Geology topics

W.E. Seyfried

Publications and source records attributed to W.E. Seyfried.

6 recordsLinked to original sources

USGS2012

Mineralization, alteration, and hydrothermal metamorphism of the ophiolite-hosted Turner-Albright sulfide deposit, southwestern Oregon

The Turner-Albright sulfide deposit, part of the Josephine ophiolite, formed on and below the seafloor during Late Jurassic volcanism at a back arc spreading center. Ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition. The uppermost massive sulfide formed on the seafloor at hydrothermal vents, similar to sulfide-depositing hot springs on modern spreading centers. The bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization. Infiltration of hydrothermal fluid into the hyalo-clastite altered the rock to quartz + Fe-chlorite and quartz + sericite + Fe-chlorite. Intense alteration resulted in replacement of the protolith by quartz + sulfide. The fluid responsible for the hydrothermal alteration was evolved seawater with low p H and Mg and high Fe. Sulfide δ 534 S values average 4.7‰ and indicate contribution of sulfur from both basalt and seawater sulfate. The average value of sulfide δ 34 S and the difference between sulfide and contemporaneous seawater sulfate δ 34 S values are similar to ophiolite-hosted sulfide deposits in Cyprus. Chlorite-rich hydrothermally altered basalt has δ 18 O values of 3.0–6.8‰ due to high-temperature (250°–350°C) reaction with a seawater-derived ore fluid. Quartz in silicified basalt has higher δ 18 O values of 12.3–15.5‰, probably reflecting lower-temperature recrystallization of amorphous silica formed during initial alteration of basalt. Mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore-transporting hydrothermal fluid, but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust. This subseafloor metamorphism produced a mineral assemblage typical of prehnite-pumpellyite facies metamorphism. Exchange with altered seawater increased the whole-rock δ 18 O of the basalts to values of 9.4–11.2‰.

Journal of Geophysical Research Solid Earth↗

USGS2004

Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge

The Bent Hill massive sulfide deposit and ODP Mound deposit in Middle Valley at the northernmost end of the Juan de Fuca Ridge are two of the largest modern seafloor hydrothermal deposits yet explored. Trace metal concentrations of sulfide minerals, determined by laser-ablation ICP-MS, were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid-mineral reactions during sulfide deposition. Detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits. Trace elements (Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi) analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins. Enrichments of some trace metals (Pb, Sb, Cd, Ag) in sphalerite at the margins of the deposits are best explained by dissolution/reprecipitation processes consistent with secondary remineralization. Results of reaction-path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater. Model results are consistent with patterns of observed mineral paragenesis and help to identify conditions (pH, redox, temperature) that may be responsible for variations in trace metal concentrations in primary and secondary minerals. Differences in trace metal distributions throughout a single deposit and between nearby deposits at Middle Valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems. ?? 2004 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS1996

Hydrogen isotope systematics of phase separation in submarine hydrothermal systems: Experimental calibration and theoretical models

Hydrogen isotope fractionation factors were measured for coexisting brines and vapors formed by phase separation of NaCl/H2O fluids at temperatures ranging from 399-450??C and pressures from 277-397 bars. It was found that brines are depleted in D compared to coexisting vapors at all conditions studied. The magnitude of hydrogen isotope fractionation is dependent on the relative amounts of Cl in the two phases and can be empirically correlated to pressure using the following relationship: 1000 ln ??(vap-brine) = 2.54(??0.83) + 2.87(??0.69) x log (??P), where ??(vap-brine) is the fractionation factor and ??P is a pressure term representing distance from the critical curve in the NaCl/H2O system. The effect of phase separation on hydrogen isotope distribution in subseafloor hydrothermal systems depends on a number of factors, including whether phase separation is induced by heating at depth or by decompression of hydrothermal fluids ascending to the seafloor. Phase separation in most subseafloor systems appears to be a simple process driven by heating of seawater to conditions within the two-phase region, followed by segregation and entrainment of brine or vapor into a seawater dominated system. Resulting vent fluids exhibit large ranges in Cl concentration with no measurable effect on ??D. Possible exceptions to this include hydrothermal fluids venting at Axial and 9??N on the East Pacific Rise. High ??D values of low Cl fluids venting at Axial are consistent with phase separation taking place at relatively shallow levels in the oceanic crust while negative ??D values in some low Cl fluids venting at 9??N suggest involvement of a magmatic fluid component or phase separation of D-depleted brines derived during previous hydrothermal activity.

Geochimica et Cosmochimica Acta↗

USGS1994

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 H 2 S, CO 2 , CH 4 , and C organic , to solution, and recrystallization of the sediment to an assemblage containing albitic plagioclase, quartz, pyrrhotite, and calcite. The δ 34 S cdt "> δ 34 S cdt values of dissolved H 2 S 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 SO 4 , H 2 S is derived from both the transformation of pyrite to pyrrhotite and S released during the degradation of organic matter. In the presence of seawater SO 4 , reduction of SO 4 contributes directly to H 2 S production. Sedimentary organic matter acts as the reducing agent during pyrite and SO 4 reduction. Requisite acidity for the reduction of SO 4 is provided by Mg fixation during early-stage sediment alteration and by albite and calcite formation in Mg-free solutions. Organically derived CH 4 was characterized by δ 13 C pdb "> δ13Cpdb values ranging between −20.8 and −23.1‰, whereas δ 13 C pdb "> δ 13 C pdb values for dissolved C organic ranged between −14.8 and −17.7%. Mass balance calculations indicate that δ 13 C values for organically derived CO 2 were ≥ − 14.8%. Residual solid sedimentary organic C showed small (≤ 0.7‰) depletions in 13 C relative to the starting sediment. The 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 NH 3 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.

Baja California Sur↗

USGS1981

Experimental seawater-basalt interaction at 300°C, 500 bars, chemical exchange, secondary mineral formation and implications for the transport of heavy metals

Seawater and NaCl solutions were reacted with basalt (basalt glass and diabase) for several months at 300°C, 500 bars and a water/rock ratio of 10. During reaction, seawater was significantly modified, increasing in Ca, H 2 S, CO 2 . SiO 2 , K. Fe, Mn. Ba, Al and H + , and decreasing in Mg and SO 4 . Basalt glass was completely replaced by smectite, wairakite, anhydrite and hematite, and diabase was partially replaced by mixed layered smectite-chlorite, anhydrite and magnetite (?). Diabase was altered more slowly than basalt glass and the corresponding changes in seawater chemistry were less pronounced. Basalt glass reacted with a 0.45 m NaCl solution resulted in the formation of smectite, albite. truscottite and wairakite. Solutions from this experiment were characterized by a relatively high pH and dominated by Ca for Na exchange reactions. At no point in this experiment were heavy metals solubilized, in contrast to the seawater experiments. This behavior illustrates the fundamental importance of seawater chemistry to heavy-metal solubility; that is, the removal of Mg from seawater generates acidity which maintains heavy metals in solution. Apparently seawater chlorinity is not capable of enhancing heavy-metal solubility by chloride complexing. Seafloor heavy-metal deposits can result from the following: Seawater-basalt interaction at moderate temperature (&sim;-300°C and high effective water/rock ratios; or at relatively high temperatures (&sim;-400°C) and low (e.g.< 10) water/rock ratios.

Geochimica et Cosmochimica Acta↗

USGS1979

Low temperature basalt alteration by sea water: an experimental study at 70°C and 150°C

Basaltic glass and diabase were reacted with seawater at 70°C at 1 bar and 150°C at 500 bars to determine fluid composition and alteration mineralogy. All experiments were performed at a water/ rock mass ratio of 10. The changes in seawater chemistry depended on temperature and crystallinity of the basalt. The experiment at 70°C produced a slight but continuous loss of Mg, Na and K and enrichment of Ca and SiO 2 in the seawater while pH decreased slowly. At 150°C, in contrast, Mg and SO 4 were quickly and quantitatively removed while Ca, SiO 2 , Na, K, Fe, Mn and Ba were added to the seawater. pH rose to values between 5.5 and 6.5 after an initial drop to lower values. Basalt glass reacted more extensively at 150°C than diabase. Smectite was the major alteration product (iron-rich saponite) at 150°C for both the glass and diabase experiments. Smectite from the diabase experiment was well crystallized while that from the glass experiment was poorly crystallized. The smectites are similar to smectites found in altered oceanic ophiolitic basalts.

Geochimica et Cosmochimica Acta↗