Hydrothermal quartz vug from the Mid-Atlantic Ridge

A site dredged on the east wall of the rift valley of the Mid-Atlantic Ridge at lat 24°21′N is characterized by a low in residual magnetic intensity similar to that observed at the TAG Hydrothermal Field in a structurally equivalent position 250 km to the north. Tholeiitic basalts that have undergone hydrothermal alteration, forming chlorites, analcime, calcite, and quartz-rich rocks, were recovered. The quartz is a late remobilization product, occurring in cracks and vugs as massive fillings and euhedral crystals. Oxygen-isotope thermometry indicates that the quartz was deposited from hydrothermal solutions comprising either sea water at 200 °C or primary water at 330 °C, consistent with a history of intense hydrothermal activity and the use of the magnetic low as a signature to identify such activity.