DISTRIBUTIONS OF HYDROTHERMAL ACTIVITY ALONG THE MID-ATLANTIC RIDGE -INTERPLAY OF MAGMATIC AND TECTONIC CONTROLS

Hydrothermal activity has been investigated along three different sect ions 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 alon g these three sections of the MAR compares well with the predictions o f a model in which frequency of venting is linearly related to ridge-c rest spreading-rate. At the scale of individual study areas, however, departure from the model is observed by up to an order of magnitude. V enting is anomalously rare along the Reykjanes Ridge but anomalously a bundant 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 whic h predominates may not give rise to conventional black-smoker hydrothe rmal 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 penetratin g active faults with the requisite heat supply being supported through some combination of along-axis magmatic intrusions and thermal releas e associated with the serpentinisation of crustal peridotites. (C) 199 8 Elsevier Science B.V. All rights reserved.