William E. Seyfried

Reduction of CO2 during serpentinization of olivine at 300 °C and 500 bar

CO 2 reduction processes occurring during experimental serpentinization of olivine at 300 °C and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas. Data reveal that conversion of Fe(II) in olivine to Fe(III) in magnetite during serpentinization leads to production of H 2 and conversion of dissolved CO 2 to reduced-C species including methane, ethane, propane, and an amorphous carbonaceous phase. Hydrocarbon gases generated in the process fit a Schulz-Flory distribution consistent with catalysis by mineral reactants or products. Magnetite is inferred to be the catalyst for methanization during serpentinization, because it has been previously shown to accelerate Fischer-Tropsch synthesis of methane in industrial applications involving mixtures of H 2 and CO 2 . The carbonaceous phase was predominantly aliphatic, but had a significant aromatic component. Although this phase should ultimately be converted to hydrocarbon gases and graphite, if full thermodynamic equilibrium were established, its formation in these experiments indicates that the pathway for reduction of CO 2 during serpentinization processes is complex and involves a series of metastable intermediates.

Hydrothermal alteration of basalt by seawater under seawater-dominated conditions

Fresh mid-ocean ridge basalt glass and diabase have been reacted with seawater at 150–300°C, 500 bar, and water/rock mass ratios of 50, 62, and 125, using experimental apparatus which allowed on-line sampling of solution to monitor reaction progress. These experiments characterize reaction under what we have called “seawater-dominated” conditions of hydrothermal alteration. In an experiment at 300°C, basalt glass undergoing alteration removed nearly all Mg2+ from an amount of seawater 50 times its own mass. In the process, the glass was converted entirely to mixed-layer smectite-chlorite, anhydrite, and minor hematite. Removal of Mg from seawater occurred as a Mg(OH)2 component incorporated into the secondary clay. This produced a precipitous drop in solution pH early in the experiment, accompanied by a dramatic increase in the concentrations of Fe, Mn, and Zn in solution. As Mg removal neared completion and the glass was hydrolyzed, pH rose again and heavy metal concentrations dropped. At water/rock ratios of 62 and 125 and 150–300°C, the mineral assemblage produced was similar to that at a water/rock ratio of 50. Solution chemistry, however, contrasted with the earlier experiment in that Mg concentrations in solution were greater and pH lower. This caused significant leaching of heavy metals. At 300°C nearly all of the Na, Ca, Cu, Zn, and CO2 and most of the K, Ba, Sr, and Mn were leached from the silicates. H2S, Al, Si, and possibly Co were also significantly mobilized, whereas V, Cr, and Ni were not. Little or no seawater sulfate was reduced. Although submarine hot spring solutions sampled to date along mid-ocean ridges clearly come from rock-dominated hydrothermal systems, evidence from ocean floor metabasalts and from heat flow studies indicates that seawater-dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures.

Alteration of the oceanic crust: Implications for geochemical cycles of lithium and boron

Abstract Fresh tholeiitic basalt glass has been reacted with seawater at 150°C, (water/rock mass ratio of 10), and fresh diabase has been reacted with a Na-K-Ca-Cl fluid at 375°C (water/rock mass ratios of 1, 2, and 5) to understand better the role of temperature, basalt composition, and water/rock mass ratio on the direction and magnitude of B and Li exchange during basalt alteration. At 150°C, slight but nevertheless significant amounts of B and Li were removed from seawater and incorporated into a dominantly smectite alteration phase. At 375°C, however, B and Li were leached from basalt. B behaved as a “soluble” element and attained concentrations in solution limited only by the B concentration in basalt and the water/rock mass ratio. Li, however, was less mobile. For example, at water/rock mass ratios of 1, 2, and 5, the percent of Li leached from basalt was 58, 70, and 92% respectively. This suggests some mineralogic control on Li mobility during hydrothermal alteration of basalt, especially at low-water/rock mass ratios. In general, these results, as well as those for B, are consistent with the temperature-dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids. Based on the distribution and chemistry of products of seafloor weathering, low (≤ 150°C) and high-temperature hydrothermal alteration of basalt, and the chemistry of ridge crest hydrothermal fluids, it was estimated that alteration of the oceanic crust is a Li source for seawater. This is not true for B, however, since the hot spring flux estimated for B is balanced by low-temperature basalt alteration. These data, coupled with B and Li flux estimates for other processes ( e.g. , continental weathering, clay mineral adsorption, authigenic silicate formation and formation of siliceous skeletal material) yield new insight into the B and Li geochemical cycles. Calculations performed here indicate relatively good agreement between the magnitude of B and Li sources and sinks. The geochemical cycle of B, however, may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones. Serpentinites are conspicuously enriched in B and if the B source for these rocks is seawater, then an additional B sink exists which must be integrated into the B geochemical cycle. However, until more data are available in terms of areal extent of serpentinization, serpentite chemistry and isotopic composition, the importance of B in these rocks with respect to the B geochemical cycle remains speculative at best.

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 SiO2 in the seawater while pH decreased slowly. At 150°C, in contrast, Mg and SO4 were quickly and quantitatively removed while Ca, SiO2, 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.

Hydrothermal serpentinization of peridotite within the oceanic crust: Experimental investigations of mineralogy and major element chemistry

Abstract Mantle derived ultramafic rocks form a significant portion of lithosphere created at slow-spreading mid-ocean ridges. These rocks are ubiquitously serpentinized, due at least in part to interaction with seawater, at temperatures below approximately 500°C. To evaluate reaction pathways, primary mineral reaction rates, major element exchange between rock and solution, and alteration mineral formation, interaction of equigranular peridotites with seawater and seawater derived solutions has been investigated experimentally at 200°C and 300°C, 500 bars. Seawater chemistry changed greatly during the experiments. Initially, the concentrations of Mg, Ca, and SO4 decreased, as did pH. During Iherzolite experiments, however, the trend of dissolved Ca concentrations reversed with time, first decreasing, then increasing. pH also increased during the latter part of the experiments. Mg, Ca, SiO2, Fe, Cl and ΣCO2 decreased as pH increased FeII oxidation is shown to be affected by solution pH, being greatly enhanced under alkaline conditions. Resulting solution composition and reaction pathway are dependent on initial solution composition, particularly initial concentrations of Mg in solution. Consistent with changes in solution chemistry, the peridotites were significantly altered. Substantial amounts of olivine, relatively minor amounts of diopside and all the enstatite dissolved. Alteration products included serpentine + anhydrite ± magnesium hydroxide sulfate hydrate ± magnetite ± brucite ± tremolite-actinolite or truscottite. From the changes in solution chemistry and examination of the alteration products, the reaction rates (moles per unit time) of olivine to enstatite to diopside during 300°C Iherzolite-seawater experiments are estimated to be approximately 1.0/1.0/0.1. These rates correspond to constant surface area rates of 1.5:5:1 (moles per unit time per unit surface area), which are consistent with experimental data on the dissolution kinetics of these minerals and emphasize the importance of initial rock texture on reaction rates.

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, H2S, CO2. SiO2, K. Fe, Mn. Ba, Al and H+, and decreasing in Mg and SO4. 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: 1. (a) Seawater-basalt interaction at moderate temperature (∼-300°C and high effective water/rock ratios; or 2. (b) at relatively high temperatures (∼-400°C) and low (e.g.< 10) water/rock ratios.

Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: An experimental study at 400°C, 500 bars

Olivine (Fo89), orthopyroxene (En85), and clinopyroxene (Di89) were reacted, individually and in combinations, with NaCl-MgCl2 at 400°C, 500 bars to better assess alteration and mass transfer in ultramafic-hosted hydrothermal systems at mid-ocean ridges. Data indicate that temperature plays a key role in mineral solubility and kinetic processes, which influence the compositional evolution of the fluid. At the temperature and pressure of the experiments, the rate of olivine hydrolysis is sluggish as indicated by the limited extent of mass transfer between the fluid and mineral and absence of hydrous alteration phases. In contrast, reactions involving pyroxenes proceed rapidly, which result in significant increases in dissolved Ca, SiO2, Fe and H2, and formation of SiO2-rich secondary minerals (talc and tremolite) and magnetite. SiO2 release from pyroxene occurs in non-stoichiometric proportions and is a critical factor governing the stability of secondary minerals, with attendant effects on fluid chemistry. Magnetite and talc-fluid equilibria were used to calculate fluid pH at elevated temperatures and pressures. In general, pH is relatively low in the orthopyroxene- and clinopyroxene-bearing experiments due to constraints imposed by talc-fluid and talc-tremolite-fluid equilibria, respectively. Even in experiments where the olivine/pyroxene ratio is as great as 3, which is typical for abyssal peridotite, the low pH and high Fe concentrations are maintained. This is in sharp contrast to theoretical predictions assuming full equilibrium in the MgO-CaO-FeO-Fe2O3-SiO2-Na2O-H2O-HCl system at 400°C, 500 bars. Ultramafic-hosted hydrothermal systems, such as the recently discovered Rainbow system at 36°13.80′N, 33°54.12′W on the Mid-Atlantic Ridge, indicate reaction processes in keeping with results of the present experiments, as suggested by vent fluid chemistry and temperature. In particular, relatively high SiO2, Ca, H2, and Fe concentrations characterize the Rainbow vent fluids. Indeed, Fe concentrations are the highest of any vent system yet discovered and require a relatively low pH in the subseafloor reaction zone from which the fluids are derived. This, together with the SiO2 concentrations of the vent fluids, strongly indicates fluid buffering by silica-rich phases produced during pyroxene dissolution, the likely abundant presence of olivine notwithstanding. Time-series observations at Rainbow are clearly needed to better constrain the temporal evolution of hydrothermal alteration processes of ultramafic rocks in subseafloor reaction zones. In the absence of events permitting fluid continuous access to fresh rock, pyroxene will ultimately be consumed and vent fluids may then reflect changes imposed by bulk compositional constraints characteristic of ultramafic bodies at depth, which would be in better agreement with theoretical phase relations for the fully equilibrated system.

Formation of massive sulfide deposits on oceanic ridge crests: Incremental reaction models for mixing between hydrothermal solutions and seawater

Equilibrium path calculations have been used to model mixing between hot (350°C) hydrothermal solutions and ambient seawater, in an attempt to simulate mineral precipitation at seafloor vents. These calculations predict temperatures of precipitation, paragenetic sequence of minerals, and chemical composition of chimney deposits associated with vents on the seafloor at 21°N, EPR. Assuming sulfate-sulfide disequilibrium during mixing, the paragenetic sequence revealed is: chalcopyrite, anhydrite, pyrrhotite, pyrite, sphalerite, graphite, and barite. When sulfate-sulfide equilibria is permitted during mixing, however, reduction of small amounts of sulfate results in early precipitation of pyrite and a sequence of Cu-rich sulfide minerals (chalcopyrite-bornite-chalcocite-covellite). This sequence is analogous to that observed in thin chimney walls. The calculations indicate that sulfide mineral precipitation occurs in response to both cooling and change in composition of the hydrothermal solutions as a result of mixing. Varying the amount of mixing with respect to temperature, simulating conductive heating of seawater prior to mixing, results in only minor variations in the sequence and abundance of precipitated phases. Anhydrite precipitation during mixing occurs early, which is consistent with formation of an anhydrite leading edge of chimney structures. Similarly, extrapolation of warm spring data from Galapagos to zero SO4 concentration suggests anhydrite formation due to mixing with seawater beneath the seafloor, most likely below the level of reactive calcareous sediments. Subsequent interaction of the mixed hydrothermal solution with those sediments results in elevated and variable Ca concentrations estimated for end-member solutions from the Galapagos. Precipitation of Mg hydroxide sulfate hydrate in the walls of the vent chimneys at 21°N, EPR, occurs as a result of conductive heating of ambient seawater with only very minor amounts of mixing. In contrast, precipitation of amorphous silica in the vents must be due to conductive cooling of the hydrothermal solutions. Thus, incremental reaction calculations demonstrate that reactions occurring in and associated with venting ridge crest hydrothermal solutions can be effectively modeled using the thermodynamic data and reaction modeling codes available today. Departures from equilibrium required to accurately model the mixing process are easily accommodated and consistent with data from the vents and vent forming materials.

Seawater-peridotite interaction at 300°C and 500 bars: implications for the origin of oceanic serpentinites

Abstract An experiment has been performed reacting seawater with fresh peridotite (80% olivine, Fo 90 and ∼- 15% enstatitic orthopyroxene En 95 and minor clinopyroxene and spinel) at 300°C, 500 bars and water/rock mass ratio of 20. The duration of the experiment was approximately 1500 hr. Seawater chemistry was appreciably modified during the experiment. Mg, Ca, Sr, SO 4 and H 2 O were removed, while H 2 S (aq) , Fe, Mn and Zn were added. H 2 S (aq) resulted from the inorganic reduction of seawater SO 4 . pH was initially acid (2.8), but then rose slowly to a value of 5.2. The aqueous concentrations of Na, K, Cl and boron (B) changed little from that in seawater prior to reaction. However, as the solution was cooled to room temperature at the end of the experiment, the B concentration decreased. This suggests that the B content of oceanic serpentinites may be the result of retrograde reactions between a previously serpentinized body and ‘cold’ seawater. The primary minerals in the peridotite were replaced to varying degrees by serpentine (lizardite), magnetite. Mg-hydroxysulfate, anhydrite and possibly pyrite and sphalerite. Mg-hydroxysulfate and much anhydrite dissolved on quench. The alteration mineral assemblage generated during this experiment is consistent with that predicted from equilibrium phase relations and is similar in chemical composition, mineralogy and paragenesis to that reported for oceanic serpentinites.

Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges.

Abstract Recent advances in experimental and theoretical geochemistry have made it possible to assess both homogeneous and heterogeneous equilibria involving a wide range of aqueous species at temperatures and pressures appropriate to model hydrothermal alteration processes at mid-ocean ridges. We have combined selected aspects of the chemistry of hot spring fluids with constraints imposed by a geologically reasonable assemblage of minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2OrAl2O3-SiO2-H2O-HCl-H2S to assess the effect of temperature on the composition of the aqueous phase and the activities of mineral components in plagioclase and epidote solid solutions. Assuming ƒO 2(g) and ƒS 2(g) controlled by pyrite-pyrrhotite-magnetite equilibria, a constant dissolved Ca concentration, and a dissolved Cl concentration equivalent to that of seawater, increasing temperature from 250 to 400°C at 500 bars results in systematic changes in the composition of mineral phases, which in turn constrain pH and the distribution of aqueous species. The model predicts that dissolved concentrations of Fe, SiO2, K, H2S, and H2 increase, while Na and pH(25°c) decrease with increasing temperature. pH(in-situ) decreases slightly with increasing temperature, and has a value of 5.16 at 400°C. Dissolved Mg concentrations do not exceed 1 mmolal at any temperature investigated. Allowing for differences in pressure and total dissolved Cl, the predicted effect of temperature on fluid chemistry is in good agreement with results from basalt alteration and plagioclase + epidote + quartz recrystallization experiments, and in some cases, with results from hot spring fluids at mid-ocean ridges. Some vent fluids, in particular NGS (EPR, 21°N) and vent-4 (EPR, 11°N), however, reveal pH and/or dissolved Fe, H2S, H2, and SiO2 concentrations, which are difficult to reconcile with measured temperatures in comparison with results of temperature dependent mineral solubility calculations, and suggest that these fluids have lost heat by conduction on ascent to the seafloor. That many hot spring vent fluids are characterized by variable degrees of conductive heat loss renders measured temperatures unreliable as indicators of the maximum temperature of subseafloor hydrothermal alteration processes. The implications of this are significant for hot spring fluids which reveal large Cl variations relative to seawater, since likely mechanisms to account for such variability typically require temperatures in excess of those inferred for subseafloor reaction zones by simply correcting measured temperatures for the effects of adiabatic cooling.