Reactive flow of mixed CO₂–H₂O fluid and progress of calc‐silicate reactions in contact metamorphic aureoles: insights from two‐dimensional numerical modelling

Abstract

Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO2 and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO2–H2O fluid and kinetically controlled progress of calc‐silicate reactions using a two‐dimensional, finite‐element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO2 strongly affects fluid‐flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO2–H2O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO2‐poor fluid in the outer aureole. Metamorphic CO2‐rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P‐T‐XCO2conditions of calc‐silicate reactions. The progress of low‐ to medium‐grade (phlogopite‐ to diopside‐forming) reactions is mainly driven by heat as the CO2 concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high‐grade wollastonite‐forming reaction is mainly driven by infiltration of chemically out‐of‐equilibrium, CO2‐poor fluid during late‐stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO2‐rich fluid dominates in the inner aureole during early heating, whereas CO2‐poor fluid prevails at or after peak temperature is reached. Low‐grade metamorphic rocks are predicted to record the presence of CO2‐rich fluid, and high‐grade rocks reflect the presence of CO2‐poor fluid, consistent with geological observations in many calc‐silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.

Reaction‐induced nucleation and growth v. grain coarsening in contact metamorphic, impure carbonates

“…In analogy with the findings and modelling of Cui et al. (2003) and Nabelek (2007) with respect to the fluid evolution in carbonate rocks, we infer that the low temperature samples (or the early steps of the contact metamorphic evolution) had low a CO2 , which increased owing to the production of CO 2 and reached average values ∼0.5 at high temperatures (Fig.…”

Fluid evolution and kinetics of metamorphic reactions in calc-silicate contact aureoles—From H2O to CO2 and back

“…The numerical calculations were done in two dimensions using the U.S. Geological Survey fi nite-element computer code SUTRA (Voss, 1984), which was modifi ed to include calculation of metamorphic reactions (Cui et al, 2003). The model domain is 12 by 8 km with 50 m grid spacing and its top at 3 km depth (Fig.…”

“…The calculation of heat transfer included advective, diffusive, and source components (i.e., heats of crystallization and reactions). The calculation of mass transport considered gradients in XCO 2 fl uid and fl uid sources (magmatic, metamorphic, and sedimentary) and diffusive dispersion of CO 2 (D = 2 × 10 -8 m 2 /s), as described by Cui et al (2003). The rate of reaction progress, R, is given by Lasaga and Rye (1993):…”

Host-rock deformation during the emplacement of the Mourne Mountains granite pluton: Insights from the regional fracture pattern

“…Numerical models indicate that several thousands of years are required for host rock located ~1 km from the intrusion to reach peak contact-metamorphic conditions (cf. Cui et al, 2003;Douglas et al, 2016). Notably, the emplacement-related deformation is primarily observed in the distal samples (relative to the granite) of the Glen River traverse (samples GRT-10-15).…”