Compositions, growth mechanisms, and temporal relations of hydrothermal sulfide‐sulfate‐silica chimneys at the northern Cleft segment, Juan de Fuca Ridge

Abstract

Three active hydrothermal vents forming sulfide mounds and chimneys (Monolith, Fountain, and Pipe Organ) and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern Cleft segment, Juan de Fuca Ridge. The formation of zoned tubular Curich chimneys (type I) on the Monolith sulfide mound is related to focused flow of high‐temperature (to 328°C) fluid. Bulbous chimneys (type II or “beehives”) at the Monolith and Fountain vents are products of diffuse high‐temperature (to 315°C) discharge. A broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite‐rich shells. Columnar Zn‐sulfide‐rich chimneys with narrow channelways (type III) are constructed where focused and relatively low‐temperature (261°C) fluid vents directly from the basalt substrate. The bulk chemistry (low Cu; high Pb, Ag, and SiO2 contents), mineralogy (pyrite‐marcasite‐wurtzite‐amorphous silica‐anglesite), colloform and filamentous textures, and oxygen isotope characteristics of inactive (type IV) chimneys indicate a low‐temperature (<250°C) origin involving diffuse and sluggish flow patterns and conductive cooling. Seafloor observations and 210Pb data indicate that (1) type IV chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet 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.

Geochemical tracers of processes affecting the formation of seafloor hydrothermal fluids and deposits in the Manus back-arc basin

“…Consequently, they have lower buoyancies and slower fluid flow rates relative to higher temperature fluids. These slower fluid rates correlate with the formation of more porous, bulbous chimneys, rather than cylindrical or pipe-like chimneys (Koski et al, 1994). compositions in diffuser-type chimneys.…”

“…and clear fluids often exhibit very different morphologies and compositions relative to their highest temperature counterparts (Haymon and Kastner, 1981;Koski et al, 1994). …”

“…The latter affects the styles of interaction and mixing among rising high-temperature hydrothermal fluids and locally entrained seawater in proximity to the seafloor. Differences in temperature, fluid composition (e.g., pH, metal and H 2 S concentrations) and style of fluid flow significantly affect the morphology and composition of metal-sulfide vent deposits at the seafloor (Koski et al, 1985;Koski et al, 1994;Tivey et al, 1999;Kristall et al, 2006). Overall, metal-sulfide deposits forming at mid-ocean ridge spreading centers tend to be dominated by Fe-sulfides (pyrite, pyrrhotite), Cu-Fesulfides (chalcopyrite) and Zn-sulfides (sphalerite, wurtzite).…”

An investigation into the characteristics and sources of light emission at deep-sea hydrothermal vents

“…Sphalerite precipitates from hydrothermal fluid as it cools to temperatures ranging from 180°C to 255°C [Haymon, 1983;Baron, 1998]. It is also found in the middle and outer portions of the chimney walls (both on the East Pacific Rise and the Juan de Fuca Ridge [Haymon, 1983;Tivey and McDuff, 1990;Koski et al, 1994].…”

“…Unlike chimneys, beehives have a bulbous morphology and high porosity (-30% [Koski et al, 1994]). The structure itself is primarily a matrix of anhydrite, with lesser amounts of pyrite, chalcopyrite, sphalerite, and wurtzite.…”