Abstract
To investigate the factors controlling the composition of geothermal fluids, we have studied the hydrothermal transformation of plagioclase → neogenic minerals at 180°C. At equilibrium this transformation is predicted thermodynamically to produce the assemblage quartz, kaolinite, laumontite, low temperature albite. Synthetic anorthite (An100) and labradorite (An50) were spiked with strontium and reacted with two different aqueous solutions: one near-equilibrium with the secondary minerals, the other far-from-equilibrium. The following results were obtained: 1) At initial condition near-equilibrium, the first neogenic minerals formed during labradorite dissolution were quartz - prehnite - kaolinite - low temperature albite and, during anorthite dissolution, quartz - prehnite - kaolinite. A later transformation of prehnite → smectite occurred after 6 months of reaction (for both An50 and An100). 2) At initial conditions far-from-equilibrium, smectite formed after 1 day of reaction. Strontium and calcium are released stoichiometrically during plagioclase dissolution, and the Sr/Ca partition coefficient between the solution and the combined neogenic phases is 1 at 180°C
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