A Si‐Cl geothermobarometer for the reaction zone of high‐temperature, basaltic‐hosted mid‐ocean ridge hydrothermal systems

Abstract

[1] The chemical composition of mid-ocean ridge hydrothermal vent fluids is thought to reflect conditions within a deep-seated reaction zone. Although temperature and pressure conditions within this region are key parameters that characterize the subseafloor hydrothermal regime and the cooling of mid-ocean ridges, they are poorly constrained. In this paper, we developed a model in which high-temperature, vapor-type (low-salinity) vent fluid silica (Si) and chlorine (Cl) concentrations can be used to define lines in pressuretemperature space whose intersection is used to estimate conditions at the top of the reaction zone, under the simplifying assumption that Si and Cl reflect a common point of equilibration. We apply this model to various basaltic-hosted mid-ocean ridge sites. Results suggest a minimal variation in inferred temperatures, ranging from 415 to 445°C. This lends support to the fluxibility model in which upwelling hydrothermal plumes rise at temperatures that maximize the energy flux. Quartz precipitation due to reequilibration during upflow tends to lower temperature and pressure estimates and can artificially indicate shallower transition from reaction to upflow zone. However, maximum equilibration pressures are site-dependent and compare well with depth to magma chamber imaged by seismic studies. This suggests that vapors circulate close to magma chambers and is difficult to reconcile with models in which mid-ocean ridge hydrothermal circulation occurs in two layers with a substantial layer of convecting brine. Accordingly, equilibration pressure predicted by our model can also be used to infer the depth of the magma chamber at sites where seismic data are not available but where vapor-like fluids have been collected and analyzed.Components: 5327 words, 5 figures, 1 table.Keywords: hydrothermal vents; mid-ocean ridges; reaction zone; silica; chlorine; AMC depth.

Fe isotope fractionation during phase separation in the NaCl–H2O system: An experimental study with implications for seafloor hydrothermal vents

“…1, green squares). Region 2 is broadly consistent with P-T conditions associated with basalt-hosted hydrothermal systems when impacted by subseafloor magmatic intrusions (Fontaine et al, 2009;Fornari et al, 2012;Pester et al, 2014).…”

A new Fe/Mn geothermometer for hydrothermal systems: Implications for high-salinity fluids at 13°N on the East Pacific Rise

“…Where available, these efforts can be aided by geophysical data, which provide constraints on maximum depth of circulation if an axial magma chamber (AMC) is observed. For example, Fontaine et al (2009) demonstrated good agreement between AMC depth and maximum corresponding hydrostatic pressures derived using Si-Cl systematics. Deep crustal fluids, when encountering the given heat source that ultimately drives hydrothermal circulation, routinely intersect the two-phase boundary (Bischoff and Rosenbauer, 1984).…”

Subseafloor phase equilibria in high-temperature hydrothermal fluids of the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17′N)

“…Dash-dot line represents the specific two-phase boundary of seawater (3.2 wt% NaCl) where deviation from the critical curve (solid line) is observed and the star is the critical point of seawater (Bischoff and Rosenbauer, 1988). The box denoted "Si-Cl" represents the pressure-temperature range of reaction derived for Lucky Strike by Fontaine et al (2009) using previously reported data, assuming quartz saturation was achieved in/near the two-phase region. While the fluids may have phase separated at temperatures >$430°C, Fe/Mn have equilibrated at lower temperatures (350-385°C).…”

“…The Fe/Mn temperatures can be used in conjunction with dissolved Si, Cl and geophysical data to constrain possible subseafloor reaction conditions in pressure-temperature space (Bischoff and Rosenbauer, 1985;Foustoukos and Seyfried, 2007c;Fontaine et al, 2009;Pester et al, 2011). The extent of chloride variability in Lucky Strike fluids is sufficiently small that phase separation is likely occurring at conditions that do not significantly deviate from the critical curve in the NaCl-H 2 O system.…”

“…6). While the physical process of phase separation can be decoupled from fluid-mineral equilibria, it is often assumed that quartz saturation is attained at pressure/temperature conditions equivalent to those that induce phase separation (Foustoukos and Seyfried, 2007c;Fontaine et al, 2009). This suggests temperature maxima of $430-475°C and associated pressures between the observed depth of fault penetration and the top of the AMC at Lucky Strike ($410-480 bars).…”