Fluid Inclusion Petrography and Microthermometry of the Middle Valley Hydrothermal System, Northern Juan de Fuca Ridge

Abstract

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-wairakiteepidote 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 melting temperatures indicate that Ca, Na, and Cl are the dominant ionic species present in the inclusion fluids. Salinities for most inclusion fluids range between 2.5 and 7.0 equivalent weight percent NaCl. Most analyses are between 3 and 4.5 eq. wt% NaCl and similar to ambient bottom water, pore fluids, and vent fluid from Site 858. Trapped fluids are modified seawater, and there is no evidence for a significant magmatic fluid component. Oxygen isotopic compositions for fluids from which calcite concretions were precipitated, calculated from isotopic analyses of carbonates formed at low temperatures (133° to 158°C from fluid inclusions), are significantly enriched in 18 O (δ 18 θ = +9.3‰ to +13.2‰), likely due to reaction with subsurface sediments at low water/rock ratios. Calcite that formed at higher temperatures (233°C) in hydrothermally altered sediment was precipitated from fluid only slightly enriched in 18 O (δ 18 θ = +0.4%o). Estimated carbon isotope compositions of the fluid vary between δ 13 C = -7.0%e and -35.4‰ and are similar to the measured range for vent fluids.

Fluid inclusion studies as a guide to the temperature regime within the TAG hydrothermal mound, 26°N, Mid–Atlantic Ridge

“…The salinity of fluid inclusions in samples from the TAG mound varies from 0.6 to 2 times seawater (3.2 wt% NaCl equivalent). Similar salinity variations in fluid inclusions have been found in samples from other mid-ocean ridge sites (Nehlig, 1991;Peter et al, 1994) and in samples from the upflow zone of these systems (Delaney et al, 1987;Vanko, 1988, Vanko et al, 1992Kelley et al, 1993;Saccocia and Gillis, 1995). Hydrothermal fluids at mid-ocean ridge sites also display similar salinity variations, where values from <10% to >230% seawater occur (see summary in de Ronde [1995]).…”

“…Fluid inclusions from anhydrite in samples collected near the surface of the Kremlin area (TAG-2) are characterized by trapping temperatures in the range from 266° to 321°C, which is comparable to the exit temperatures of the presently venting white smoker fluids (270°-300°C, . There is generally a good agreement between average trapping temperatures obtained from fluid inclusion measurements and the actual temperature of venting in the same hydrothermal systems (Le Bel and Oudin, 1982;Brett et al, 1987;Hannington and Scott, 1988;Peter and Scott, 1988;Leitch, 1991), although in most cases, except for Middle Valley (Peter et al, 1994), only surface samples have been studied. Trapping temperatures in the stockwork zone underlying the Black Smoker Complex are generally higher than 375°C.…”

Petrographic, Geochemical, and Fluid Inclusion Evidence for the Origin of Siliceous Cap Rocks Above Volcanic-Hosted Massive Sulfide Deposits at Myra Falls, Vancouver Island, British Columbia, Canada

“…The salinity and temperatures increase slightly with depth in the Battle and HW orebodies, similar to the Kuroko deposits (e.g., Bryndzia et al, 1983;Roedder, 1984), and in modern sea-floor hydrothermal systems such as the TAG mound (Petersen et al, 2000) and Middle Valley (Peter et al, 1994). The salinity and temperatures increase slightly with depth in the Battle and HW orebodies, similar to the Kuroko deposits (e.g., Bryndzia et al, 1983;Roedder, 1984), and in modern sea-floor hydrothermal systems such as the TAG mound (Petersen et al, 2000) and Middle Valley (Peter et al, 1994).…”

Sea-Floor Tectonics and Submarine Hydrothermal Systems

“…The lower venting temperatures compared to sediment-free mid-ocean ridges (e.g., 317°C in the Guaymas basin, 276°C at Middle Valley, 217°C in the Escanaba trough) and the higher pH account for the lower metal concentrations at the sea floor. However, the presence of a high-temperature Cu-rich zone beneath the Bent Hill sulfide deposit at Middle Valley together with fluid inclusion and isotopic data confirm that temperatures below the sea floor were on the order of 350°to 400°C Peter et al, 1994;Teagle and Alt, 2004), similar to end-member vent fluids on nearby sediment-free ridges.…”