Michael J. Mottl

Geomicrobiology of Deep-Sea Hydrothermal Vents

During the cycling of seawater through the earths crust along the mid-ocean ridge system, geothermal energy is transferred into chemical energy in the form of reduced inorganic compounds. These compounds are derived from the reaction of seawater with crustal rocks at high temperatures and are emitted from warm (≤25�C) and hot (∼350�C) submarine vents at depths of 2000 to 3000 meters. Chemolithotrophic bacteria use these reduced chemical species as sources of energy for the reduction of carbon dioxide (assimilation) to organic carbon. These bacteria form the base of the food chain, which permits copious populations of certain specifically adapted invertebrates to grow in the immediate vicinity of the vents. Such highly prolific, although narrowly localized, deep-sea communities are thus maintained primarily by terrestrial rather than by solar energy. Reduced sulfur compounds appear to represent the major electron donors for aerobic microbial metabolism, but methane-, hydrogen-, iron-, and manganese-oxidizing bacteria have also been found. Methanogenic, sulfur-respiring, and extremely thermophilic isolates carry out anaerobic chemosynthesis. Bacteria grow most abundantly in the shallow crust where upwelling hot, reducing hydrothermal fluid mixes with downwelling cold, oxygenated seawater. The predominant production of biomass, however, is the result of symbiotic associations between chemolithotrophic bacteria and certain invertebrates, which have also been found as fossils in Cretaceous sulfide ores of ophiolite deposits.

Metabasalts, axial hot springs, and the structure of hydrothermal systems at mid-ocean ridges

Knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid-ocean ridge. Studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the Mid-Atlantic Ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy, mineral abundances, and mineral compositions, especially for chlorite. These relationships can be interpreted in terms of the distribution of alteration with respect to time, temperature, and water/rock ratio in young crust. Assemblages of chl-ab-ep-act, chl-ab-ep-act-qtz, chl-ab-qtz, and chl-qtz are produced at successively higher effective sea-water to rock ratios within the temperature range of the greenschist facies (−250 to 450 °C). Toward higher ratios, chlorites tend to become more Mg-rich. Pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending, rather than the ascending, limb of a convection system; this type of alteration, which reflects moderately high water/rock ratios, may characteristically occur above a still partially molten magma chamber. As the magma chamber solidifies and is then penetrated, more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water/rock ratios, producing solutions such as those recovered from springs on the Galapagos Rift and the East Pacific Rise at 21°N. Localized upwelling limbs of the convection system can produce veins filled with quartz, sulfides, and Fe-rich chlorite.

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.

Hydrothermal circulation through mid-ocean ridge flanks: Fluxes of heat and magnesium☆

Thermally driven convection of seawater occurs through oceanic crust of all ages, at the seafloor spreading axis, on mid-ocean ridge flanks, and in the ocean basins. At the ridge axis and on the flanks, circulating seawater produces a major chemical exchange between the oceans and the crust. Based on heat budget constraints and the composition of hot springs, between 10 and 40% of the river flux of Mg can be taken up during high-temperature alteration of the basaltic crust along the ridge axis. Most of the hydrothermal heat loss, however, occurs on the mid-ocean ridge flanks, where the temperatures are lower and the seawater flux correspondingly larger. The estimated heat loss on the flanks is so large that upwelling must occur over a large fraction (5–30%) of the seafloor less than 65 Ma in age, if temperatures are < 20°C and seepage velocities are on the order of 10 to 100 cm/y. The circulating seawater needs to lose on average less than 1–2% of its Mg content in order to solve the Mg mass balance for the oceans. Chemical fluxes through mid-ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date. The most critical parameter for characterizing crustal conditions is temperature in basement, which is a function of crustal age, basement topography, and sediment thickness and permeability. Basement temperature, which can usually be inferred from heat flow and seismic reflection surveys, largely determines the change in the composition of seawater circulating through basement. This change can be inferred, in turn, from profiles of sediment porewater chemistry. All sites studied to date with temperatures at the sediment-basement interface ≤ 25°C have a large component of advective heat loss and show only a small ( 80%) Mg loss. Both types of sites may be important for chemical fluxes. Whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust; only small changes are needed. Whether the warmer sites are important depends on how much heat is lost by advection in this type of setting. The warmer sites could produce significant chemical fluxes even if they are scarce: they could account for the entire river input of Mg to the oceans even if they represent only 8–20% of the total advective heat loss on ridge flanks.

Gradients in the composition of hydrothermal fluids from the Endeavour segment vent field: Phase separation and brine loss

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47°57′N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on ∼10 large sulfide structures, which are localized along faults and fault intersections across the vent field. Each sulfide structure has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given structure. However, there are large gradients in fluid composition across the vent field, with endmember chlorinity increasing from ∼255 mmol/kg in the SW to 505 mmol/kg in the NE. End-member concentrations of major elements are well correlated with chlorinity, and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids. The gradients in composition across the vent field and measured vent fluid temperatures >400°C are consistent with supercritical phase separation and loss of brine phase below the seafloor. The factor-of-2 variation in CO2 (and H2S) is larger than expected for loss of a very high-chlorinity brine. Concentrations of iron and manganese are not positively correlated with chlorinity, suggesting that temperature and pH are more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter, and high boron concentrations point to a sedimentary source.

Chemical exchange during hydrothermal alteration of basalt by seawater—II. Experimental results for Fe, Mn, and sulfur species

Abstract Fresh mid-ocean ridge basalts of varying crystallinity and an andesite were reacted with seawater and with a Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water rock mass ratios of one to three were used and durations ranged from two to twenty months. The concentrations of Fe, Mn, and reduced and oxidized sulfur species in solution reached steady state in most of the experiments at 400–500°C, but not in those at 200–300°. The concentrations of Fe and Mn were a few ppm at 200–300° and increased greatly with temperature between 300 and 500°. The low values at 200–300° are probably related to the uptake of Fe and Mn by smectite at the in situ pH, which was slightly acid at 200° and slightly alkaline at 300°. The quench pH values decreased with increasing temperature above 300°. The only reliable data for the concentration of Zn in solution were obtained at 400°, where values 1–2 ppm were found. Copper was extensively leached from basalt and andesite and was deposited as part of a Cu-Au alloy in the capsule walls or, in some experiments, as chalcopyrite. Reduced sulfur was readily leached from basalt into solution, and was also produced by the reduction of seawater sulfate by ferrous iron derived from the basalts. The proportion of seawater sulfate which was reduced in the experiments with a water rock ratio of one varied from 5–10% at 300°C to > 95% at 500°. The rate of sulfate reduction depended on the run temperature, on the crystallinity and initial sulfur content of the rocks used as starting materials, and on the water rock ratio. The final concentration of reduced sulfur in solution increased greatly with temperature, and generally exceeded that of Fe on a molal basis. The oxide-sulfide assemblages produced in the experiments resemble those in the basalt-seawater geothermal system at Reykjanes, Iceland, and in hydrothermally altered basalts and gabbros from the oceanic crust; they include pyrite, pyrrhotite. chalcopyrite, hematite, and probably magnetite. The particular assemblage varied systematically with the temperature, rock type, and crystallinity of each run. Anhydrite precipitated in all experiments with seawater, at all temperatures from 200–500°C. However, its persistence to the end of the runs was apparently metastable, as it should have reacted with the final solutions to produce pyrite or pyrrhotite.

Composition of pore and spring waters from Baby Bare: global implications of geochemical fluxes from a ridge flank hydrothermal system

Warm hydrothermal springs were discovered on Baby Bare, which is an isolated basement outcrop on 3.5 Ma-old crust on the eastern flank of the Juan de Fuca Ridge. We have sampled these spring waters from a manned submersible, along with associated sediment pore waters from 48 gravity and piston cores. Systematic variations in the chemical composition of these waters indicate that hydrothermal reactions in basement at moderate temperatures (63°C in uppermost basement at this site) remove Na, K, Li, Rb, Mg, TCO2, alkalinity, and phosphate from the circulating seawater and leach Ca, Sr, Si, B, and Mn from the oceanic crust; and that reactions with the turbidite sediment surrounding Baby Bare remove Na, Li, Mg, Ca, Sr, and sulfate from the pore water while producing ammonium and Si and both producing and consuming phosphate, nitrate, alkalinity, Mn, and Fe. K, Rb, and B are relatively unreactive in the sediment column. These data confirm the earlier inference that sediment pore waters from areas of upwelling can be used to estimate the composition of altered seawater in the underlying basement, even for those elements that are reactive in the sediment column or are affected by sampling artifacts. The composition of altered seawater in basement at Baby Bare is similar to the inferred composition of 58°C formation water from crust nearly twice as old (5.9 Ma) on the southern flank of the Costa Rica Rift. The Baby Bare fluids also exhibit the same directions of net elemental transfer between basalt and seawater as solutions produced in laboratory experiments at a similar temperature, and complement compositional changes from seawater observed in seafloor basalts altered at cool to moderate temperatures. The common parameter among the two ridge flanks and experiments is temperature, suggesting that the residence time of seawater in basement at the two ridge-flank sites is sufficiently long for the solutions to equilibrate with altered basalt. This conclusion is supported by compilations of data from other ridge-flank sites, which show a systematic relationship between inferred basement water composition and temperature. We use the Baby Bare spring waters along with constraints from the riverine flux of Mg to estimate upper limits on the global fluxes of 14 elements at warm ridge-flank sites such as Baby Bare. Maximum calculated fluxes of Mg, Ca, sulfate, B, and K may equal or exceed 25% of the riverine flux, and such sites may represent the missing, high K/Rb sink required for the K budget. Additional fluxes from/to the crust on ridge flanks are expected from cool (<20–25°C) hydrothermal sites.

Mariana blueschist mud volcanism: Implications for conditions within the subduction zone

Several recently discovered active mud volcanoes on the nonaccretionary Mariana convergent plate margin are erupting slab-derived fluids, serpentine mud, and metamorphosed rocks from depths of as great as 25 km. Blueschist materials from the metamorphosed subducted plate are contained in the muds. Pore fluids indicate a depth dependence for decarbonation. The mud volcanoes record in situ conditions along the decollement, including pressure and temperature conditions and physical properties within the subduction zone. Similar mud-flow material occurs worldwide as “sedimentary serpentinite” deposits in accreted fragments of former convergent margins, making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized.

Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge

We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin . Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO 2 , sulfate, K, Li, U, O 2 , nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H 2 S, H 2 , CH 4 , 222 Rn, and 226 Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4–13 L/s and 2–3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes.