Equilibrium and kinetic studies in the system magnesium oxide-silicon dioxide-water

Abstract

The dehydration kinetics of the reaction Talc = 3Enstatite + SiO₂ + H₂O have been experimentally determined as a function of grain size using synthetic and natural talc at 1 bar, 775 - 977°C. The experimental data on the 10-15 pm grain size can be described by a second order rate expression, with rate constant k = 1.98 (10⁴)exp( -EₐIRT min⁻¹, where the activation energy Eₐ = 372 ± 7 kJ/mol. The 1-2 pm and < O.lpm grain sizes of natural talc also follow the second-order rate law with faster dehydration rates but there is no significant difference in rates between the 1-2 μm and < 0.1 μm size fractions. Transmission electron microscopic studies indicate a topotactic relation between talc and product enstatite suggesting an inhomogeneous dehydration mechanism. From available reaction kinetic data and analysis of their crystal structural properties, the time scale of formation of hydrous phyllosilicates in the solar nebula has been evaluated. The results indicate that contrary to the currently held notion, there is no kinetic barrier to the formation of hydrous phyllosilicates within the lifetime of the primitive solar nebula (∼ 1 Ma). The polymorphic transformation α-quartz ≷ coesite and the equilibrium Talc = 3Enstatite + Quartz/Coesite + H₂O have been experimentally investigated up to 40 kbar in the piston cylinder apparatus. The latter has been calibrated by studying the variation of nominal pressure versus piston intrusion as a function of time and by insitu melting point determination of Liel as a function of pressure. The experimentally determined dehydration equilibrium of talc is around 150°C higher than that predicted from existing thermodynamic data bases. A set of mutually compatible thermodynamic properties of talc, quartz, coesite and enstatite have been derived from the experimental data and used to calculate the equilibrium Talc + Kyanite = Pyrope + Coesite + Vapor. The calculated boundary agrees very well with that determined experimentally. Thermal modelling of a subducting oceanic plate and the newly determined phase equilibria relations indicate that talc is stable to depths of 220-250 kIn in subduction regimes. The H₂O release accompanying talc dehydration at upper mantle conditions may have significant impact on our understanding of the petrological and geophysical processes in the upper mantle.