Distributions of hydrothermal activity along the Mid-Atlantic Ridge: interplay of magmatic and tectonic controls

Hydrothermal activity has been investigated along three different sections of the slow-spreading Mid-Atlantic Ridge (MAR): 11 degrees 20'-30 degrees N, 36-38 degrees N and the Reykjanes Ridge, 57 degrees 45'-63 degrees 06'N. When considered in total, the incidence of venting along these three sections of the MAR compares well with the predictions of a model in which frequency of venting is linearly related to ridge-crest spreading-rate. At the scale of individual study areas, however, departure from the model is observed by up to an order of magnitude. Venting is anomalously rare along the Reykjanes Ridge but anomalously abundant along the MAR 36-38 degrees N. Whilst such variability may be within the error of the linear spreading-rate model, we note that the interplay between magmatic and tectonic processes also differs between the three study areas. In the case of the Reykjanes Ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling/investigative strategy because the style of venting which predominates may not give rise to conventional black-smoker hydrothermal plumes. Along the oblique and broadly segmented MAR 36-38 degrees N, we propose that vigorous hydrothermal venting in broad segment-end non-transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along-axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites. (C) 1998 Elsevier Science B.V. All rights reserved.