Mass Transfer in Diffuse Low-Temperature Hydrothermal Systems

Diffuse low-temperature venting (<100° C) carries 50-90% of heat transferred at mid-ocean ridges from the lithosphere to the hydrosphere, implying that mass transfer associated with this type of fluid flow may be significant. Alteration of ocean crust by circulating seawater is manifested by the breakdown of primary minerals and deposition of secondary minerals in voids. Diffuse fluids vent through basalt or sulfide when spatially associated with high-temperature vents and only through basalt when located distally from these vents. Diffuse fluids react with the basaltic crust through which they pass, however, chemical and mineralogical effects of these reactions has not been explicitly examined. The goal of this project is to examine chemical fluxes associated with these reactions in two hydrothermal systems on the Juan de Fuca Ridge, each in different stages of evolution. Chemical fluxes between basalt, secondary minerals and hydrothermal fluids will be quantified in the context of the systems environmental parameters, age and geological setting. This is the first study to directly link hydrothermal fluid chemistry with secondary minerals deposited by low-temperature alteration. Samples collected were in direct contact with diffuse hydrothermal fluids and where possible, fluid temperatures were taken simultaneously with sample collection. The East Field at Axial Volcano represents a young end-member of a hydrothermal system where an eruption took place in January, 1998. Basalt samples collected are very fresh with alteration manifested as surface coatings. Preliminary chemical results indicate depletion of Si by 1 wt. %, compared to samples of the 1998 lava flow collected away from vents. Secondary mineralogy indicates subsurface hydrothermal fluids in this region are warmer than those vented at the seafloor. Barite and anhydrite indicate subsurface mixing of hotter fluids with fresh, oxygenated seawater for these vents. The Endeavour hydrothermal system ( ∼5000-8000 years old) represents a mature end-member for this study. Samples are more altered than those from Axial, with alteration penetrating into sample interiors and more extensive surface coatings. Geochemical modeling of basalt, secondary mineral and fluid chemistries will be used to predict subsurface conditions for reactions that occurred.