CHANGES IN THE HYDROTHERMAL SYSTEM AT LOIHI SEAMOUNT AFTER THE FORMATION OF PELE PIT IN 1996

Sulfide and sulfate minerals recovered from Loihi Seamount during PISC ES V submersible dives in October 1996 are the first occurrence of hig h-temperature hydrothermal mineralization documented for ocean island volcanoes, Millimeter-to centimeter-sized particles recovered in talus deposits on the north rim of the newly formed Pele's pit consist of a ggregates of pyrite, marcasite (?), pyrrhotite, wurtzite, and barite, The barite occurs as single, clear, euhedral crystals or as acicular, radial crystal clusters on sulfide or rock fragments. The mineral asse mblage and the composition of the sulfide phases are similar to those of hydrothermal sulfide from mid-ocean spreading centers. The sulfide assemblage confirms the presence of high-temperature fluids (>250 degr ees C) in the Loihi hydrothermal system. The sulfide samples are not p ieces of broken chimney or basal mound structures, or stockwork from l ower levels in the hydrothermal system. The euhedral crystals and homo geneous compositions suggest rapid precipitation from a high-temperatu re plume injected into cold seawater along fractures and fissures that define the north wall of the new pit crater formed during the seismic swarm in July and August 1996.