ALBERT

Description: The active Trans-Atlantic Geotraverse (TAG) hydrothermal mound is a mature submarine massive sulfide deposit at the slow-spreading Mid-Atlantic Ridge at 26°N. Fluid inclusion measurements were conducted on quartz and anhydrite from six boreholes drilled in different areas of the mound to characterize the fluids responsible for the deposition of sulfide-silica breccias and anhydrite and to investigate the vertical and horizontal temperature zonation within an actively forming hydrothermal system. Fluid inclusions in both host minerals are generally two phase liquid/vapor inclusions that homogenize into the liquid phase. Trapping temperatures for quartz and anhydrite from the TAG mound range from 212° to 390°C. Salinities vary from 1.9 to 6.2 wt% NaCl equivalent for anhydrite and from 4.0 to 6.0 wt% NaCl equivalent for quartz. This salinity variation is probably best explained by supercritical phase separation at temperatures above 450°C with subsequent remixing of the liquid and the vapor phase during ascent. A zone of anhydrite-rich precipitates recovered at 20 to 35 mbsf below the central Black Smoker Complex (TAG-1) is characterized by trapping temperatures averaging 348°C for anhydrite and 358°C for quartz, which is slightly below the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360°-366°C). Breccias in the stockwork zone underlying the anhydrite zone were formed at slightly higher temperatures ranging from 327°- 381°C for quartz and from 349° to 384°C for anhydrite. Trapping temperatures vary strongly between different areas of the mound. Fluid inclusions in quartz and anhydrite from the central Black Smoker Complex are characterized by a narrow range of trapping temperatures, whereas other areas drilled on the mound were influenced by lower temperature hydrothermal fluids percolating through the mound or by local entrainment of seawater into the mound. White smokers venting on the southeastern side of the TAG mound are characterized by exit temperatures of 270°-300°C, (Kremlin area, TAG-2). Fluid inclusion measurements in quartz and anhydrite from this area give trapping temperatures in the range of 266°-375°C with a distinct peak around 340°C, only somewhat lower than results for the Black Smoker Complex. Trapping temperatures in anhydrite-hosted fluid inclusions in this area show a strong vertical temperature increase. The west side of the mound (TAG-4) is characterized by trapping temperatures ranging from 212° to 390°C showing evidence for seawater entrainment or overprinting by lower temperature hydrothermal events at the sulfide/basalt interface. Samples from the northern side of the mound (TAG-5) exhibit trapping temperatures in the range from 258°-383°C with a strong vertical temperature increase, indicating additional hightemperature upflow at the northern margin of the mound outside the central Black Smoker Complex.

Description: Eighty-five bulk samples consisting of varying proportions of pyrite, silica, and anhydrite and 82 mineral separates (pyrite, chalcopyrite) from the TAG hydrothermal mound were analyzed using Neutron Activation Analyses (INAA), Inductively Coupled Plasma Emission Spectrometry (ICP-ES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and sulfur-isotopic methods. The samples were collected from five different areas of the Trans-Atlantic Geotraverse (TAG) mound during Ocean Drilling Program Leg 158. The chemistry of the bulk samples is dominated by high Fe (average 30.6 wt%, n = 57) and S concentrations (average 42.0 wt%, n = 50), reflecting the high amount of pyrite in these rocks. High Ca (up to 11.5 wt%, n = 57) and SiO2 values (up to 49.8 wt%, n = 50) indicate the presence of anhydrite-rich zones in the center of the mound, and pyritesilica breccias, silicified wallrock breccias, and paragonitized basalt breccias deeper in the system. The Cu and Zn concentrations vary from 〈0.01 to 12.2 wt% Cu (average 2.4 wt%, n = 57) and from 〈0.01 to 4.1 wt% Zn (average 0.4 wt%, n = 57), with highest values commonly occurring in the uppermost 20 m of the mound. Most trace-element concentrations are relatively low compared to other mid-ocean ridge hydrothermal sites and average 0.5 ppm Au, 43 ppm As, 234 ppm Co, 2 ppm Sb, 14 ppm Se (n = 85), 9 ppm Ag, 11 ppm Cd, and 59 ppm Pb (n = 57). Gold, Ag, Cd, Pb, and Sb behave similarly to Cu and Zn and are enriched close to the surface of the mound. This is interpreted as evidence for zone refining, a process in which elements that are mobilized from previously deposited sulfides in the interior of the mound by later hydrothermal fluids are transported to the surface, where they reprecipitate as a result of mixing with ambient seawater. The trace-element composition of pyrite and chalcopyrite separates is similar to the bulk geochemistry. However, down to about 50 mbsf, Au, As, Sb, and Mo values in pyrite separates are generally higher than in bulk samples and chalcopyrite separates. Below this depth, these elements appear to be enriched in chalcopyrite separates. Cobalt is typically more enriched in pyrite than in chalcopyrite throughout. A major difference between pyrite and chalcopyrite separates is the strong enrichment of Se in chalcopyrite at the top of the mound, whereas pyrite separates show a moderate increase of Se with depth. Sulfur-isotopic values for bulk sulfides from the interior of the TAG mound vary from +4.6‰ to +8.2‰, with an average of +6.4 ‰ d34S (n = 49). These values do not change significantly downhole, but samples collected from the top of the mound appear to have somewhat lower d34S values than samples from the interior. The average d34S value for TAG sulfides is about 3‰ higher than for most other sulfides generated at sediment-free mid-ocean ridges (average 3.2‰, n = 501). This is largely attributed to thermochemical sulfate (anhydrite) reduction by hightemperature hydrothermal fluids upwelling through the interior of the TAG mound.