Recent Investigation of In-Situ pH in Hydrothermal Vent Fluids at Main Endeavour Field (MEF) and ASHES Vent Field (ASHES): Implications for Dynamic Changes in Subseafloor Hydrothermal System

In-situ pH is among the key factors affecting chemical reactions involved with fluid-rock interaction and metal transport in hydrothermal systems. A small variation in pH will often result in a large difference in dissolved metal concentrations. For instance, at 400^oC, a decrease of ~0.15 pH unit will cause dissolved Fe concentration to double in fluid coexisting with a Fe-bearing mineral assemblage. This parameter also offers us an opportunity to better understand processes controlling the temporal evolution of hydrothermal vent fluid chemistry at mid-ocean ridges. During our recent cruise AT 26-17 with newly upgraded DSV2 Alvin, in-situ measurements of pH were carried out along with gas-tight sampling of vent fluids. Our efforts were focused at MEF and ASHES on the Juan de Fuca Ridge. These hydrothermal systems have been shown to be particularly responsive to subseafloor seismic and magmatic events. The measured fluid temperature was approximately 333˚C and 300˚C at Dante vent orifice of MEF and Inferno vent orifice of ASHES, respectively. The corresponding measured in-situ pH values for both vents are: 4.94 and 4.88, respectively. Dissolved gases and other species were also measured from gas-tight fluid samples providing a means of comparison with the in-situ data.

As we have known the earthquake and magmatic activity often places the system at higher temperature and more reducing conditions in connection with a new evolutionary cycle. Comparing these relatively low in-situ pH values with those measured in the past, especially with the ones obtained at MEF in 1999 after an intense swarm of earthquakes, we see the system trending towards more acidic conditions along with decreasing temperature and dissolved H₂ and H₂S. Taking an example from Dante vent site, in-situ pH value of 5.15 was recorded with a measured temperature of 363^oC two month after the event in 1999, which gives 0.2 pH unit greater than the more recent data. Measured dissolved H₂ and H₂S concentrations in 1999 were also much higher. Altogether these results reveal an ongoing trend toward more oxidizing and acidic stage along with cooling.

More results will be discussed in this presentation with the emphasis on the factors that may have played the role in acidifying the system during the evolution cycle and its effect to metal mobility.