Rapid variations in fluid chemistry constrain hydrothermal phase separation at the Main Endeavour Field

Abstract

Previous work at the Main Endeavour Field (MEF) has shown that chloride concentration in high‐temperature vent fluids has not exceeded 510 mmol/kg (94% of seawater), which is consistent with brine condensation and loss at depth, followed by upward flow of a vapor phase toward the seafloor. Magmatic and seismic events have been shown to affect fluid temperature and composition and these effects help narrow the possibilities for sub‐surface processes. However, chloride‐temperature data alone are insufficient to determine details of phase separation in the upflow zone. Here we use variation in chloride and gas content in a set of fluid samples collected over several days from one sulfide chimney structure in the MEF to constrain processes of mixing and phase separation. The combination of gas (primarily magmatic CO2 and seawater‐derived Ar) and chloride data, indicate that neither variation in the amount of brine lost, nor mixing of the vapor phase produced at depth with variable quantities of (i) brine or (ii) altered gas rich seawater that has not undergone phase separation, can explain the co‐variation of gas and chloride content. The gas‐chloride data require additional phase separation of the ascending vapor‐like fluid. Mixing and gas partitioning calculations show that near‐critical temperature and pressure conditions can produce the fluid compositions observed at Sully vent as a vapor‐liquid conjugate pair or as vapor‐liquid pair with some remixing, and that the gas partition coefficients implied agree with theoretically predicted values.

A Long‐Term Earthquake Catalog for the Endeavour Segment: Constraints on the Extensional Cycle and Evidence for Hydrothermal Venting Supported by Propagating Rifts

“…The estimated heat flux for the entirety of the active Endeavour hydrothermal fields is ∼800 MW with a mean temperature of ∼300°C (Kellogg, 2011). The concentration of CO 2 within the vent fluids for periods not impacted by dike emplacements is on the order of 15–20 mmol/kg, with 80% of the output believed to be sourced from melt (Love et al., 2017; Proskurowski et al., 2004). Taking a change in enthalpy of 1,600 kJ kg −1 between 0 and 350°C at 220 bars for a 3.2 wt.% NaCl solution (Anderko & Pitzer, 1993), we estimate that the VFs emit 10,000–14,000 tons of melt‐derived CO 2 per year.…”

Diversity of magmatism, hydrothermal processes and microbial interactions at mid-ocean ridges

“…Pronounced changes in seismic activity, permeability, heat flux, temperature and vent fluid chemistry result from intermittent dyking and volcanic eruptive or tectonic events at MORs 40,86,108,110 . Large increases in CO 2 could precede volcanic events, as documented at the Main Endeavour Field following an earthquake swarm in 1999 40,111,112 . Lava interacting with hydrothermal circulation cells produces extreme changes in water-rock reactions and the nature and degree of phase separation.…”

Canada’s Internet-Connected Ocean

“…In these same settings, Coogan et al (2017) revealed insights about near-vent chemical processes in plume while Jackson et al (2017) described diffuse hydrothermal flow using sound. Love et al (2017) showed that there were variations in fluid chemistry that constrained hydrothermal vent phase separation. In 2017, Xu published three studies with co-authors that show the impact of tides on hydrothermal vents, further advancing the quantification of hydrothermal venting, and advanced the techniques in using acoustic imaging to study venting (Xu et al, 2017a,b,c).…”