Basalt - fluid interactions at subcritical and supercritical conditions: An experimental study

To investigate the interaction between fluids and basalt at subcritical, near-supercritical, and supercritical hydrothermal conditions (350-400˚C/500 bar), eight experiments have been conducted. These used a continuous-flow, high temperature and pressure hydrothermal apparatus. The basalt was reacted with three fluids: distilled water; geothermal brine; and natural seawater. Two further experiments used only seawater as a control to determine its behaviour without the influence of basalt.

With distilled water, the fluid chemistry results show elevated SiO2, K, Cl, SO4, and H2S in solution for the first 12 days of both experiments. This is due to volcanic glass dissolution. After glass was removed, fluid composition was controlled by the remaining rock minerals. At 400˚C, the secondary mineral assemblage at the bottom of the Reactor (fluid entry point) is composed of grossular, wollastonite, anorthite, and chlorite. These results show the effectiveness of distilled water, which lacks any alkali cations, at removing Na and K rapidly from the rock. At the top of the Reactor (fluid exit point) the secondary minerals are anorthite and celadonite. At 350˚C, the secondary mineral assemblage at the bottom is anorthite and chlorite, while celadonite is the dominant secondary mineral at the top. In both experiments, celadonite replaces solely olivine. The formation of celadonite through reaction with distilled water shows that it can be formed by the interaction of deuteric water and basalt without addition of other components.

The geothermal brine contains high concentrations of SiO2, K, SO4, Na, Cl and has an acidic pH. At 400˚C, fluid chemistry displays elevated SiO2 concentrations for approximately two weeks due to glass dissolution. At 350˚C, SiO2 concentration is initially high after temperature increase, but decreases gradually over the remainder of the experiment. At 400˚C, the secondary mineral assemblage at the bottom of the Reactor is composed of anhydrite and biotite, while at the top of the Reactor, smectite is the only secondary mineral. At 350˚C, anhydrite and smectite are found at the bottom, while only smectite is found at the top. The lack of biotite at 350˚C suggests this mineral’s precipitation kinetics are too slow to outcompete chlorite precipitation.

The seawater-only experiments were conducted as controls to determine its behaviour during heat-up and provide the input solution composition for the seawater-basalt experiments. Both seawater-only experiments (377˚C and 342˚C) show the precipitation of anhydrite, caminite and brucite due to their retrograde solubilities. The effluent solutions are greatly depleted in Ca, Mg and SO4.

In the seawater-basalt experiments at near-supercritical (400˚C) and subcritical conditions (350˚C), elevated SiO2 concentrations due to glass dissolution are not observed. This is attributed to rapid secondary mineral precipitation. Fluid chemistry and mass balance calculations show almost complete removal of SO4, and in particular, Mg, from the seawater while Ca shows a considerable loss from the rock. Three mineralization fronts were identified: (1) glass dissolution; (2) chloritization; and (3) anhydrite precipitation. In both experiments, there is a switch from chloritization to smectitization. This is accompanied by a decrease in Mg/Fe ratio in smectite. This mineral was also found at the top of both experiments, but its composition was more reflective of the rock.

In terms of reactivity, the order of phases from most to least reactive is glass – olivine – clinopyroxene – plagioclase – Fe-Ti oxide. For the aluminosilicate phases this is attributed their respective Al contents. The seawater-basalt experiments also emphasise the fast rate of reaction at which Mg is fixed by the rock, which is conjectured to take less than a few hours.

Considering all experiments, the distilled water results show a rock control on fluid chemistry while in the remaining basalt experiments, the chemistry is largely controlled by the fluid.

Temperatures calculated using standard Na/K geothermometer did not estimate, in most cases, values close to the experimental temperature. This is due to the inability of the rock to sufficiently adjust the Na/K ratio given the secondary mineral assemblages that form.