Geotectonic setting of hydrothermal activity on the summit of Lucky Strike Seamount (37,17'N, Mid-Atlantic Ridge)

Humphris, S.E., Fornari, D.J., Scheirer, D.S., German, C.R. and Parson, L.M. (2002) Geotectonic setting of hydrothermal activity on the summit of Lucky Strike Seamount (37,17'N, Mid-Atlantic Ridge). Geochemistry, Geophysics, Geosystems, 3 (8), art.1049. (doi:10.1029/2001GC000284).

Abstract

We have investigated the relations between volcanic, tectonic, and hydrothermal activity on Lucky Strike Seamount (37°17?N, Mid-Atlantic Ridge) using a nested survey strategy involving collection of data from different deep-sea mapping systems. The highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center. Hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake. Isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic (pillow lava) and hydrothermal (sulfide rubble and hydrothermally cemented breccias) substrates. The large volume of sulfide rubble, together with the nature of the sulfide structures, indicates that hydrothermal activity has been episodic but ongoing for a long period of time (hundreds to thousands of years). On the basis of the distribution of hydrothermal deposits, we propose a model of alternation between tectonic and volcanic control on hydrothermalism at Lucky Strike Seamount. Midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high-temperature hydrothermal activity over long time periods. During periods of amagmatic extension, active faulting within the summit depression provides multiple, near-surface fluid flow pathways for discharge of high-temperature fluids and widespread deposition of massive sulfides. During eruptive events, rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits. The new sheet flows form a cap on the hydrothermal system, and fluid upflow is reorganized. Discharge of high-temperature fluids is restricted to isolated sites with relatively high permeability, for example, the edges of the lava lake. Much of the upwelling hydrothermal fluid pools in the subsurface, conductively cools, and mixes with entrained seawater before discharging as widespread low-temperature diffuse flow. Hyaloclastites become cemented, further augmenting the sealing of the system. Present-day activity at Lucky Strike Seamount represents this locally volcanically controlled phase of activity, despite the segment as a whole being dominantly amagmatic.

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