Fluid Mixing and Spatial Geochemical Variability in the Lost City Hydrothermal Field Chimneys

Carbonate-brucite chimneys are a characteristic of low- to moderate-temperature, ultramafic-hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30 degrees N near the Mid-Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization-derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (87Sr/86Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid-dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite-poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid-seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have delta 13C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower 87Sr/86Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate. At the Lost City hydrothermal field, warm alkaline fluids are discharging out of uplifted mantle rocks. When vent fluids mix with seawater at the seafloor, carbonate and brucite minerals form spectacular towers up to 60 m high. Systems like Lost City are important because the reaction between water and rocks provides carbon and energy sources for microbial life. However, we still do not fully understand what controls the mineralogy and geochemistry of the Lost City hydrothermal chimneys. In this paper, we suggest that the extent of mixing between the hydrothermal fluids and seawater influences the mineralogy and geochemistry of the chimneys. Calcite, which was previously thought to form only during alteration of aragonite by seawater, can also form during seawater-hydrothermal fluid mixing. Both calcite and brucite form in the interior of the chimneys where vent fluid is more dominant. Aragonite, on the other hand, forms in the exterior of the structures from seawater-rich fluids. Lastly, because minerals precipitate rapidly during fluid mixing, the stable isotope geochemistry of the carbonates mostly records the composition and temperature of seawater and not the mixed fluid. Thus, care should be exercised in interpreting mineral geochemical data from similar systems. The mineralogy and geochemistry of Lost City chimneys are controlled by the extent of mixing between hydrothermal fluids and seawater Brucite and calcite precipitate in vent fluid dominated zones while aragonite forms in the exterior of the structures in seawater-rich zones Carbonates precipitate in isotopic disequilibrium and record the O and C stable isotope composition of seawater dissolved inorganic carbon