Rare earth element concentrations and geochemistry of carbonate minerals from the Logatchev Hydrothermal Field and the Gakkel Ridge

• Main Authors: Eickmann, Benjamin, Bach, Wolfgang, Rosner, Martin, Peckmann, Jörn
• Format: Other/Unknown Material
• Language: English
• Published: PANGAEA 2009
• Subjects: ARK-XVII/2; Center for Marine Environmental Sciences; DERIDGE; Dredge; pipe; DRG_P; From Mantle to Ocean: Energy-; Material- and Life-cycles at Spreading Axes; HYDROMAR1; M60/3; M60/3-49-GTV; M60/3-53-ROV-2; M60/3-54-GTV; M60/3-67-GTV; MARUM; Meteor (1986); Mid-Atlantic Ridge at 10-15°N; Polarstern; PS59/238; PS59/249; PS59 AMORE; Remote operated vehicle; ROV; Television-Grab; TVG; Arctic; Mid-Atlantic Ridge; Gakkel Ridge
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.746502; https://doi.org/10.1594/PANGAEA.746502

The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like 87Sr/86Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~100 °C), and yet its 87Sr/86Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine-talc fault schists form north of the LHF. These calcites show unradiogenic 87Sr/86Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the ...