21. FLUID INCLUSION PETROGRAPHY AND MICROTHERMOMETRY OF THE MIDDLE VALLEY

Middle Valley is a hydrothermally active, sediment-covered rift at the northernmost end of the Juan de Fuca Ridge. Two hydrothermal centers are known from previous work: (1) a 60-m-high sediment mound with a 35-m-high inactive sulfide mound and two 20-m-high sulfide mounds 330 m to the south, one of which is known to be active, and (2) several mounds with attendant active hydrothermal chimneys. These sites (Sites 856 and 858, respectively), as well as other adjacent areas (Sites 857 and 855), were drilled during Leg 139 of the Ocean Drilling Program. Fluid inclusion petrographic observations and microthermometric measurements were made on a variety of samples and minerals recovered from these cores: (1) quartz from hydrothermally altered sediment; (2) low iron sphalerite and interstitial dolomite in massive sulfide; (3) calcite-sulfide veins cross-cutting sediment; (4) calcite and anhydrite concretions in sediment; (5) anhydrite veins cross-cutting sediment; and (6) wairakite and quartz veins cross-cutting mafic sills and sediment. Trapping temperatures of fluid inclusions in hydrothermal alteration minerals precipitated with massive sulfides range between 90 ° and 338°C. Fluid inclusions in calcite in carbonate concretions indicate these concretions formed between 112 ° and 192°C. Anhydrite in veins and concretions was precipitated between 137 ° and 311 °C. Quartz-wairakite-epidote veins in mafic sills and hydrothermally altered sediment were precipitated between 210 ° and 350°C. For all inclusions, there is a general increase in minimum trapping temperatures with increasing subsurface depth for all sites, with temperatures ranging from around 100°C at 2400 meters below sea level to around 275°C at 3100 mbsl. Eutectic and hydrohalite meltin