Sulfide Formation Related to Changes in the Hydrothermal System on Loihi Seamount, Hawai'i, Following the Seismic Event in 1996

Hydrothermal sulfide and sulfate minerals were discovered in sediment samples collected on the Rapid Response Cruise to Loihi Seamount, following a large swarm (>4000) of earthquakes in the summer of 1996. PISCES V submersible dives, con-ducted two months after the seismic event, discovered that part of the summit had collapsed to form a new pit crater. The sudden collapse of the summit crater resulted in a cataclysmic discharge of hydrothermal fluid, ejecting magmatic gases and sulfide crystals. The presence of wurtzite, pyrrhotite, and chalcopyrite indicate high-temperature fluids (>250°C), the first such occur-rence documented for an ocean-island volcano. The composition of the sulfide minerals is similar to that of sulfides from black smokers at mid-ocean ridges. Samples from barite-rich mounds, built after the initial cataclysmic discharge of the hydrothermal plume, resemble white smoker deposits from mid-ocean ridges. The 34S of the sulfide is lower than that of minerals from massive sulfide deposits formed on oceanic basalt, suggesting that the Loihi basalt has lost sulfur as magmatic SO2. Dissolution and oxidation of the ejected high-temperature sulfides, observed over the two years following the event, suggest that these minerals are not easily preserved. Similar crystals forming at deeper levels in the hydrothermal system probably are ejected