Mobility of the Elements in Metamorphism

The mobilities of the elements are discussed in terms of their thermodynamic and kinetic properties. The contributions of lattice diffusion and intergranular vapor transport are compared, and the observational data are interpreted in terms of several simplified models. It is concluded that, while lattice diffusion is ineffective as a means of extensive mass transport, it is of considerable importance in bringing about local equilibrium when it is increasingly favored by differential stresses during recrystallization creep. Existing data seem to indicate that by and large the compositions of individual metamorphic minerals vary continuously with distance but that this variation is far less than the variation in modal phase content which gives rise to gneissic banding. The nature of these variations may be interpreted using a quasi-equilibrium model of diffusion between small volumes of rock which are regarded as being in local equilibrium. According to the model the compositional gradients may be established by the diffusion of certain critical molecular or ionic species. Application is made to the variation of the atomic fraction Mg/(Mg + Fe2+) in cummingtonite as a function of the flux of H2, and in this case the model yields a variation curve qualitatively similar to those observed. Analogous mechanisms are also proposed to explain continuous variations in feldspars and micas. The problem of the long-range transport of isotopes is discussed using the illustration of H2O18 and H2O16. It is concluded that, in general, the ratio of the fluxes of the two species may be approximated by the relation \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$J_{{\mathrm{H_{2}O^{18}}}}/J_{{\mathrm{H_{2}O^{16}}}} = 1/500$$\end{document}. Consequently, the exchange of these isotopes over a distance should be less rapid than the equilibration over the same distance of hydration reactions. This result appears to diminish greatly the probability of occurrence of widespread fluids of constant O18/O16 ratio and is in accord with some recent data. Finally, it is concluded that the rigid classification of many chemical constituents of rocks as "mobile" or "inert" has little basis in theory or experiment. In particular, this arbitrary classification has led to error and confusion with respect to certain constituents such as oxygen.