METAMORPHIC CO2 PRODUCTION: A NEGLECTED CONTRIBUTION TO GLOBAL CARBON MODELS (Invited Presentation)

Perhaps the most significant consequence of fluid flow during metamorphism is release of CO₂ to the Earth’s surface by metamorphic decarbonation reactions. The flux of metamorphic CO₂ may comprise a significant fraction of the CO₂ flux from the solid Earth and needs to be taken into account by models of long-term climate change. Methods for estimating palaeo-fluid fluxes, and thus CO₂ fluxes, in metamorphic rocks have evolved substantially over the last 30 years. The most reliable are those that involve calculation of fluid fluxes needed to drive decarbonation reactions. For example Ferry et al. (2013) in Journal of Petrology estimate time-integrated fluid fluxes of ~ 10² to 10³ m³.m^-2 in metamorphic terrains in Maine and Vermont and Skelton (2011) in Geology estimates time-integrated fluid fluxes of ~ 10 to 50 m³.m² in the Dalradian metamorphic belt in south west Scotland. However these are minimum time-integrated fluxes and some estimates are as high as 10⁵ m³.m^-2. Given the CO₂ contents of the metamorphic fluids, even the lower estimates imply release up to ~ 10¹² mol CO₂ year from orogenic belts the size of the Himalayas. Such a flux is ~ 15% of the solid Earth CO₂degassing rates and would have a significant impact on global climate. However there are large uncertainties related to the magnitude of the time integrated fluid fluxes, the duration of the fluid flow events and the volume of metamorphic crust subject to the fluid fluxes.

Alternatively it is possible to assess metamorphic CO₂ fluxes from active metamorphic belts. Again such estimates are uncertain because of the difficulty of measuring distributed fluxes. In the Himalayas direct measurements of CO₂ emissions and estimates of CO₂/H₂O ratios in hot springs, coupled with calculation of hot spring outputs from measurements of river chemistries, give a range of values up to ~ 1x10¹² mol CO₂/yr implying climatically significant emissions. Given the difficulties in quantifying fluxes from ancient metamorphic belts, let alone the current problems in understanding the timescales, heat source and rates of metamorphism, perhaps metamorphic petrologists should pay more attention to studying active metamorphism.