Numerical simulation of kinetically-controlled calc-silicate reactions and fluid flow with transient permeability around crystallizing plutons

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ID: 301113
2009
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Abstract
Numerical simulations were performed to examine the relationships between variable contact-aureole permeability, the kinetics of calc-silicate reactions, and the fluxes of mixed CO~2~-H~2~O fluids around a crystallizing granite laccolith. The role of magmatic water was explicitly considered. The Notch Peak contact-metamorphic aureole in Utah was used to define the stratigraphy of the model domain and rock compositions. Reactions that were considered include the major isograd-defining reactions that occurred in the Notch Peak aureole. The half-space model domain had the laccolith, 2 km thick at the middle apex. Only 1-phase fluid flow was considered. Results show that the evolution of the fluid flow-field is highly dependent on the pressure (*P*) boundary condition at the top of the model domain. When *P* at the top boundary is allowed to increase, *P* in most of the top half of the domain eventually exceeds the lithostatic pressure plus the assumed tensile strength of rocks of 15 MPa. This boundary condition simulates an unvented flow-system. A more realistic boundary condition, one that simulates a system that is able to vent to the surface, is when *P* at the top boundary is held at a hydrostatic pressure. In this case, the flow-field is determined largely by pressure gradients between the overpressured magmatic fluid exsolving from the pluton and the lower pressures at the domain boundaries. Fracturing is predicted to occur early after pluton emplacement as the pore fluid is heated and metamorphic reactions produce CO~2~. Although fracturing and reaction-enhanced porosity and permeability influence the local flow-field, the domain-scale flow-field is controlled by long-distance pressure gradients. The domain-scale flow-field and temperature distribution impose the largest control on the distribution of major mineral assemblages in the metamorphic aureole. Transient changes in permeability due to fracturing and volume changes in the solid matrix that accompany reactions have a smaller control on the distribution of minerals. The simulations predict significant overstepping and coeval progress of metamorphic reactions. Reaction rates range from 5×10^−10^ to 10^−14^ kmol/m^2^/sec, depending on the actual *P*-*T*-*X*CO~2~^f^ conditions and the abundance of the rate-controlling mineral. Throughout the metamorphic aureole, *X*CO~2~^f^ approaches 1 as CO~2~ evolved by reactions displaces H~2~O. High pore pressures prevent magmatic H~2~O from infiltrating the aureole until pressures drop when reactions are approaching completion. Consequently, only in the inner aureole, which is eventually infiltrated by magmatic H~2~O, can minerals such as wollastonite and vesuvianite be produced. An integrated H~2~O flux of 3×10^4^ kmol/m^2^ is required to produce the width of the model wollastonite zone by 20 ky. Because of pressure gradients, CO~2~ that is produced in the inner aureole flows outward into colder rocks even before these rocks are heated. The *T*-*X*CO~2~^f^ paths in outer aureole rocks cut across the H~2~O-CO~2~ solvus, which predicts that in nature H~2~O and CO~2~ should unmix and behave as two separate fluid phases. The average yearly CO~2~ flux at the top of the model domain, 2.3×10^3^ mol/m^2^/y, is comparable to fluxes of metamorphic carbonic fluids in active geothermal fields.
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openalex_W2142539158 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Peter I. Nábělek
Journal american journal of science
Year 2009
DOI
10.2475/07.2009.01
URL
https://ajsonline.org/article/61785.pdf
Keywords Keywords not found

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