Hydrography and high‐temperature heat flux of the Rainbow hydrothermal site (36°14′N, Mid‐Atlantic Ridge)

Abstract

Abstract. On the basis of an extensive set of conductivity-temperature-depth, lowered acoustic Doppler current profiler (LADCP), and nephelometry profiles and tow-yos the hydrography, flow field, and particle plume of the Rainbow hydrothermal site on the Mid-Atlantic Ridge are analyzed. In the rift valley the water column is less dense and stratified than both eastern and western off-ridge water, with T/S characteristics consistent with inflow across a sill from the east. The buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m. The bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m. Large-amplitude internal waves consistent with tidal forcing are observed near the sill, but LADCP measurements suggest that the tidal signal is not strong enough to lead to flow reversal at plume depth. Above 2000 rn the mean current across the topography generates lee waves which radiate energy upward and downstream. Density-averaged light-scattering profiles show the hydrothermal particle plume to be Gaussian in depth, even in the near field, where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest. The temperature anomalies associated with the mean near-source particle plume are of order -$ x 10 -3 øC; that is, the plume is cold/fresh as expected from the background hydrography of the deep Atlantic. Using the flow field, light-scattering, and hydrographic anomaly observations, the heat flux associated with the hydrothermal particle plume at Rainbow is estimated to lie between 1 and $ GW.

Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge

“…8, 9; Dosso et al 1999). In contrast, the present day deep-water circulation in the Rainbow rift valley is characterized by an inflow of North East Atlantic Deep Water, from the eastern ridge flank, with ε Nd value of -12.4 (Thurnherr and Richards 2001;Thurnherr et al 2002;Lacan 2002) The non-mixing trend in the semi-massive sulphides is generated by the high ε Nd and Co/Ni values and low 1/Nd. The emplacement of a gabbroic body (with N-MORB affinity) within the serpentinites can provide heat and elements which easily explain the isotope signatures and all other peculiarities found in the Rainbow vent field.…”

Serpentinization and heat generation: constraints from Lost City and Rainbow hydrothermal systems 1 1Associate editor: J. C. Alt

“…The source of heat for alteration of ultramafic-hosted hydrothermal systems has often been linked directly to the exothermic nature of the olivine to serpentine reaction (Fyfe, 1974;Kelley et al, 2001;Bach et al, 2002;Lowell and Rona, 2002;Schroeder et al, 2002). This is unlikely for the Rainbow hydrothermal system, however, owing to the high temperatures (Ͼ364°C) and flow rates (500 kg/s) of the vent fluids (Thurnherr and Richards, 2001;Bach et al, 2002;Douville et al, 2002;Seyfried et al, in press), together with its on-axis location, which makes a magmatic heat source more likely. The same may not be the case for the off-axis Lost City hydrothermal system, as has been emphasized in a number of recent studies (Kelley et al, 2001;Bach et al, 2002;Lowell and Rona, 2002;Schroeder et al, 2002).…”

Strontium and oxygen isotopic profiles through 3 km of hydrothermally altered oceanic crust in the Reykjanes Geothermal System, Iceland

“…The size of the Reykjanes system (130 ± 16 MW; Fridriksson et al, 2006) is also more similar to the smaller systems associated with fast and intermediate spreading ridges than the massive systems associated with the MAR (i.e. the mafic intrusion driven Rainbow Hydrothermal Field 2.3 GW; Thurnherr and Richards, 2001). For these reasons we have chosen to focus on comparisons of hydrothermal systems from fast spreading ridges rather than slow spreading ridges.…”