Seyfried

Chemical Evolution of Seafloor Hydrothermal Vent fluids: Constraints from In-Situ Chemical Sensor Data and Results of Experimental Studies at Elevated Temperatures and Pressures

W.E. Seyfried¹* & K. Ding¹

Corresponding author: wes@umn.edu
¹University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN

Abstract:
Redox and pH are critical parameters that play a key role in constraining the direction and magnitude of mass transfer during hydrothermal alteration processes at mid ocean ridges. Accordingly, changes in temperature, pressure and/or phase separation effects in the root zones of seafloor hydrothermal systems can result in predictably large changes in pH and dissolved H2 and H2S. Recent investigations at, EPR 9˚N (01/08) using in-situ chemical sensors together with gas tight fluid samples, with which the chemical sensor data can be compared, indicate moderately reducing conditions and pH values approximately 0.5 units below neutrality at vent conditions. This is consistent with phase equilibria constraints imposed by plagioclase ± epidote ± Mg-chlorite + anhydrite ± tremolite +quartz at temperatures in excess of 400˚C and low to moderate pressure. Experimental and theoretical data indicate that dissolved silica is especially important in pH lowering processes which are needed to explain the observed inventory of dissolved metals in vent fluids. pH and redox parameters, however, are highly non-conservative and can be greatly affected by mixing with seawater at vents underscoring the need for deployment of in-situ chemical sensors for increasing lengths of time.

Contributions to Integration and Synthesis:
The wealth of high quality vent fluid chemical data that have been obtained for different Ridge 2000 IS-Sites can be integrated with geophysical and biological observations to understand better the cause and effect of subseafloor hydrothermal activity. OBS data targeting hydrothermal recharge and reaction zones in the crust can help to constrain deep-seated chemical and physical conditions relevant to the origin and evolution of vent fluid chemistry, whereas biological observations involving microbial diversity constrain the flux of chemicals and the effect of seawater-vent fluid mixing phenomena at the seafloor. Moreover, the vent fluid chemical and temperature data obtained in the aftermath of subseafloor magmatic activity, for example, provide information critical to understanding mass-transport and kinetic processes at the magma-hydrothermal interface. To understand quantitatively the partitioning of chemicals and control of pH and redox under these conditions, however, requires thermodynamic and kinetic data for minerals at conditions beyond which these data presently exist. Thus, an important component hindering system integration involves the dearth of mass transfer models and supporting data capable of operation in unusually complex chemical and physical systems.