FY 1994

Geochemistry of north Cleft segment vent fluids: Temporal changes in chlorinity and their possible relation to recent volcanism

Butterfield, D.A., and G.J. Massoth

J. Geophys. Res., 99(B3), 4951–4968 (1994)

Hydrothermal vent fluids from the North Cleft segment of the Juan de Fuca Ridge between 44°54 and 45°00 N were sampled in 1988, 1990, 1991, and 1992. In 1988, chloride-depleted (relative to seawater) diffuse fluids with low dissolved metal concentrations (relative to other mid-ocean ridge (MOR) fluids) were sampled over a distance of ~10 km along axis. In 1990 and 1991, both high-temperature (>200°C) and diffuse vents were consistently metal- and chloride-enriched. The end-member compositions of high-temperature vents and nearby diffuse vents were very similar. There were small but significant correlated changes in diffuse and high-temperature composition from 1990 to 1991. The large-scale but temporary venting of low-chlorinity fluids over the entire North Cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride-enriched fluids. It is possible that a volcanic event along the North Cleft segment prior to 1987, for which there is firm geologic evidence, initially caused a boiling event which resulted in the preferential venting of vapor-enriched fluids through 1988, followed by a transition to brine-enriched fluids by 1990. High iron, low sodium, and low Sr/Ca ratios in the high-chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event. The absence of chloride-depleted fluids from 1990 onward, and the systematics of lithium, boron, and manganese with chloride in the high-temperature fluids from North Cleft suggest that the evolution toward lower chlorinity at Monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater.