Crystallization of hydrous magmas - Calculation of associated thermal effects, volatile fluxes, and isotopic alteration


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Crystallization of hydrous magmas - Calculation of associated thermal effects, volatile fluxes, and isotopic alteration
Journal of Geology
Podladchikov Y.Y., Wickham S.M.
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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 tum 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 T-X(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, M(sat), 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 M(sat) 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 dominated 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.
Create date
09/10/2012 19:50
Last modification date
20/08/2019 15:58
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