Natural systems evidence for the alteration of clay under alkaline conditions: An example from Searles Lake, California

Mudstones containing smectite have been altered under mildly alkaline conditions (9 < pH < 10) at Searles Lake, California over a 3 million-year time period. Published mineralogical investigations have shown that up to 70% of detrital clay may be replaced by authigenic minerals such as Fe-illite, analcime, and K-feldspar over timescales < 100 ka. This natural alteration has been simulated incorporating time-dependent boundary conditions of sedimentation and fluid composition, a Pitzer model for activities of aqueous species, and a coupled hydrogeological model for time-dependent flow in the sediment layers. Kinetic dissolution of detrital smectite under alkaline conditions was described using one of two models based on departure from thermodynamic equilibrium or by an empirical rate dependent upon aqueous Si concentrations. Secondary mineral growth was modelled with rates estimated using mineralogical data from the Searles Lake drillcore. The zonal pattern of smectite dissolution observed at Searles Lake was reproduced reasonably well by the ‘Cama TST’ model of montmorillonite dissolution; the other models strongly overestimated clay dissolution. However, the rate of primary montmorillonite dissolution was also a strong function of the assumed rate of growth of secondary minerals. The amount of K-feldspar growth was overestimated in each of the models, whereas analcime (comprising up to 10% of the Searles Lake drillcore composition in some zones) was absent from the results of all the model simulations. This assessment provides a test of the accuracy and reliability of published data in the application of models of smectite dissolution in the long-term and demonstrates that smectite will be reactive under mildly alkaline conditions under conditions of advective fluid transport.