Analysis of fluid inclusions in seafloor hydrothermal precipitates: testing and application of an integrated GC/IC technique

The principal research objective was to test an integrated gas (GC) and ion chromatographic (IC) technique for analysis of trapped fluids in seafloor hydrothermal precipitates and to compare the results with independently analyzed vent fluids. Twenty-three samples of chalcopyrite, pyrite, sphalerite, barite and anhydrite from hydrothermal chimneys from four seafloor sites, Axial Seamount (Juan de Fuca Ridge), the Vai Lili vent field (Lau Basin), the TAG hydrothermal field (Mid-Atlantic Ridge), and the 21°N vent field (East Pacific Rise), as well as of pyrite/marcasite and quartz from the TAG mound, were analyzed. A new type of blank for sulphides and sulphates was also developed. Water contents obtained by GC analysis are in agreement with known compositions of seafloor hydrothermal solutions. Also, the volatiles occur in the same order of abundance as in vent fluids CO2>N2(±CO±Ar±O2)≥CH4>COS>C2–C3 hydrocarbons. For all analyzed hydrothermal fluids, the mean Cl− concentrations are similar to those for the respective measured vent fluid compositions. Furthermore, the GC/IC results are directly comparable to salinity data determined by microthermometric methods. NH4+ can be enriched and is most probably formed by decomposition of organic matter. Volatile concentrations in fluid inclusions and data from vent fluids differ significantly for some of the investigated sites. Furthermore, the GC data indicate systematic variations regarding the volatile contents of trapped fluids from different active seafloor hydrothermal systems. Fluid inclusion volatile data of samples from the ASHES vent field at Axial Seamount define an inclined array consistent with phase separation.