Hydrothermal activity at slow-spreading mid-ocean ridges : evidence from chlorine in basalt
Hydrothermal activity can be traced by the chlorine (Cl) concentrations of erupted mid-ocean ridge basalts, if their magmas assimilated hydrothermally altered, Cl-rich crust. In contrast to fast-spreading ridges, this process is not observed so far at slow-spreading ridges, due to their intrinsically lower basaltic Cl contents (<200 ppm). This study involves the development of a new, high precise (<2 ppm SD) technique to measure Cl by electron microprobe, which is applied to basaltic glasses and mineral melt-inclusions from the southern Mid-Atlantic Ridge, the Red Sea Rift and the Gakkel Ridge (all <3.5 cm/yr). Addition of hydrothermal Cl is determined by comparing Cl with major and trace element concentrations. Chemical thermobarometry is used to obtain depth information on hydrothermal circulation. Cl data is compared to geological settings (e.g. hydrothermal sites, volcanoes, faults) to examine their relationships and to assess basaltic Cl as prospection tool for hydrothermal vents.