Chemical exchange during hydrothermal alteration of basalt by seawater—I. Experimental results for major and minor components of seawater

Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO₂, SO₄), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO₂, Ca, K, Ba, B and CO₂ were leached from basalt and enriched in solution. SiO₂ concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.     

Chemistry of springs across the Mariana forearc shows progressive devolatilization of the subducting plate

Cold springs upwelling through large serpentinite mud volcanoes in the outer half of the Mariana forearc provide a unique window into processes of devolatilization of the subducting Pacific Plate. We have sampled upwelling pore waters with lower chlorinity than seawater from six sites on five serpentinite mud volcanoes, by conventional gravity and piston coring, by push coring from the ROV Jason, by drilling on ODP Legs 125 and 195, and by manned submersible. The sites range from 13°47′N to 19°33′N and 52 to 90 km from the Mariana trench axis, corresponding to approximate depths to the top of the downgoing plate of 16 to 29 km. The composition of the springs varies systematically over this distance: nearer the trench the upwelling waters have much higher Ca and Sr than seawater and much lower carbonate alkalinity, sulfate, Na/Cl, K, Rb, and B. Farther from the trench the waters show the opposite trends relative to seawater. Chlorinity is consistently lower than in seawater and shows large variations that are not systematic with distance from the trench. Cs is consistently higher than in seawater and increases with distance from the trench. All of the waters have high pH and are heavily depleted in Mg, Si, Li, F, and ⁸⁷Sr/⁸⁶Sr relative to seawater. They tend to be enriched in O¹⁸/O¹⁶. Except for ODP drilling, none of the cores was long enough to produce an asymptotic compositional trend with depth. We have inferred the end-member compositions of the upwelling waters by extrapolation against Mg. At two sites we were able to compare data from gravity cores with data from drill cores or push cores collected at springs to estimate the effects of reactions that occur at shallow depth below the seafloor, on mixing of the upwelling waters with seawater. These effects are different for sites high in dissolved Ca, nearer the trench, vs. those high in alkalinity, farther from the trench. Common to both are large losses from solution of 1) Ca, as CaCO₃ and in exchange for Na; 2) Mg, in exchange for Na or Ca and as brucite; 3) sulfate, probably reduced by microbes or possibly precipitated as gypsum; 4) Sr, Ba, Si, and F. Na is consistently leached from the solids into solution, whereas K and O¹⁸/O¹⁶ are relatively unreactive.We infer that the upwelling waters are uniformly saturated with CaCO₃ and that the excess H₂O and the trends in Ca, Sr, alkalinity, and sulfate with distance from the trench result from introduction of H₂O and dissolved carbonate and sulfate from an external source, the sediment and altered basalt at the top of the subducting plate. The concurrent trends in Na/Cl, B, Cs, and especially K and Rb indicate that these species originate from the top of the subducting plate in response to increasing temperature. These systematic variations across the outer forearc imply that the solutions ascend more or less vertically from the source region and do not travel long distances laterally along the décollement before ascending. Based on leaching of K, the 150°C isotherm is crossed approximately beneath Big Blue Seamount at a depth of ∼22 km below the seafloor, 70 km behind the trench. By this point it appears that carbonate dissolution has joined dehydration as a significant process at the top of the subducting plate.     

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

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.     

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

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. $\text{Water}\text{rock}$ 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 $\text{water}\text{rock}$ 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 $\text{water}\text{rock}$ 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.     

Water and astrobiology

Water is formed from two of the three most abundant elements in the universe and so is abundant in interstellar space, in our Solar System, and on Earth, where it is an essential compound for the existence of life as we know it. Water ice acts as a substrate and reactant in interstellar clouds of gas and dust, enabling the formation of organic compounds that are important precursors to life and that eventually became incorporated into comets and asteroids in the early Solar System. Laboratory experiments have allowed us to infer the reaction pathways and mechanisms by which some of these compounds are formed. In these reactions, water can act as an energy transfer medium, increasing product yields, or it can lower yields by diluting reaction centers. Water can also destroy organic compounds when water ice decomposes under ionizing radiation and the decomposition products attack the compounds; whether this happens depends critically on temperature and structure of the ice, whether crystalline or amorphous. Ice structure and temperature also largely determine its gas content. As the solar nebula collapsed, icy mantles on interstellar grains probably sublimated and then recondensed onto other grains, thus influencing the transport of energy, mass, and angular momentum in the disk. Icy grains also influenced the temperature structure of the disk because they influence mean disk opacity. Outside the “snow line” at 3–5 AU icy grains accreted to become part of comets and planetesimals that occupy the region of the outer planets, the Kuiper belt, and the Oort cloud. Water was acquired by the growing Earth by several mechanisms. Evidence from noble gas isotopes indicates that Earth achieved sufficient mass fast enough to capture an early H-rich atmosphere from the Solar nebula itself. Although the remnant of this primary atmosphere is now found only in the mantle, it may also reside in the core, which could contain most of the H on Earth (or none at all). The bulk silicate Earth contains only 500–1100 ppm H₂O, an amount small enough to explain by “wet” accretion, although most of it probably accumulated with the latter half of Earth's mass from wetter planetary embryos originating beyond 1.5 AU. Degassing on impact delivered water to Earth's surface, where it dissolved into a magma ocean, a process that likely saved it from loss during subsequent catastrophic impacts such as the Moon-forming giant impact, which resulted in >99% loss of the noble gas inventory. Although most of Earth's water probably came from meteoritic material, the depletion on Earth of Xe relative to Kr strongly suggests a role for comets. The role of water in supporting life is an essential one on Earth and probably elsewhere, given the unusual properties of water compared with other potentially abundant compounds. Its dipolarity, high boiling point and heat of vaporization and, for ice, melting temperature; its expansion on freezing; and its solvent properties make it an ideal medium for life. Life originated early on Earth, indicating an abundance of water, nutrients, precursor molecules, substrates, and appropriate physical and chemical conditions. Life adapted quickly to (and may have originated in) extreme environments, of heat, cold, dryness, saltiness, and acidity. This adaptation to extreme conditions bodes well for the prospect of finding life elsewhere in our Solar System and in planetary systems around other stars.     

Chemistry of hot springs along the Eastern Lau Spreading Center

The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ⁷Li increase southward, and δ¹¹B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H₂S, Li, Na/Cl, Fe, Mn, and ⁸⁷Sr/⁸⁶Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H₂S, CO₂, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.