Chemistry of hydrothermal vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: Geochemical controls in the aftermath of June 1999 seismic events

Abstract

[1] In June 1999, an intense swarm of earthquakes occurred on the Endeavour segment of the Juan de Fuca Ridge influencing hydrothermal activity in and around the Main Endeavour Field (MEF). Here we report the dissolved concentrations of 31 species from five high-temperature vents sampled 3 months after the seismic event. The spatial variability of vent fluid chemistry is extreme. Vapor-dominated vent fluids at Cantilever and Sully sites have high measured temperatures (375°-379°C), high dissolved gas and boron concentrations, but low SiO 2 . Modeling results indicate that these fluids can be accounted for by supercritical phase separation and brine condensation. Other vent fluids have moderate temperatures (340°-366°C) and chloride concentrations (208-426 mmol/kg), and may result from mixing of supercritical, vapor-rich fluids with evolved seawater. Phase equilibria calculations indicate that in addition to chloride, redox, temperature, and especially pressure play key roles in accounting for compositional variability of vent fluids at MEF. In comparison with earlier (1988) data, the 1999 data set reveals significantly lower chloride concentrations and higher boron, whereas alkali and alkaline earth cations are lower by 10-20% in keeping with chloride decrease. That dissolved chloride, boron, and other elements returned to preevent levels when again sampled in 2000 provide additional data documenting the inherently dynamic nature of hydrothermal systems at mid-ocean ridges.

Chemistry of hot springs along the Eastern Lau Spreading Center

“…Enrichments and depletions of Cl relative to seawater in mid-ocean ridge vent fluids are typically attributed to phase separation (Butterfield et al, 1994, Von Damm et al, 1997Seewald et al 2003;Seyfried et al, 2003). Maximum measured temperatures at the Mariner vent field (363C) are very close to the two-phase boundary for seawater at the seafloor depth of 1920 m (365C; Bischoff and Rosenbauer, 1985), consistent with on-going phase separation at this location.…”

“…These characteristics are similar to those that prevailed at Vai Lili in 1989 . Although differences in fluid composition are expected at Mariner due to higher temperatures of water-rock reaction and active phase separation at and below the seafloor, the extreme levels of acidity and associated metal mobility are not observed in basalt-hosted mid-ocean ridge hydrothermal systems at similar or higher temperatures that are also experiencing active phase separation (Butterfield et al, 1994;Von Damm et al, 1997;Seyfried et al, 2003). Because the substrate at Mariner is similar to that at Tui Malila (except for nearby dacites at Mariner), the substantial difference in fluid composition suggests a fundamental shift in geochemical processes regulating fluid composition.…”

Subsurface hydrothermal processes and the bioenergetics of chemolithoautotrophy at the shallow-sea vents off Panarea Island (Italy)

“…Recent evidence suggests that supercritical fluids occurring near a magma chamber can ‗compact' without crossing the two-phase solvus (i.e., do not boil), thus creating very high salinity fluids crucial for development of high sulfide epithermal mineral deposits (Hedenquist and Lowenstern, 1994;Heinrich et al, 2004). However, subcritical phase separation below the critical point of seawater occurs as a result of decompression during upflow, wherever the 2-phase boundary is intercepted, producing ~3.2 wt% NaCl liquid coexisting with a low-salinity vapor (Berndt et al, 2001;Foustoukos and Seyfried, 2007;Seyfried et al, 2003). This process is essentially boiling, creating a gas-rich vapor phase and residual brine.…”

Sulfur and lead isotopic compositions of massive sulfides from deep-sea hydrothermal systems: Implications for ore genesis and fluid circulation

“…Another possible explanation is changing Pb isotope compositions in the hydrothermal fluids. In general, the changes in fluid paths caused by a magmatic/diking event may lead to changes in the chemistry of hydrothermal fluid (Rubin et al, 1994;Von Damm et al, 1995;Fornari et al, 1998;Seyfried et al, 2003).…”