COMPOSITIONS, GROWTH MECHANISMS, AND TEMPORAL RELATIONS OF HYDROTHERMAL SULFIDE-SULFATE-SILICA CHIMNEYS AT THE NORTHERN CLEFT SEGMENT, JUAN-DE-FUCA RIDGE

Three active hydrothermal vents forming sulfide mounds and chimneys (M onolith, Fountain, and Pipe Organ) and more widely distributed inactiv e chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern Cleft segment, Juan de Fuc a Ridge. The formation of zoned tubular Cu-rich chimneys (type I) on t he Monolith sulfide mound is related to focused flow of high-temperatu re (to 328-degrees-C) fluid. Bulbous chimneys (type II or ''beehives'' ) at the Monolith and Fountain vents are products of diffuse' high-tem perature (to 315-degrees-C) discharge. A broader zone of vigorous mixi ng between the hydrothermal fluid and seawater results in quench cryst allization of anhydrite-rich shells. Columnar Zn-sulfide-rich chimneys with narrow channelways (type III) are constructed where focused and relatively low-temperature (261-degrees-C) fluid vents directly from t he basalt substrate. The bulk chemistry (low Cu; high Pb, Ag, and SiO2 contents), mineralogy (pyrite-marcasite-wurtzite-amorphous silica-ang lesite), colloform and filamentous textures, and oxygen isotope charac teristics of inactive (type IV) chimneys indicate a low-temperature (< 250-degrees-C) origin involving diffuse and sluggish flow patterns and conductive cooling. Seafloor observations and Pb-210 data indicate th at (1) type IV chimneys are products of an earlier period of hydrother mal activity that ended no more than 60 years ago but prior to the she et flow eruption, (2) the high-temperature Monolith and Fountain vents are manifestations of the same heating event (shallow emplacement of magma) that led to the sheet flow eruption and recent megaplumes, and (3) the Pipe Organ Vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption.