4. GEOCHEMISTRY AND SULFUR-ISOTOPIC COMPOSITION OF THE TAG HYDROTHERMAL MOUND, MID-ATLANTIC RIDGE, 26°N1Peter M. Herzig,2 Sven Petersen,2 and Mark D. Hannington3 |
ABSTRACTEighty-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 pyrite-silica 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 ‰ delta34S (n = 49). These values do not change significantly downhole, but samples collected from the top of the mound appear to have somewhat lower delta34S values than samples from the interior. The average delta34S 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 high-temperature hydrothermal fluids upwelling through the interior of the TAG mound. |
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