Crystallization of Hydrous Magmas: Calculation of Associated Thermal Effects, Volatile Fluxes, and Isotopic Alteration

The metamorphism and accompanying isotopic alteration of country rock in contact with an instantaneously emplaced sheet-like body of hydrous magma has been studied using a one-dimensional analytical solution and numerical modeling. The model includes a consideration of the complete crystallization and degassing history of the magma, coupled with conductive and convective heat flow and mass transfer in the porous country rock, and in the magma layer itself. The dynamics of cooling of the magma determine the velocity with which the solidus point (solidification front) moves downward, and this in turn gives (by conservation of fluid mass) the magnitude of the flux of aqueous magmatic fluid that is released to flow upward through the country rock. The fluid flux is therefore variable because it depends upon the temperature evolution of the magma, and this allows us to make several new statements about the P-T-X_H₂O paths of rocks undergoing contact metamorphism as a function of distance from the contact and the temperature, composition, and water content of the intruded magma. In previous studies of metamorphic fluid flow, the fluid flux has usually been assumed constant, or only the integrated effects of the fluid flux were studied. The parameterization of H₂O phase diagrams of hydrous magmas at fixed pressure is discussed in detail and a simple scheme is developed. For a given magmatic water content, we suggest a description of the variation of the melt fraction with temperature that includes only one dimensionless parameter, Mₛₐₜ, which represents the fraction of melt generated close to the solidus temperature during melting. This parameterization allows us to calculate the evolution of temperature and volatile flux during magma crystallization. The results of the numerical calculations are shown to depend upon Mₛₐₜ and two other dimensionless parameters: the Stefan number (Ste) and the solidus temperature (T'ₛₒₗ) of the magma. Polynomials are given that describe the numerically calculated contact temperature, the contact temperature gradient, the timing of fluid release and the crystallization time as a function of these three parameters. We discuss the application of the results to natural situations and use them to classify the temporal evolution of metamorphic and isotopic zonation in contact metamorphic aureoles. Three types of time-temperature-fluid infiltration trajectory are recognized within the aureole, and these define a series of zones, following in a specific sequence moving away from the contact. Identification of such zones in metamorphic terranes allows assessment of the plausibility of magmas as the principal fluid sources, and documentation of their relative width provides a means to quantify various aspects of the crystallization history. We demonstrate that the maximum width of an oxygen isotope alteration zone caused by magmatic volatiles is unlikely to exceed 1 km, and most natural situations will involve much smaller alteration zones (e.g., tens to hundreds of meters). Because we are able to predict the thermal and isotopic effects exclusively due to outward directed flow of magmatic fluids, we can use our results to distinguish such situations from others which involve inward directed flow of externally derived fluids. Because systems domi-nated by magmatic fluids are likely to be more common at greater depth in the crust, our model may be particularly appropriate for metamorphism in the deep crust, adjacent to underplated magmas. Such volatile fluxes will exert an important influence on deep crustal melting processes.