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

Abstract

Mineral assemblages in contact-metamorphic aureoles are the products of the interplay between heat transfer and fl uid fl ow induced by intrusion of magma. In wall rocks containing carbonate and silicate minerals, metamorphic reactions produce CO 2 , which then becomes part of the hydrodynamic system. Although observed assemblages are the ultimate products of T-XCO 2 fl uid -t paths in aureole rocks, the complexities of the paths, and hence the evolution of a hydrodynamic system, are diffi cult to decipher from them. Numerical simulations were conducted to model the mineralogical evolution in a hydrodynamic system and to evaluate the extent and effects of reaction retardation. Simulations reveal that fl uid composition in the inner aureole evolves rapidly toward high XCO 2 fl uid as the rocks heat up before an appreciable amount of water is exsolved out of the pluton. After local fl uid pressure drops when early reactions are coming to completion, infi ltration of magmatic water becomes signifi cant and can drive production of typical inner aureole minerals such as wollastonite. Fluid compositions in the outer aureole refl ect largely (1) the initial CO 2 production, as fl uids are driven down the pressure gradients from the inner aureole, and (2) the subsequent infi ltration of magmatic H 2 O. Simulations also suggest temperature overstepping of the onset of reactions and retarded consumption of reactant minerals, which leads to coeval metastable reactions. However, the fi nal simulated mineral assemblage in the inner aureole refl ects equilibration with H 2 O-rich fl uids that is usually seen in the fi eld, although evidence for kinetic retardation may be preserved in some rocks, especially in the outer aureole.

Clumped isotope thermometry of calcite and dolomite in a contact metamorphic environment

“…The highest-grade argillite layers (wollastonite-bearing assemblages) are highly variable in δ 18 O, even at similar distances from the contact with the pluton, suggesting channelized fluid flow, and/or variations in mixing ratios of isotopically distinct magmatic and/or meteoric fluids. These findings, and the results from 2-D numerical models of thermal and fluid evolution in the aureole, have been cited as evidence for down-temperature fluid flow with a dominantly vertical component (i.e., up and away from the laccolith, perpendicular to bedding) (Nabelek, 2007;2002;Nabelek et al, 1992). Although it has been argued that the distribution of whole-rock δ 18 O within 500 m of the contact and the observed location of the diopside-in isograd are better explained by a model of up-temperature, horizontal fluid flow (Ferry and Dipple, 1992), such conclusions are inconsistent with the results of hydrodynamic models of fluid flux in metamorphic terranes (Ferry et al, 2013).…”

How contact metamorphism can trigger global climate changes: Modeling gas generation around igneous sills in sedimentary basins

“…Nabelek, 2007). An overstepping temperature needed for dehydration reactions are estimated from kinetic modeling to be rarely more than a few degrees, with a maximum of 40°C (Walther and Wood, 1984), and would only lead to a minor shift of the equilibrium reaction conditions (Fig.…”

“…Gas generation around igneous sills Isherwood, 2003;Nabelek, 2007). Mineral reactions in sedimentary host-rocks around sheet intrusions, which cools faster and have less heat available relative to plutons, are comparatively poorly documented.…”

Metamorphic evolution of the contact aureole of the Dipilto Batholith, Eastern Chortis Terrane, Nicaragua

“…Published numerical simulations for intrusions of granitoid plutons into the metasedimentary lithologies (Nabelek 2007(Nabelek , 2009 showed complex evolution of mixed CO 2 -H 2 O fluids coexisting with marbles. Wollastonite-rich rocks occur in narrow zones next to the contact with granitoids, having been formed as a consequence of a magmatic fluid infiltration.…”