Hydrothermal processes along mid-ocean ridges: an experimental investigation

An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200°C and 500°C at 500-800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na�, Mg��, SO₄[superscipt =], Cl� solution to a reducing, acidic, Na�, Ca��, Cl� solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid approach saturation values with respect to quartz at all temperatures; however, Al abundances remain very low (~0.5 ppm). Montmorillonite, nontronite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed. These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and be important in the genesis of certain Fe-Cu sulfide ore deposits. The process could also affect the geochemical budgets of certain elements and exert substantial control on the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si and H to the oceans

Elemental changes and alteration recorded by basaltic drill core samples recovered from in situ temperatures up to 345°C in the active, seawater-recharged Reykjanes geothermal system, Iceland

Hydrothermal activity results in element exchanges between seawater and oceanic crust that contribute to many aspects of ocean chemistry; therefore, improving knowledge of the associated chemical processes is of global significance. Hydrothermally altered basaltic drill core samples from the seawater-recharged Reykjanes geothermal system in Iceland record elemental gains and losses similar to those observed in samples of hydrothermally altered oceanic crust. At Reykjanes, rocks originally emplaced on the seafloor were buried by continued volcanism and subsided to the current depths (>2250 m below surface). These rocks integrate temperature-dependent elemental gains and losses from multiple stages of hydrothermal alteration that correspond to chemical exchanges observed in rocks from different crustal levels of submarine hydrothermal systems. Specifically, these lithologies have gained U, Mg, Zn, and Pb and have lost K, Rb, Ba, Cu, and light rare earth elements (La through Eu). Alteration and elemental gains and losses in lithologies emplaced on the seafloor can only be explained by a complex multistage hydrothermal alteration history. Reykjanes dolerite intrusions record alteration similar to that reported for the sheeted dike section of several examples of oceanic crust. Specifically, Reykjanes dolerites have lost K, Rb, Ba, and Pb, and gained Cu. The Reykjanes drill core samples provide a unique analog for seawater-oceanic crust reactions actively occurring at high temperatures (275–345°C) beneath a seafloor hydrothermal system