Abstract
AbstractIn 2016, temperature recorders were recovered, temperatures were measured, and fluid samples were collected from Vent 1, a high temperature (338°C) hydrothermal discharge site on the southern Cleft Segment of the Juan de Fuca Ridge. Coupled with previous sampling efforts, this collection represents a 32‐year record of discharge from a single chimney structure, the longest record to date. Remarkably, the fluid has remained brine‐dominated for more than three decades. This brine formed during phase separation and segregation prior to initial observations in 1984. Although the chloride concentration of the discharging fluid has decreased with time, the fluid temperature has remained nearly constant for at least 3.3 years and probably for 15 or even 22 years. Compositions of the discharging fluids are consistent with inputs from a deep‐sourced brine, which was last equilibrated at >400°C at a depth consistent with the base of the sheeted dikes and the brittle‐ductile transition. This brine mixed (diffusion or dispersion) with a likely non‐phase‐separated, hydrothermal fluid prior to discharge. A survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol/kg shows, with the exception of Fe, a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions, spreading rates, and water and magma depths. Calculated deep‐sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas. A better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals.
Highlights
New oceanic crust forms with the intrusion of magma, which is cooled by seawater and drives convective circulation beneath the seafloor
In 2016, temperature recorders were recovered, temperatures were measured, and fluid samples were collected from Vent 1, a high temperature (338°C) hydrothermal discharge site on the southern Cleft Segment of the Juan de Fuca Ridge
The initial work conducted on southern Cleft Segment concluded that the high chloride concentration in the high-temperature fluid that discharged from the Plume site was, in part, a result of subsurface phase separation and segregation (Von Damm & Bischoff, 1987)
Summary
New oceanic crust forms with the intrusion of magma, which is cooled by seawater and drives convective circulation beneath the seafloor Such circulation allows for the exchange of heat and elements prior to discharging hydrothermal fluids at the seafloor. These hydrothermal fluids impact ocean biogeochemistry and support primary production for one of the Earth's biomes (e.g., Humphris & Klein, 2018; Mullineaux et al, 2018). Intruded magma provides a thermal boundary ( 1,200°C) at depth, and depending on the path of fluid circulation, circulating seawater can be heated sufficiently to result in phase separation, forming coexisting vapor and brine (or liquid) phases (Bischoff & Rosenbauer, 1987; Delaney et al, 1987; Von Damm & Bischoff, 1987). Segregation of these two phases has been observed on a range of scales from a single structure (Von Damm et al, 2003) to adjacent structures (Massoth et al, 1989) and ridge segments (Butterfield & Massoth, 1994)
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