ18О, δ13С, δ88Sr and 87Sr/86Sr) during submarine precipitation of inorganic carbonates is considered using the Lost City hydrothermal field as an example of “natural laboratory”. During the carbonate precipitation, the isotope composition, T, and pH of hydrothermal solution change due to mixing of the “end member” Lost City fluid with ocean water. The composition of DIC and carbonates (Cc, Arag) equilibrated with mixed hydrothermal solution was calculated in the isotopic coordinates 87Sr/86Sr–103(Sr/Ca), 87Sr/86Sr–δ18О, 87Sr/86Sr–δ13С, and 87Sr/86Sr–δ88Sr. The observed isotope composition of the Lost City field carbonates is compared with the calculated equilibrium lines. The disequilibrium values of δ18О, δ13С, and δ88Sr in the Cс(Arag) are result of rapid precipitation of carbonates from a hydrothermal fluid under T and pH gradient. The δ18О values of most chimney samples vary around the “DIC–water” equilibrium curve with a slight shift to the Сс(Arag)–water equilibrium. The values of δ13С of chimney carbonates fall between the calculated curves of δ13С (DIC) and equilibrium Сс and Arag. The kinetic isotopic shift Δ88Sr established in chimney carbonates is close to available experimental data on the synthesis of calcite and aragonite from aqueous solutions.