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 ana lyzed 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 degreesN vent f ield (East Pacific Rise), as well as of pyrite/marcasite and quartz from th e TAG mound, were analyzed. A new type of blank for sulphides and sulphates was also developed. Water contents obtained by GC analysis are in agreemen t with known compositions of seafloor hydrothermal solutions. Also, the vol atiles occur in the same order of abundance as in vent fluids CO2 > N-2(+/- CO +/- Ar +/- O-2) greater than or equal to CH4 > COS > C-2-C-3 hydrocarbo ns. For all analyzed hydrothermal fluids, the mean Cl- concentrations are s imilar to those for the respective measured vent fluid compositions. Furthe rmore, the GC/IC results are directly comparable to salinity data determine d by microthermometric methods. NH: can be enriched and is most probably fo rmed by decomposition of organic matter. Volatile concentrations in fluid i nclusions and data from vent fluids differ significantly for some of the in vestigated sites. Furthermore, the GC data indicate systematic variations r egarding the volatile contents of trapped fluids from different active seaf loor hydrothermal systems. Fluid inclusion volatile data of samples from th e ASHES vent field at Axial Seamount define an inclined array consistent wi th phase separation. (C) 2001 Elsevier Science B.V. All rights reserved.