HR: 17:10h
AN: V42G-05    [PDF]
TI: Unexpected Temperature-Fluid Composition Paths in Kinetic Models of Contact Metamorphism of Siliceous Dolomites
AU: * Bolton, E W
EM: edward.bolton@yale.edu
AF: Yale University, Dept. Geology and Geophysics, P.O. Box 208109, New Haven, CT 06520-8109 United States
AU: Luttge, A
EM: aluttge@mail.rice.edu
AF: Rice University, Dept. Earth Science, P.O. Box 1892, Houston, TX 77251-1892 United States
AU: Rye, D M
EM: danny.rye@yale.edu
AF: Yale University, Dept. Geology and Geophysics, P.O. Box 208109, New Haven, CT 06520-8109 United States
AU: Ague, J J
EM: jay.ague@yale.edu
AF: Yale University, Dept. Geology and Geophysics, P.O. Box 208109, New Haven, CT 06520-8109 United States
AB: We present new results from our two-dimensional kinetic model of contact metamorphism. The model includes 8 minerals and 15 reactions in the system defined by CaO-SiO$_2$-MgO-H$_2$O-CO$_2$. Initially, a matrix of dolomite and quartz is present in a vertical 2D cross section at a depth equivalent of 3 kbar at the top of the layer. The model pluton is introduced by gradually heating, then cooling, a portion of the lower boundary. Overall reaction rates are based on experimental data and depend on the departure of the Gibbs free energy from equilibrium as calculated from the Berman thermodynamic database along with data for supercritical CO$_2$-H$_2$O mixtures from Kerrick and Jacobs. Reaction rates also depend on the evolving mineral surface areas, and both stable and metastable reactions are necessarily included in this kinetic model. In addition to reaction and fluid generation rate calculations, we model the fluid flow, dispersion, thermal evolution, compaction, grain-size changes, and dynamic permeability. We primarily consider no fluid expulsion from the pluton itself, although, within the layer, reaction induced fluid release due to decarbonation reactions can be dramatic. Key factors that influence the style of flow (buoyancy drive verses drive from reaction induced fluid release-devolatilization) are the grain size and heating rates. The grain size effect is dominated by permeability dependencies, and to a lesser extent by the influence of surface areas on reaction rates. As has also been observed by other field and modeling studies, several styles of flow around the pluton are realized, including up temperature flow toward the pluton (buoyancy drive), and down-temperature flow away from the pluton (devolatilization drive). Several unexpected T-X paths (temperature-fluid composition) are generated by the kinetic model within the 2D domain. In some cases, dramatic overstepping of univariant curves is observed, even though kinetic rates of metamorphic reactions from the experimental studies are considered fast. In other cases, fluid rich in CO$_2$ created in one zone is driven toward neighboring zones where T-X evolution paths lie below univariant curves while temperature increases. Needs for future models include better data for kinetics of mineral dissolution and precipitation, solubilities in CO$_2$-rich fluids, and nucleation kinetics, as well as models for matrix fracture and improved numerical resolution.
DE: 3660 Metamorphic petrology
DE: 8045 Role of fluids
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting