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
in metamorphic terrains in Maine and Vermont and Skelton (2011) in Geology estimates time-integrated fluid fluxes of ~ 10 to 50 m3
in the Dalradian metamorphic belt in south west Scotland. However these are minimum time-integrated fluxes and some estimates are as high as 105
. Given the CO2
contents of the metamorphic fluids, even the lower estimates imply release up to ~ 1012
year from orogenic belts the size of the Himalayas. Such a flux is ~ 15% of the solid Earth CO2
degassing 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.