Rock-fluid interactions in a temperature gradient: biotite granodiorite + H 2O

Abstract

A biotite granodiorite was reacted in a controlled temperature gradient with distilled water for 60 days at 1/3 kbar P Tot = P H 2 o . Polished rock prisms were placed in the gradient at 72, 119, 161, 209, 270, and 310°C. Scanning electron microscope and microprobe analyses show the appearance of several secondary phases: Ca montmorillonite at 72°C and 119°C; zeolite, either stillbite or heulandite, at 161°C; and another zeolite, thomsonite, at higher temperatures. Solution analyses show a constant composition after about 2 weeks of reaction. The phase assemblages observed in this experiment are connected by reaction relationships. The reactions show the transition from clay to zeolite I (stilbite or heulandite) to zeolite II (thomsonite) with increasing temperature and decreasing chemical potential of silica. Additional relations can be used to predict mineral assemblages not directly observed in this experiment but which may occur at different pressures and temperatures as well as at different chemical potentials of water and silica.