| Title | Basalt-seawater interaction at near-supercritical conditions (400˚C, 500 bar): Hydrothermal alteration in the sub-seafloor |
|---|---|
| Authors | M. Passarella, B.W. Mountain, T.M. Seward |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Experimental geochemistry, near-supercritical, hydrothermal, seawater, sulfate mineral precipitation; elemental flux, trace elements |
| Abstract | Experiments are currently underway to simulate basalt-seawater interaction using a continuous flow reactor at near- supercritical conditions for seawater (400˚C, 500 bar). These conditions are typical of the supercritical reaction zone of mid-ocean ridge hydrothermal systems. The experimental set-up consists of two pressure vessels in series: a preheater vessel with a high temperature gradient (40 – 377˚C) containing zirconia (ZrO2) beads and a reactor vessel at 400˚C to contain the rock material. In the experiment reported here, seawater was pumped through the system while being heated to 400˚C, 500 bar without rock material in the reactor vessel. The seawater was first filtered to remove microflora and then deoxygenated. Continuous flow was maintained at 1 ml hr-1 during the experiment. Chemical analysis show a loss of Ca (from 360 to 75 mg kg-1), Mg (from 1010 to 370 mg kg-1) and SO4 (from 2020 to 120 mg kg-1) from the seawater. SEM analysis confirm the presence of anhydrite, Mg-sulfate minerals (epsomite and/or caminite) and brucite. Mineral saturation indices show that the reacted seawater is in equilibrium with brucite and anhydrite at 400oC. The seawater is depleted in calcium, magnesium and sulfate due to the precipitation of retrograde solubility phases. This is consistent with the seawater having a very low pH (3.0). It is conjectured that this process occurs within hours. Further experiments are underway to react this modified seawater for basalt material. |