CO₂ degassing and melting of metasomatized mantle lithosphere during rifting-Numerical study

Reactivation of metasomatized mantle lithosphere may occur during continental extension, which is an important component of plate tectonics. The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO₂. Therefore, partial melting of these metasomatized domains may play a crucial role in the global carbon cycle. However, little is known about this process and up until now few numerical constraints are available. Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting, CO₂ degassing and melting. We test 4 lithospheric thicknesses:90, 110, 130 and 200 km with a 10 km thick metasomatized layer at the base using CO₂ of 2 wt.% in the bulk composition. The carbonate enriched layer is stable below~3 GPa (>110 km) for a temperature of 1300℃; therefore, we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick. The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting, whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension. The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young (shallow) and of the North Atlantic Rift for old (thick) lithosphere.