Characteristics of magma‐driven hydrothermal systems at oceanic spreading centers

Abstract

[1] We use one-and two-limb single-pass models to de termine vent field characteristics such as mass flow rate Q, bulk permeability in the discharge zone k d , thickness of the conductive boundary layer at the base of the system d, magma replenishment rate, and residence time in the discharge zone. Data on vent temperature, vent field area, heat output, and the surface area and depth of the subaxial magma chamber (AMC) constrain the models. The results give Q~100 kg/s, k d~1 0 À13 m 2 , and d~10 m, essentially independent of spreading rate, and detailed characteristics of the AMC. In addition, we find no correlation between heat output at individual vent fields and spreading rate or depth to the AMC. We conclude that high-temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of AMC depth. Using data on partitioning of heat flow between focused and diffuse flow, we find that 80-90% of the hydrothermal heat output is derived from high-temperature fluid, even though much of the heat output discharges as low-temperature fluid. In some cases, diffuse flow fluids may exhibit considerable conductive cooling or heating. By assuming conservative mixing of diffuse flow fluids at East Pacific Rise 9 50 0 N, we find that most transport of metals such as Fe and Mn occurs in diffuse flow and that CO 2 , H 2 , and CH 4 are taken up by microbial activity.

Thermal response of mid-ocean ridge hydrothermal systems to perturbations

“…In Ramondenc et al (2008), however, the mass flow rate for EPR 9150 0 N was assumed to be between 14 and 26 kg/s, which is much lower than the value of $ 80 kg/s estimated by Germanovich et al (2011) and Lowell et al (2013). These values resulted in a lower heat flux and permeability than estimated for EPR.…”

“…Because the events were considered to be thermal cracking events taking place at the magma-hydrothermal boundary layer Sohn et al, 1998), this would imply that fluids travel several thousand meters in the crust in a few days or hours. Germanovich et al (2011) and Lowell et al (2013) show, however, that a typical mass flow rate for high temperature hydrothermal systems of $ 100 kg/s would yield a hydrothermal fluid speed $10 À 5 -10 À 6 m/s over the area of the hydrothermal discharge zone. To traverse $1000 m at these speeds would require several weeks, if not months, even if the speed is increased by an order of magnitude as a result of the event.…”

“…He noted that such permeability value was considerably less than typically thought for high temperature hydrothermal systems (e.g., Germanovich, 1994, 2004;Wilcock and McNabb, 1996) and hence, favored the thermal perturbation mechanism. The analysis of Wilcock (2004) was done before heat flow data were available to constrain the flow rate, permeability, and other parameters related to the EPR hydrothermal system (Ramondenc et al, 2006;Lowell et al, 2013).…”

A preliminary 1‐D model investigation of tidal variations of temperature and chlorinity at the Grotto mound, Endeavour Segment, Juan de Fuca Ridge

“…where w is the interstitial upflow velocity, Q5292 kg/s is the estimated mass flux within the discharge zone beneath the MEF [Lowell et al, 2013], q v 5646 kg=m 3 is the vapor density, A is the area of the horizontal cross section of the upper discharge zone in layer 2A, which is assumed to equal the area of the vent field: 6310 4 m 2 [Lowell et al, 2013], and /50:2 is the crustal porosity of layer 2A [Crone and Wilcock, 2005]. The result: w53:8310 25 m/s suggests that it will take approximately 154 days for the vapor to rise through the 500 m thick layer 2A.…”

“…where Q5292 kg/s is the estimated mass flux within the discharge zone beneath the MEF [Lowell et al, 2013], q v 5646 kg=m 3 is vapor density, and A56310 4 m 2 is the area of horizontal cross section of discharge zone, which is assumed to equal the area of the vent field. In the meantime, the chlorinity signals are advected at the speed of interstitial flows, which is related to w T as…”