Numerical Modeling of Melting-Induced Decarbonation During the Flattening of the Subducted Slab Within the Mantle Transition Zone

Abstract

Deep carbon fundamentally modifies the physical properties of solids and melts, thereby affecting partial melting and compositional differentiation within the mantle. Experimental petrological studies suggest that the carbonated oceanic crust in the subducted slab may undergo carbon-induced partial melting and decarbonation in the deep upper mantle. Although slab geotherms in cold subduction zones fall below the experimentally obtained solidus of the carbonated oceanic crust, as many subducted slabs can stagnate within the mantle transition zone (MTZ), they could be warmed up during the elongated residence time. However, quantitative geodynamic studies regarding the effect of such slab stagnation on slab decarbonation through partial melting are lacking. To fill this research gap, we employ 2D numerical modeling to investigate the potential decarbonation behaviors of subducted slabs due to carbon-induced melting during their flattening within the MTZ. Our results demonstrate that the decarbonation rate of a stagnated slab that is hot and carbonate-rich can surpass 75%, whereas for a cold and carbonate-poor slab, for example, the present-day Pacific plate, the decarbonation rate is limited to a few percent. Hence, we suggest that geochemical signatures of Cenozoic intraplate basalts in eastern China and seismological observations of the low-velocity layer in the Northeast Asian upper mantle can be traced back to an early phase of subduction when the subducted slab was moderately hot and carbonate-rich. Our geodynamic model provides a quantitative constraint on the deep carbon cycle associated with subduction, especially in the deep upper mantle.