Fabrice J. Fontaine

Hydrothermal circulation at slow-spreading mid-ocean ridges: The role of along-axis variations in axial lithospheric thickness

At several ridge segments along the slow-spreading Mid-Atlantic Ridge, the lithosphere appears to be cooled by centrally located, isolated hydrothermal fields, hundreds of meters wide, extracting as much as 1000 MW from the lithosphere and hosting very large (>106 m3) sulfide edifices. These fields are possibly fueled by subseafloor hydrothermal cells cooling and leaching the lithosphere up to a few tens of kilometers along axis. However, the detailed mechanisms by which such hydrothermal heat extraction takes place are not well constrained. It is postulated that melt focusing and preferred cooling near transforms result in a thinner lithosphere at the center of slow-spreading ridge segments. In this configuration, and with a depth of penetration controlled by brittle lithospheric thickness, the base of the hydrothermal system is not at constant depth. Here we present models of along-axis hydrothermal circulation showing that pressure gradients generated along this basal slope influence flow dynamics. We show that the size of hydrothermal cells increases with the basal slope α. For α 15°–20°, the circulation reaches steady state and is composed of a single cell with a broad recharge and a focused discharge. Although our models make several simplifying assumptions, we propose that along-axis variations in lithosphere thickness associated with the magmatic and tectonic segmentation of slow-spreading ridges should favor the formation of large and centrally located vent fields, mining heat on several kilometers along axis. We also predict that more short-lived and weaker vent fields may develop away from the segment center.

Along-axis hydrothermal flow at the axis of slow spreading Mid-Ocean Ridges: Insights from numerical models of the Lucky Strike vent field (MAR)

The processes and efficiency of hydrothermal heat extraction along the axis of mid-ocean ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high-temperature mid-ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid-Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma-rich slow spreading environment. Using three-dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across-axis permeability variations. Models with across-axis permeability gradients produce along-axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across-axis permeability gradients are also likely to occur at faster spreading ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.

Can high-temperature, high-heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast-spreading Mid-Ocean Ridges?

We present numerical models to explore possible couplings along the axis of fast-spreading ridges, between hydrothermal convection in the upper crust and magmatic flow in the lower crust. In an end-member category of models corresponding to effective viscosities μM lower than 1013 Pa.s in a melt-rich lower crustal along-axis corridor and permeability k not exceeding ∼10−16 m2 in the upper crust, the hot, melt-rich, gabbroic lower crust convects as a viscous fluid, with convection rolls parallel to the ridge axis. In these models, we show that the magmatic-hydrothermal interface settles at realistic depths for fast ridges, i.e., 1–2 km below seafloor. Convection cells in both horizons are strongly coupled and kilometer-wide hydrothermal upflows/plumes, spaced by 8–10 km, arise on top of the magmatic upflows. Such magmatic-hydrothermal convective couplings may explain the distribution of vent fields along the East (EPR) and South-East Pacific Rise (SEPR). The lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt-rich regions/pockets in the axial melt lenses of EPR and SEPR. Crystallization of this melt provides the necessary latent heat to sustain permanent ∼100 MW vents fields. Our models also contribute to current discussions on how the lower crust forms at fast ridges: they provide a possible mechanism for focused transport of melt-rich crystal mushes from moho level to the axial melt lens where they further crystallize, feed eruptions, and are transported both along and off-axis to produce the lower crust.