COMPOSITIONS, GROWTH MECHANISMS, AND TEMPORAL RELATIONS OF HYDROTHERMAL SULFIDE-SULFATE-SILICA CHIMNEYS AT THE NORTHERN CLEFT SEGMENT, JUAN-DE-FUCA RIDGE
Ra. Koski et al., COMPOSITIONS, GROWTH MECHANISMS, AND TEMPORAL RELATIONS OF HYDROTHERMAL SULFIDE-SULFATE-SILICA CHIMNEYS AT THE NORTHERN CLEFT SEGMENT, JUAN-DE-FUCA RIDGE, J GEO R-SOL, 99(B3), 1994, pp. 4813-4832
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.