Fluid elemental and stable isotope composition of the Nibelungen hydrothermal field (8°18′S, Mid-Atlantic Ridge): Constraints on fluid-rock interaction in heterogeneous lithosphere

Depending on the geological setting, the interaction of submarine hydrothermal fluids with the host rock leads to distinct energy and mass transfers between the lithosphere and the hydrosphere. The Nibelungen hydrothermal field is located at 8 degrees 18'S, about 9 km off-axis of the Mid-Atlantic Ridge (MAR). At 3000 m water depth, 372 degrees C hot, acidic fluids emanate directly from the bottom, without visible sulfide chimney formation. Hydrothermal fluids obtained in 2009 are characterized by low H2S concentrations (1.1 mM). a depletion of B (192 mu M) relative to seawater, lower Si (13.7 mM) and Li (391 mu M) concentrations relative to basaltic-hosted hydrothermal systems and a large positive Eu anomaly, and display a distinct stable isotope signature of hydrogen (Delta H-2(H2O) = 7.6-8.7%.) and of oxygen (Delta O-18(H2O) = 2.2-2.4 parts per thousand) The heavy hydrogen isotopic signature of the Nibelungen fluids is a specific feature of ultramafic-hosted hydrothermal systems and is mainly controlled by the formation of OH-bearing alteration minerals like serpentine, brucite, and tremolite during pervasive serpentinization. New isotopic data obtained for the ultramafic-hosted Logatchev I field at 14 degrees 45'N, MAR (Delta H-2(H2O) = 3.8-4.2%.) display a similar trend, being clearly distinguished from other, mafic-hosted hydrothermal systems at the MAR. The fluid geochemistry at Nibelungen kept stable since the first sampling campaign in 2006 and is evident for a hybrid alteration of mafic and ultramafic rocks in the subseafloor. Whereas the ultramafic-fingerprint parameters Si, Li, B, Eu anomaly and Delta H-2(H2O) distinguish the Nibelungen field from other hydrothermal systems venting in basaltic settings at similar physico-chemical conditions and are related to the interaction with mantle rocks, the relatively high concentrations of trace alkali elements, Pb, and TI can only be attributed to the alteration of melt-derived gabbroic rocks. The elemental and isotopic composition of the fluid suggest a multi-step alteration sequence: (1) low- to medium-temperature alteration of gabbroic rocks, (2) pervasive serpentinization at moderate to high temperatures, and (3) limited high-temperature interaction with basaltic rocks during final ascent of the fluid. The integrated water/rock ratio for the Nibelungen hydrothermal system is about 0.5. The fluid compositional fingerprint at Nibelungen is similar to the ultramafic-hosted Logatchev I fluids with respect to key parameters. Some compositional differences can be ascribed to different alteration temperatures and other fluid pathways involving a variety of source rocks, higher water/rock ratios, and sulfide precipitation in the sub-seafloor at Logatchev I. (C) 2010 Elsevier B.V. All rights reserved.