HR: 1330h
AN: B12A-0764    [PDF]
TI: Reconciling Geophysical and Hydrothermal Data Concerning the Transfer of Heat from the Lithosphere to the Oceans at Fast and Slow Spreading-Centres
AU: * German, C R
EM: cge@soc.soton.ac.uk
AF: Southampton Oceanographic Centre, European Way, Southampton, SO14 3ZH United Kingdom
AU: Lin, J
EM: jlin@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Understanding the complex interplay between the geophysical and geological processes active at Mid-Ocean Ridges and their interplay with the overlying ocean through submarine hydrothermal circulation remains a fundamental goal of international mid-ocean ridge research. Here we reflect on some aspects of the current state of the art (and limits thereto) in our understanding of the transfer of heat from the interior of the Earth to the Ocean. Specifically, we focus upon the cooling of the upper ocean crust and its possible relationship to heat-transfer via high-temperature hydrothermal circulation close to the ridge-axis. For fast- (and intermediate-)spreading ridges we propose a simple conceptual model in which ridge extension in the upper crust is achieved via episodic ca.30-50-km-long diking events with a repeat period at any given location of ca.50 years and a "quantum" extensional dimension of ca.5m (ca.3m on intermediate ridges). We argue that the heat available from cooling of such a system to the roof of an axial magma chamber could be removed by (i) ca.5% instantaneous heat loss through "Event" plumes and (ii) ca.20% rapid discharge (order 5 yrs.) through rapid cooling of the upper ca.500 m of the crust via diminishing but vigorous (order 10 GW to 1GW) high-temperature hydrothermal flow. The residual heat available from cooling of the lower section of the dike, as far as the roof of the AMC (iii) would then be sufficient to sustain further high temperature flow in the form of discrete, long-lived (decadal), but less-vigorous (order 100 MW) high-temperature fields sited every ca.10 km along-axis. On the slow spreading Mid-Atlantic Ridge (MAR) our understanding of the mechanisms for the formation of long-lived tectonically-hosted fields such as TAG and especially Rainbow are more problematic. The apparent heat-flux required at these sites appears to be at least one order of magnitude greater than can readily be explained. The slow-spreading ridges exhibit much greater irregularity and episodic focussing of heat sources in space and time. This much is evident, for example, from variable off-axis seafloor morphology/residual gravity anomaly and from the activity over 1-2 Ma of individual mega-mullion detachment fault surfaces. The latter are associated with much thinner apparent crustal thickness, hence, an apparent deficit in magmatic heat release during their formation. This degree of apparent focussing of heat sources spatially and temporally along the MAR could readily facilitate the sustained 2.3 GW flow calculated for Rainbow, over 8-12 ky.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 4832 Hydrothermal systems
DE: 8135 Hydrothermal systems (8424)
DE: 8424 Hydrothermal systems (8135)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting