FLUIDS IN REGIONAL METAMORPHIC ENVIRONMENTS: CURRENT RESEARCH CHALLENGES

It is now well established that large volumes of fluid can flow through regional metamorphic rocks during mountain building events. Nonetheless, fundamental questions remain regarding the amounts, length scales, and processes of mass and heat transfer by fluids. Devolatilization of thick metasedimentary sequences should produce fluid fluxes of the order of 10³ m³ (fluid) m² (rock) on a time-integrated basis. Focusing of flow into shear zones, fracture zones, or other conduits may produce fluxes a factor of 10 to 100 greater. These fluxes have the potential to transport significant mass, but there are still relatively few detailed mass balance studies of metamorphic systems. Alkali and alkaline earth elements, ore-forming elements, REE, and a host of other elements have been shown to be mobile in various mid and lower crustal environments. However, a wide range of behavior is observed, and it is unclear if patterns of element mobility can be generalized to the regional scale of mountain belts, or if they vary more locally from outcrop to outcrop. Theoretical considerations indicate that fluids can be effective agents of heat transfer if fluxes are large and/or timescales of flow are short. On the other hand, actual field examples of regional metamorphic heat transfer by fluids are rare and often controversial. Traditional conductive models of regional metamorphism envision peak thermal events that last 10-50 million years. However, recent work that models diffusion profiles in metamorphic minerals indicates that timescales of peak heating in the Barrovian type locality (Soctland) were only a few hundred thousand years (Ague and Baxter, 2006). These results suggest rapid fluid release, element transport, and heat transport. Moreover, rapid volatile release of greenhouse gases such as CO₂ has the potential to transiently influence climate. Devolatilized rocks provide unequivocal evidence for fluid transport, but where do these fluids ultimately end up? Is flow dominantly upward and toward the surface, or is transient convection possible in the middle and lower crust? A major challenge is to trace flow pathways at the crustal scale from the deep roots of mountain belts, to shallow hydrologic systems such as ore-forming environments.