The Iceland Deep Drilling Project (IDDP): (IV) Interpretations of Black Smoker Fluid Compositions
Abstract
One scientific goal of the IDDP is to understand high-temperature reaction zones such as those that feed hydrothermal fluids to active mid-ocean ridge black smoker vents. Smoker fluids emerge from a reservoir of composition, pressure and temperature resembling those expected in a supercritical IDDP well in the Reykjanes geothermal system. We have reconstructed black smoker fluids based on published analyses, and then computed mineral saturation indices, log(Q/K), for a wide range of P-T conditions, from which we identify a pressure and temperature where a group of probable alteration minerals equilibrated with the fluid. The estimated reservoir conditions commonly reflect approximately 60°C of cooling at the vent in excess of that from adiabatic decompression. Saturation index surfaces distinctly converge to zero in a narrow range of pressure and temperature, but the small angle of intersection of most curves yield substantial uncertainty, especially in pressure. Feldspars, quartz, garnet, actinolite, wairakite and chlorite have a stronger pressure dependence than do others, so they become the principal indicators of pressure, which is especially reflected in pH and silica solubility. An accurate reconstructed in situ pH is essential. In reconstructing fluids, we recompute pH to high P-T starting from the pH measured on shipboard in cooled fluid samples. Aside from temperature effects, the pH in such samples is elevated by mixing with cold seawater and lowered by precipitation of vent sulfides. To examine our understanding of pH, we scrutinized the saturation states of sulfides in the reconstructed vent fluids. For example, in 21°N EPR HG vent, we find that sphalerite, chalcopyrite, and pyrrhotite are approximately saturated at the vent conditions (350°C, 260bar), and that pyrite is supersaturated and bornite is undersaturated. The former three are common vent sulfides. In the same fluid, silicates indicate reservoir conditions of approximately 450°C and 600 bar, at which conditions the sulfides are substantially undersaturated. These findings indicate that pH and concentrations of metals and sulfide measured in vent fluids are depressed by sulfide precipitation at and near the vent, thus an accurate estimate of the reservoir fluid properties requires a 're-dissolution' of metals and sulfide into the fluid, limited by saturation at reservoir P and T with sphalerite, chalcopyrite and pyrite, which are common accessory minerals in seafloor-altered basalts.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V41B2072R
- Keywords:
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- 8410 Geochemical modeling (1009;
- 3610);
- 8412 Reactions and phase equilibria (1012;
- 3612);
- 8424 Hydrothermal systems (0450;
- 1034;
- 3017;
- 3616;
- 4832;
- 8135)