Wayne C. Shanks

Variations in the chemical and stable isotope composition of carbon and sulfur species during organic-rich sediment alteration: An experimental and theoretical study of hydrothermal activity at guaymas basin, gulf of california

Organic-rich diatomaceous ooze was reacted with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity at 325–400°C, 400–500 bars, and fluid/sediment mass ratios of 1.56–2.35 to constrain factors regulating the abundance and stable isotope composition of C and S species during hydrothermal alteration of sediment from Guaymas Basin, Gulf of California. Alteration of inorganic and organic sedimentary components resulted in extensive exchange reactions, the release of abundant H2S, CO2, CH4, and Corganic, to solution, and recrystallization of the sediment to an assemblage containing albitic plagioclase, quartz, pyrrhotite, and calcite. n nThe δ34Scdt values of dissolved H2S varied from −10.9 to +4.3‰ during seawater-sediment interaction at 325 and 400°C and from −16.5 to −9.0‰ during Na-Ca-K-Cl fluid-sediment interaction at 325 and 375°C. In the absence of seawater SO4, H2S is derived from both the transformation of pyrite to pyrrhotite and S released during the degradation of organic matter. In the presence of seawater SO4, reduction of SO4 contributes directly to H2S production. Sedimentary organic matter acts as the reducing agent during pyrite and SO4 reduction. Requisite acidity for the reduction of SO4 is provided by Mg fixation during early-stage sediment alteration and by albite and calcite formation in Mg-free solutions. n nOrganically derived CH4 was characterized by δ13Cpdb values ranging between −20.8 and −23.1‰, whereas δ13Cpdb values for dissolved Corganic ranged between −14.8 and −17.7%. Mass balance calculations indicate that δ13C values for organically derived CO2 were ≥ − 14.8%. Residual solid sedimentary organic C showed small (≤ 0.7‰) depletions in 13C relative to the starting sediment. n nThe experimental results are consistent with the isotopic and chemical composition of natural hydrothermal fluids and minerals at Guaymas Basin and permit us to better constrain sources and sinks for C and S species in subseafloor hydrothermal systems at sediment-covered spreading centers. Our data show that the sulfur isotope composition of hydrothermal Sulfide minerals in Guaymas Basin can be explained by derivation of S from diagenetic sulfide and seawater sulfate. Basaltic S may also contribute to hydrothermal sulfide precipitates but is not required to explain their isotopic composition. Estimates of seawater/ sediment mass ratios based on sulfur isotopic composition of sulfide minerals and the abundance of dissolved NH3 in vent fluids range from 3–29 during hydrothermal circulation. Sources of C in Guaymas Basin hydrothermal fluids include thermal degradation of organic matter, bacteriogenic methane production, and dissolution of diagenetic carbonate.

Geochemistry of fluid phases and sediments: relevance to hydrothermal circulation in Middle Valley, ODP Legs 139 and 169

Geochemical and isotopic studies of pore fluids and solid phases recovered from the Dead Dog and Bent Hill hydrothermal sites in Middle Valley (Ocean Drilling Program Leg 169) have been compared with similar data obtained previously from these sites during Ocean Drilling Program Leg 139. Although generally the hydrothermal systems reflect non-steady state conditions, the data allow an assessment of the history of the hydrothermal processes. Sediment K/Al ratios as well as the distribution of anhydrite in the sediments suggest that the Dead Dog hydrothermal field has been, and still is, active. In contrast, similar data in the Bent Hill hydrothermal field indicate a waning of hydrothermal activity. Pore fluid and hydrothermal vent data in the Dead Dog hydrothermal field are similar in nature to the data collected during ODP Leg 139. In the area of the Bent Hill sulfide deposit, however, the pore water data indicate that recent wholesale flushing of the sediment column with relatively unaltered seawater has obliterated a previous record of hydrothermal activity in the pore fluids. Data from the deepest part of Hole 1035A in the Bent Hill locality show the presence of hydrothermal fluids at greater depths in this area. This suggests the origin of the hydrothermal fluids found to be emanating from Hole 1035F, which constitutes one of the first man made hydrothermal vents in the Middle Valley hydrothermal system. Similarly, CORKed Hole 858G, because of seal failures, has acted as a hydrothermal vent, with sulfide deposits forming inside the CORK.

LABORATORY STUDIES OF SPHALERITE DECOMPOSITION: APPLICATIONS TO THE WEATHERING OF MINE WASTES AND POTENTIAL EFFECTS ON WATER QUALITY

Sphalerite [(Zn,Fe)S] from Mississippi Valley type (MVT), volcanogenic massive sulfide (VMS), and polymetallic vein (PMV) ore deposits show variable rates of dissolution and resulting aqueous metals concentrations under acid conditions. An important control on dissolution rates is solubilized iron that eventually produces Fe(III), a strong oxidant of sphalerite. VMS (high Fe and Cd) and PMV (high Fe and metals) sphalerite samples showed faster rates of sphalerite dissolution (10 -9 to 10 -10 mol/L/s Zn +2 ) and Fe solubilization (10 -9 to 10 -10 mol/L/s total Fe), and higher aqueous trace metals compared to MVT sphalerite that has low Fe and trace metals; these characteristics are manifested by slower rates of sphalerite dissolution (10 -10 to 10 -11 mol/L/s Zn +2 ) and Fe solubilization (10 -12 mol/L/s total Fe), and low aqueous metals compared to VMS and PMV samples. Aqueous concentrations of major (Fe, Zn) and trace (Cu, Cd, Pb, Mn) elements depended on the original composition and rate of dissolution of sphalerite. Fine-grained, high-Fe VMS sphalerite (6.7 wt.%) leached at pH 2-3 (25° C) produced dissolved Fe and Zn approaching 50 and 180 mg/L, respectively, within 1 week; Cu and Pb were 3 and 250 uf06dg/L, respectively. The same sphalerite leached at pH 4.0 or higher took two months to yield comparable concentrations. Coarse-grained VMS sphalerite leached for several months at a pH of 4.0 yielded only 100uf020uf06dg/L Fe and 9.2 mg/L Zn. PMV sphalerite (4.2 wt.%) had a Zn solubilization rate and trace metal concentrations similar to the VMS sample, but a faster Fe solubilization rate (10 -8 mol/L/s Fetot). Low-iron sphalerite (0.2-0.3 wt.%) from MVT deposits produced Zn concentrations similar to the VMS sphalerite at pH 2.0 but lower concentrations at pH 4.0. At both pH values, aqueous Fe from the high-Fe VMS sample was 100 times that from the MVT sample. Trace metals showed a range of concentrations that generally depended on their original abundance in the solid and their solubilities in the acid solution. Because sphalerite from different ore deposits can be major sources of aqueous metals that may affect the composition of nearby surface and ground water, the potential metal contribution of non-pyritic minerals such as sphalerite should be a major consideration during all stages of economic development of an ore deposit. Additional