SULFIDE MINERAL-VENT FLUID INTERACTION DURING UPFLOW AT ODP MOUND, MIDDLE VALLEY, NORTHERN JUAN DE FUCA RIDGE

Middle Valley, northern Juan de Fuca Ridge, is a sedimented mid-ocean ridge that hosts two massive sulfide deposits: Bent Hill and ODP Mound. Although the Bent Hill deposit is no longer hydrothermally active, the ODP Mound is presently venting ~270°C degree fluids at several locations. In July, 2000 hydrothermal fluids were collected at Shiner Bock and Spire vents, and ODP Hole 1035H that started venting high-temperature fluids following drilling in 1996. Shiner Bock was active prior to 1996, but venting at Spire is a relatively recent phenomenon likely caused by penetration of a geologic seal during drilling. All three vents are characterized by similar exit temperatures and concentrations of the major ions (Na, K, Ca, Cl) and some volatiles (NH₃, CO₂ and CH₄), implying a common fluid source. Fluids from Spire, however, contain substantially higher H₂S and lower H₂ concentrations relative to fluids from 1035H and Shiner Bock.

Intervent variations in H₂S and H₂ concentrations might be attributed to reaction of fluids with a disequilbrium assemblage of sulfide minerals during upflow from deep-seated reaction zones. Because of pipe-like flow at 1035H, fluid-rock interaction is limited and fluids can flow from the base of the mound to the seafloor with minimal compositional changes. In contrast, more tortuous flow paths through the mound during upflow at Spire result in extensive interaction of fluids and substantial changes in fluid composition. Modifications to fluid composition during upflow at the relatively long-lived Shiner Bock are not observed because time integrated water:rock ratios are high and mineral assemblages that line flow paths have already equilibrated with the composition of fluids entering at the base of the mound. Reaction-path modeling indicates that reaction of ODP 1035H and Shiner Bock fluids with pyrite in the mound produces pyrrhotite and concentrations of H₂S and H₂ identical to those observed at Spire. Conversion of pyrite to pyrrhotite is consistent with mineral paragenesis in the ODP Mound and sulfur isotope variations in vent fluids. These results indicate that vent fluids actively re-equilibrate during upflow, resulting in changes in sulfide mound mineralogy and fluid composition.