ABSTRACT

Oceanographic studies including CTD survey of warm sites and bottom photography confirmed several hydrothermal fields on the summit of Loihi Seamount, Hawaii. Warm water venting at these sites forms chimneys sticking out of and veins cutting through older precipitates. The summit is covered with hydrothermal plumes which are extremely rich in methane, helium, carbon dioxide, iron and manganese, the maximum concentration of helium being 91.8nl/l, the highest so far reported for open-ocean water. The ³He/⁴He ratio of helium injected into seawater is 14 times the atmospheric (1.4 × 10^-6) and similar to that reported for Kilauea and other hotspot-volcanic gases. The ³He/heat and CO₂/heat ratios in the plume are one to two orders of magnitude greater than those at oceanic spreading centers, implying a more primitive source region for hotspot volcanism. The Loihi plumes show negative pH anomalies up to half a pH unit from ambient owing to the high injection rate of CO₂.