Compared geochemical signatures and the evolution of Menez Gwen (37 degrees 50 ' N) and Lucky Strike (37 degrees 17 ' N) hydrothermal fluids, south of the Azores Triple Junction on the Mid-Atlantic Ridge [Review]

摘要

During the DIVA 1 cruise in May 1994, a series of 19 dives was conducted using the French submersible Nautile at the topographic highs of three volcanic segments centered at 37 degrees 17'N, 37 degrees 50'N and 38 degrees 20'N, respectively, south of the Azores Triple Junction (ATJ) on the Mid-Atlantic Ridge (MAR). Hydrothermal fluids were collected at the Lucky Strike (LS) hydrothermal field, discovered at 37 degrees 17'N in 1993, and at a new hydrothermal site called Menez Gwen (MGw) discovered at 37 degrees 50'N during this cruise. Both systems are relatively shallow compared to other MAR systems with seafloor depths of 1700 and 850 m, respectively, characteristics which make them unique among the already known sites on the MAR. The characteristics of the LS fluids are the same as in 1993 and include temperatures ranging from 170 degreesC to 324 degreesC, variable chlorinities lower than seawater, low hydrogen sulfide(< 3.0 mmol/kg), high Ba concentrations (up to 80 mol/kg), low metal concentrations and high gas contents, and distinct chemical end-members indicative of significant geographic control of the venting system. In contrast, the very clear MGw fluids at 37 degrees 50'N show a rather uniform exit temperature (285 degreesC) and chemical compositions with chlorinities (360-380 mmol/kg) lower than at LS and corresponding lower concentrations of cations, H2S (1.8 mmol/kg), metals and silica (8-11.5 mmol/kg), due to the lower temperature (T) and pressure (P) of the system. In the two systems, fluid chemistry is strongly affected by phase separation. At the low-pressure conditions of these sites, phase relations in the NaCl-H2O system dictate production of an extremely low salinity. The higher salinities observed in fluids thus indicate that mixing of extremely low salinity vapor with single-phase hydrothermal seawater is the dominant process controlling Cl concentrations. Depletions and enrichments of elements in solutions are also explained by the involvement of fluid-rock reactions. The enrichment of Ba, K, Cs and Rb in the fluids is linked to the enriched character of these elements in basalts. Relatively high pH and Ca, low Li, Sr and metals are related to the highly altered basaltic substrate in the reaction zone. Low Sr/Ca and high Ca/Na ratios are explained by albitization within the crust. The increase of degassing when approaching the Azores may be related to the carbon-enriched basaltic crust near the Azores hot spot. Isotopic ratios show that CO2, CH4 and helium gases have a magmatic signature. Compared to other deeper sites on the MAR, all fluids collected at LS and MGw are gas-enriched, except for H2S and helium, whose both concentration and isotopic ratio compare well with other hydrothermal fluids. The high CH4 concentration associated with unsaturated hydrocarbons and the high CH4/He-3 ratios also suggest a contribution of CH4 generated by serpentinization of ultramafic rocks by Fischer-Tropsch catalysis of CO2 reduction. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 101]