Can metamorphic reactions influence atmospheric greenhouse gas concentrations?

• Metamorphism is an ongoing process which occurs to accommodate changes of pressure and temperature at active plate boundaries. Metamorphic reactions commonly involve the uptake, storage and/or release of a fluid which typically contains H2O, CO2 and/or CH4. These metamorphic fluids are free to migrate along fracture and/or grain boundary pathways providing a mechanism for chemical transfer both within the lithosphere and between the lithosphere and the surface systems. In this respect metamorphic fluids represent a (poorly understood) part of global C-O-H cycles and are thus a potential influence on atmospheric greenhouse gas concentrations. To assess whether metamorphic fluids can therefore significantly impact climate systems requires knowledge of fluid compositions, volumes, flux rates, flow pathways. The present day flow of metamorphic fluids can not be measured directly because this occurs at depths which exceed those attainable by scientific drilling. However, evidence of metamorphic fluid flow is often “fossilised” in ancient metamorphic terrains where rocks from the middle and lower parts of the crust or lithospheric mantle are exposed at the surface. Thus ancient metamorphic terrains provide a natural laboratory in which to study the role of the lithosphere in global C-O-H cycles. The metamorphic terrains of the SW Scottish Highlands and New England are excellent examples. These terrains host well-preserved, sometimes overprinting, yet quantifiable evidence of metamorphic fluid flow events which occurred during both extensional and collisional tectonics and in both middle and lower parts of the Earth’s crust. In these studies, time-integrated fluid fluxes are obtained by chromatographic modelling of the propagation of volatilization fronts from lithological boundaries. Only time-integrated fluid fluxes can be obtained in these studies because the rock being studies preserves a time-integrated record of metamorphic fluid flow. However, chromatographic modelling permits conversion of time-integrated fluid fluxes to time-averaged fluid fluxes by comparison with other processes (e.g. diffusion) for which rates can be determined experimentally. These time-averaged fluid fluxes can be used to obtain time-averaged carbon fluxes where fluid compositions can be constrained based on mineralogy. Time-averaged carbon (C) flux rates obtained in these studies are 0.001-10 mol-C.m-2.yr-1. This estimate can be compared with the average global carbon flux rate to the atmosphere which is 10 mol-C.m-2.yr-1 (Houghton & Hackler, 2001).

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