Heat flow variations on a slowly accreting ridge: Constraints on the hydrothermal and conductive cooling for the Lucky Strike segment (Mid‐Atlantic Ridge, 37°N)

Abstract

[1] We report 157 closely spaced heat flow measurements along the Lucky Strike segment in the MidAtlantic Ridge (MAR) for ages of the ocean floor between 0 and 11 Ma. On the eastern flank of a volcanic plateau delimiting off-axis and axial domains, the magnitude of heat flow either conforms to the predictions of conductive lithospheric cooling models or is affected by localized anomalies. On the western flank it is uniformly lower than conductive model predictions. We interpret the observed patterns of heat flow by lateral fluid circulation in a highly permeable oceanic basement. The circulation geometries are probably 3-D rather than 2-D and are determined by the configuration of the basement/sediment interface and the distribution of effectively unsedimented seamounts where water recharge can occur. Two major hydrothermal circulation systems can possibly explain the observations off-axis: the first would involve lateral pore water flow from west to east, and the second would have a reverse flow direction. The wavelengths and magnitudes of heat flow anomalies require Darcy velocities of the order of 1-4 m/year, which are similar to those proposed for fast-accreted crust elsewhere. However, a large proportion of this MAR domain remains unaffected by hydrothermal cooling, which is a relatively unusual observation but confirms the validity of conductive thermal models for seafloor ages between 5 and 10 Ma. Closer to the ridge axis (<5 Myr old crust), water circulation affects the overall axial domain, as larger proportions of basement are exposed. As much as 80-90% of the heat flux from the axial domain may be transferred to the Lucky Strike vent field, in agreement with the estimated discharge.

Crustal velocity structure of the Lucky Strike segment of the Mid‐Atlantic Ridge at 37°N from seismic refraction measurements

“…These decreased resistivities are likely related to an increase in porosity and permeability, which might play an important role in the hydrothermal fluid flow. The vents provide a significant contribution to the near‐axis heat flux [ Jean‐Baptiste et al , 1998], and the observed heat flow measurements are consistent with fluid flow from the ridge flank toward the ridge axis [ Lucazeau et al , 2006]. The magnetic anomaly observed at the Lucky Strike segment is axis parallel, and a low magnetization anomaly underneath the Lucky Strike hydrothermal field can be interpreted as demagnetization of the rocks by focalized hydrothermal flow [ Miranda et al , 2005].…”

Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

“…Maps of basement relief and sediment thickness in this area show numerous shallowly buried basement highs (Hutnak et al, 2006;Zühlsdorff et al, 2005); basement in many of these areas would have been exposed at the seafloor prior to the last several hundred thousand years of rapid sedimentation, and areas of current basement exposure (e.g., Baby Bare outcrop) would have been larger and better connected to the ocean (Hutnak & Fisher, 2007). Larger areas of basement exposure, and the greater spatial distribution of these areas, would have been permitted more efficient regional advective heat loss, as is currently seen at the western end of the Leg 168 transect (Davis, Chapman, et al, 1992;Hutnak et al, 2006), where measured heat flow is ∼20% of lithospheric predictions, and on other ridge flanks where basement outcrops are more common (e.g., Hutnak et al, 2008;Lucazeau et al, 2006;Villinger, Grevemeyer, Kaul, Hauschild, & Pfender, 2002 , 1979) and has been revisited multiple times for logging, hydrogeological studies, and other survey work (mapping, seismics, shallow coring, and heat flow) (e.g., Becker et al, 2001;Morin, Hess, & Becker, 1992;Morin, Moos, & Hess, 1992). Processes and conditions at North Pond are likely to be typical of ridge-flank hydrothermal circulation through young crust in many settings: rapid flow of cool fluids having limited opportunity to react with basement rocks and overlying sediments before being discharged to the overlying ocean.…”

Evidence for Low‐Temperature Diffuse Venting at North Pond, Western Flank of the Mid‐Atlantic Ridge

“…The situation is very different at ridge flanks where sediment cover is thin or absent or where seamounts are abundant and where recharge of cold seawater at exposed basement has been inferred in numerous studies (e.g., Heesemann et al, 2009;Kuhn et al, 2017;Le Gal et al, 2017;Lucazeau et al, 2006;Villinger et al, 2002Villinger et al, , 2017. The consequence of this fluid recharge into the upper crust is that seafloor heat flow is up to 1 magnitude lower than predicted by conductive cooling models.…”