Deepwater mantle 3 He plumes over the northern Mid-Atlantic Ridge (36°N-40°N) and the Azores Platform

International audienceAs part of a multidisciplinary project aimed at studying mid-ocean ridge processes near the Azores, fifty water column profiles were analyzed for 3 He/ 4 He ratios in dissolved helium (a well-known hydrothermal tracer) from 36°N to 40°N along the Mid-Atlantic Ridge (MAR) and over the Azores Plateau. As expected, large d 3 He anomalies could be observed over the Rainbow, Lucky Strike, and Menez Gwen hydrothermal sites. The main finding of the present study is the discovery of a large hydrothermal 3 He plume north of the Açor Fracture Zone (north AFZ site), with a CH 4 / 3 He ratio indicative of a basaltichosted hydrothermal system. Clear 3 He and CH 4 anomalies, likely corresponding to unknown venting sites too, were also detected in the Amar Minor segment and south of the Kurchatov Fracture Zone. Evidence for substantial mantle helium degassing was also observed in the deep nodal basins along the Terceira Rift. On the basis of 3 He plumes over the total length of the surveyed segments, the distribution of hydrothermal sites corresponds to a site frequency of 1.3 ± 0.2 site/100 km, in good agreement with the global vent field statistics of Baker and German (2004). For the Rainbow, Lucky Strike, and Menez Gwen sites, the application of a plume model based on the conservation of mass, heat, and momentum shows that the heat output computed by the model is only an estimation of the heat released by the focused part of the flow imputable to one single vent. Applied to the north AFZ venting site for which the height of the plume is not known precisely, the model does not allow us to discriminate between a Menez Gwen/Rainbow type of venting or a more focused vent complex such as the one observed at the TAG site (26°N)