The carbon budget of crustal reworking during continental collision: Clues from nanorocks and fluid inclusions

The source of volatiles in the continental crust is a long-standing issue. In addition to controlling the amount of melt generated during anatexis, H2O and CO2 budgets of the middle and lower siliciclastic crust are also of great importance for carbonate precipitation, ore concentration, orogenic degassing and carbon storage. Here we focus on two case studies of partially melted metamorphic rocks of crustal affinity– the Ivrea Zone in the Western Alps (0.8 GPa) and the Central Maine Terrane (1.8 GPa), USA. These terrains contain fluid inclusions and carbonbearing nanogranitoids (former melt inclusions) from which the H2O and CO2 content has been estimated via in-situ analyses. Thermodynamic modelling is used to quantify the amount of internally derived, mineral-bound bulk rock CO2 necessary to reproduce the volatile contents of these melt inclusions. The minimum amount of bulk rock CO2 present at peak metamorphic conditions is estimated at 400 ppm for the Ivrea Zone and 3000 ppm for the Central Maine Terrane. This suggests that the flux of carbon associated with the burial of siliciclastic sediments in the lower crust during the Phanerozoic is 0.2–4.4 Mt. C/yr. These values, as well as the nature of the source of the deep crustal carbon might have changed with time, with periods dominated by internal reworking rather than external inputs. The protracted growth and differentiation of the continental crust through the reworking of supracrustal materials in continental collision settings is a key element of carbon storage processes. The stability of the continental crust through time provides an ultimate, long-lasting reservoir of carbon.