SURVIVING DECARBONATION: THE CO2 BUDGET OF THE LOWER CONTINENTAL CRUST DURING ANATEXIS

The source of volatiles in the continental crust is a long-standing issue. In addition of controlling the amount of melt generated during anatexis, H₂O and CO₂ budget of the siliciclastic lower crust is also of great importance for carbonate precipitation, ore concentration, orogenic degassing and carbon storage.

Here, we focus on four case studies – the Ivrea Zone (0.8 GPa) and the Gruf Complex (0.9 GPa), Italy; the Lanterman Range (0.8 GPa), Antarctica; the Central Main Terrane (1.8 GPa), USA. All these terrains contain carbon-bearing nanogranitoids (i.e. polycrystalline melt inclusions) from which the CO₂ and H₂O content has been quantified by NanoSIMS. We determine the amount of internally derived bulk rock CO₂ necessary to reproduce the volatile content of the melt inclusions using results of experiments on volatile solubility with depth and phase equilibria modelling.

We estimate that the amount of bulk CO₂ present at peak metamorphic conditions is 56 ± 26 ppm at P < 1.0 GPa and 1132 ± 378 ppm at P ~ 2.0 GPa. The amount of CO₂ that survives pass decarbonation varies between 0.05 vol.% (< 1 GPa) and 1.5 vol.% (2.0 GPa) showing a correlation between the amount of CO₂ that can be retained in the system and pressure. Importantly, such small amount of CO₂ does not affect the volume of melt produced during anatexis. Phase equilibria modelling indicates a reduction of only 1% of the melt volume in the CO₂-present scenarios. Therefore, we argue that fluid-present partial melting is likely a common feature in the lower crust. However, the amount of fluid is so minimal, < 1 vol.%, that in appearance, fluid-absent partial melting prevails.

New constraints are emerging on the impact of crustal thickening and continental reworking on the volatile cycle. In the Phanerozoic, the abundance of carbon-rich melt inclusions coincides with changes in the atmospheric compositions. In continental subduction settings, we estimated that the flux of carbon associated to the burial of siliciclastic sediments is ~ 65 Mt C.yr^-1; similar to that of the oceanic lithosphere. Our calculation for the lower crust indicates that the average endogenic flux of carbon in collisional settings is ~ 0.5 Mt C.yr^-1. Hence, the systematic collection of information on preserved anatectic melt and fluid inclusions would open new, fruitful research avenues.