Composition and dissolution of black smoker particulates from active vents on the Juan de Fuca Ridge

During two Atlantis II/Alvin cruises to the Juan de Fuca Ridge in 1984 active high temperature (140°–284°C) vents were sampled for black smoker particulates using the Grassle Pump. Individual mineral phases were identified using standard X ray diffraction and petrographic procedures. In addition, elemental compositions and particle morphologies were determined by X ray energy spectrometry and scanning electron microscope/X ray energy spectrometry techniques. The vent particulates from the southern Juan de Fuca Ridge vent sites were highly enriched in S, Si, Fe, Zn, and Cu and were primarily composed of sphalerite, wurtzite, pyrite, pyrrhotite, barite, chalcopyrite, cubanite, hydrous iron oxides, and elemental sulfur. Two additional unidentified phases which were prevalent in the samples included an Fe-Si phase and a Ca-Si phase. The grain sizes of the individual particle phases ranged from 100 μm for the Fe-Si particles. Grain size and current meter data were used in a deposition model of individual phase dispersal. For many of the larger sulfide and sulfate particles, the model predicts dispersal to occur over length scales of only several hundreds of meters. The high-temperature black smokers from the more northerly Endeavour Segment vents were highly enriched in Fe, S, Ca, Cu, and Zn and were primarily composed of anhydrite, chalcopyrite, sphalerite, barite, sulfur, pyrite, and other less abundant metal sulfide minerals. The grain sizes of the individual particles ranged from < 10 μm to slightly larger than 500 μm. The composition and size distributions of the mineral phases are highly suggestive of high-temperature mixing between vent fluids and seawater. A series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals. The dissolution rates ranged over more than 3 orders of magnitude, from 3.2 × 10−8 cm s−1 for anhydrite to 1.2 × 10−12 cm s−1 for chalcopyrite. The results indicate that for some minerals, particularly anhydrite and marcasite, total dissolution occurs within a few hours to a few weeks of their formation. For other more stable minerals, including pyrite, sphalerite and chalcopyrite, the time required for total dissolution is much longer, and consequently, individual crystals may be expected to persist in the sediments for considerable periods of time after deposition.