Chemically Reactive Fluid Flow During Metamorphism

Abstract

▪ Abstract Stable isotopic, mineralogical, and chemical alteration in metamorphic terranes is evidence for reactive fluid flow during metamorphism. In many cases, the amount and spatial distribution of the alteration can be quantitatively interpreted using transport theory in terms of fundamental properties of metamorphic flow systems such as time-integrated flux, flow direction, and Peclet number. Many estimates of time-integrated flux in the upper and middle crust are surprisingly large, 105–106 cm3 fluid/cm2 rock; estimates for the lower crust are much smaller. Rather than pervasive and uniform, reactive fluid flow in all metamorphic environments is channelized on scales of <1–104 m. Channelization results from heterogeneous permeability structures controlled by features such as lithologic layering, contacts, folds, fractures, and faults. Consequently flow may be in the direction of either decreasing or increasing temperature or isothermal. Site-specific thermal-hydrologic models of metamorphic terranes that explicitly consider chemical reactions and dynamic permeability structures will help resolve outstanding questions with regard to the driving forces and duration of flow, metamorphic permeability distributions, and how deformation controls fluid flow.

An assessment of the role of nonlinear reaction kinetics in parameterization of metamorphic fluid flow

“…Thus, our field-based estimates tend to favor laboratory-based estimates over field-based estimates of effective net rate constants. The most likely reason for our estimates differing from previous field-based estimates is our comparatively short metamorphic fluid flow duration of 10 4.9 years, compared with over 10 7 years from previous studies [Christensen et al, 1989;Ferry and Gerdes, 1998]. The positive linear correlation…”

A Hydromechanical Model for Lower Crustal Fluid Flow

“…14.9 and 14.13). It seems probable that large-scale lateral fluid flow inferred from metamorphic field studies (e.g., Ferry and Gerdes 1998;Skelton 1996;Wing and Ferry 2007) is induced by external perturbations, such as drainage caused by tectonically-induced dilatant shear zones (Sibson 1992) or mean stress variations caused by folding (Schmalholz and Podladchikov 1999;Mancktelow 2008). The strength of these perturbations increases rock strength, thus they are likely to become important under the same conditions that embrittlement may cause a decompaction-weakening rheology (Sect.…”

“…In the former category, results from the Kola deep drilling project suggest the development of fluid compartmentalization at~8 km depth within the crust (Zharikov et al 2003). While in the latter category, the existence of permeable horizons with sublithostatic fluid pressure are essential to explain the lateral fluid flow so often inferred in metamorphic studies (Ferry and Gerdes 1998;Wing and Ferry 2007;Staude et al 2009). Counterintuitively, the non-compacting scenario is consistent with the idea that the brittle-ductile transition is coincident with the transition in crustal hydrologic regimes if faulting in the brittle domain is responsible for the high permeability of the upper crust (Zoback and Townend 2001).…”

Fluid Immiscibility in Metamorphic Rocks

“…Rock compartments in aureoles are therefore not isochemical. However, the nature of reactive fluid flow in more than 30 carbonate-bearing contact aureoles worldwide has been subject to substantial research summarized in reviews by Ferry and Gerdes (1998), Baumgartner and Valley (2001) and Ferry et al (2002). Actual or possible immiscibility during reactive fluid flow is therefore reviewed in some detail here.…”

“…The progress of fluid-producing mineral reactions has been interpreted in terms of time-integrated fluid fluxes by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. Ferry and Gerdes (1998), Baumgartner and Valley (2001) and Ferry et al (2002) provided global reviews of contact metamorphic fluid flow in metacarbonate host rocks worldwide. Models based on treatment of continuum mechanics in porous media that combine homogeneous fluid flow with mineral reactions and oxygen isotopic exchange have attempted to show that the spatial disposal of such fronts as well as the geometry of the isotopic front itself can be used to determine, or to speculate on, important parameters including: composition and sources of fluids, migration pathways and direction of fluid flow, time-integrated fluxes, fluid transport mechanisms in terms of advection/diffusion/dispersion, validity and degree of local mineralogical and isotopic equilibrium, isotope exchange kinetics, and time-spans of fluid-rock interaction (e.g.…”