Lithospheric control on the deep mantle carbon transfer in the Magadi-Natron basins, East Africa

Abstract

Mantle-derived carbon dioxide (CO₂) emissions during continental rifting are linked to climate change. The Archean cratonic mantle lithosphere has been considered a carbon reservoir that truncates ascending carbon-rich melts at its base, releasing huge amount of mantle CO₂ in the surrounding rift basins. These emissions are believed to be remobilized from the carbon-rich cratonic lithosphere. However, the mechanisms by which mantle carbon is reactivated from the craton to the surrounding rift basins remain poorly understood. Hence, we conducted petrological-thermomechanical modeling to investigate the migration and decarbonation processes of the carbonate-metasomatized mantle lithosphere (CMML) across the craton-mobile belt boundary during continental rifting. The model results show that for a thicker, lighter, and higher water (H₂O) content CMML, and a faster model extension, the CMML layer can be removed by the lateral advection and ascent from the craton margin to the rift basin formed in the adjacent mobile belt. Considering the 184 km long Magadi-Natron basins in East Africa, we observed that these processes generate a metamorphic CO₂ degassing flux of 0.19–0.56 Mt./yr across the rift basins. Based on the model results, we suggest that the carbon transit from the Archean Tanzanian craton into the Proterozoic Mozambique Belt boundary may be explained by the CMML migration processes. Our modeled CO₂ degassing flux provides lower bounds for the diffuse mantle CO₂ flux along faults in the Magadi-Natron basins.