Growth History Of Hydrothermal Black Smoker Chimneys And Fluid Inclusion Evidence For The Evolution Of Hydrothermal Fluid Within The Endeavour Segment, Juan De Fuca Ridge

Petrographic and fluid inclusion studies of 9 active chimney samples from the Endeavour Segment have been carried out, to establish their formation environment and growth history. The chimney precipitates were divided into four distinct groups according to their mineralogy and metal contents: (1)Sulfate-rich type, (2)Zn-rich type, (3)Fe-Zn rich type, (4)Cu-rich type. Sulfate-rich type chimney fragments are mainly composed of anhydrite with minor sulfides. Zn-rich type chimneys are mainly composed of wurtzite and/or sphalerite, characteristically contain the low-temperature mineral galena. They were formed in the low-medium temperature, less acidic, Zn-rich and reducing environment. Pyrrhotite and wurtzite are common minerals in Fe-Zn rich type chimneys. This type chimney was formed in the relatively medium-high temperature, acidic, more reducing environment. Cu-rich type chimneys are mainly composed of chalcopyrite and/or cubanite. They were formed in the high-temperature, acidic, Cu-rich and reducing environment. Chimneys grew from the precipitation of anhydrite for the mixing of hot fluids and cold seawater. Subsequently, Cu-Fe-Zn sulfides deposited directly, replaced and recrystallized to form different type chimneys. The parameters with the greatest potential impact minerals precipitation are fluid composition and flow rates, temperature, redox condition, pH, and biological activity.The data obtained from microthermometric analyses of fluid inclusions show that anhydrite precipitated from high-temperature fluids(217-393 ℃). At 1# Vent Cap chimney, a site of high-temperature venting(332 ℃, measured by Alvin), the homogenization temperatures of fluid inclusions range from 233 to 371 ℃. At 2# Vent Cap chimney, a site of lower-temperature venting(220-310 ℃ ), the homogenization temperatures of fluid inclusions range from 223 to 371 ℃. At 3# Vent Cap chimney, a site of lower-temperature venting(273-290 ℃ ), the homogenization temperatures of fluid inclusions range from 217 to 393 ℃. Salinities of fluid inclusions in anhydrite exhibit a relatively narrow range, from 2.7 to 4.8 wt% NaCl eq.. All of the salinities of fluid inclusions fall well within the ranges measured in mid-ocean ridge hydrothermal fluids, some higher than seawater, the others lower than seawater. The ranges of homogenization temperatures and salinities of fluid inclusions are wider than the in situ measurement, indicating various degrees of the mixing between hydrothermal fluids and seawater. Compositions of fluid in fluid inclusions are mainly H₂O and minor amounts of CO₂, CH₄+C₂H₆ and H₂S.