Hydrothermal Reaction Of Groundwater Witgh Basalt From The Entablature Of The Cohassett Flow, Grande Ronde Formation, Hanford, Washington: The Effect Of The Introduction Of Fresh Solution To The System At 200 Degrees And 300 Degrees Celsius, And 30 MPA

Abstract

Basalt from the Cohassett flow of the Columbia River Basalt Group (Southeast Washington) was reacted with synthetic Hanford groundwater in Dickson rocking autoclaves at 200° and 300°C, 30 MPa, with initial water:rock mass ratios of 50. During the experiments fresh solutions were re-injected into the reaction cells using a high-pressure chromatography pump. The re-injection experiments are intermediate between closed-system Dickson experiments and flow-through tests in that the fluid and solids can remain in contact for extended periods of time before the solution is replaced. This allows more time for equilibrium to be approached. Data from these experiments suggest the following interpretations: (1) After re-injection many solution parameters quickly (hrs-days) return to near pre-injection values. (2) The redox buffer capacity of the basalt was not exceeded, i.e. fO2 values remained near magnetite-hematite, although nominal water:rock mass ratios as high as ca. 140 were achieved by re-injection. (3) After re-injection the stable high-temperature pH value was only slightly less than the initial pH value, particularly at 300°C. (4) The silica concentration stabilized near apparent quartz saturation in one 300°C experiment, rather than the cristobalite saturation value found in closed-system experiments. (5) Ca­Na-K, quartz, and alpha-cristobalite geothermometer values from the re-injection experiments more closely model the actual experiment temperature than do values from closed­ system Dickson experiments. (6) Short-term relationships between cations appeared to be controlled by ion-exchange between the solutions and secondary clay minerals. Reaction products identified from the re-injection experiments include: Fe-smectite, illite, hematite, very minor cristobalite and possibly Ti-maghemite at 300°C, plus K-spar and analcime in 200°C and variable temperature (300° to 200°C) experiments.