Decarbonation of subducting slabs: Insight from petrological–thermomechanical modeling

Abstract

Subduction of heterogeneous lithologies (sediments and altered basalts) carries a mixture of volatile components (H2O±CO2) into the mantle, which are later mobilized during episodes of devolatilization and flux melting. Several petrologic and thermodynamic studies investigated CO2 decarbonation to better understand carbon cycling at convergent margins. A paradox arose when investigations showed little to no decarbonation along present day subduction geotherms at subarc depths despite field based observations. Sediment diapirism is invoked as one of several methods for carbon transfer from the subducting slab. We employ high-resolution 2D petrological–thermomechanical modeling to elucidate the role subduction dynamics has with respect to slab decarbonation and the sediment diapirism hypothesis. Our thermodynamic database is modified to account for H2O–CO2 binary fluids via the following lithologies: GLOSS average sediments (H2O: 7.29wt.% & CO2: 3.01wt.%), carbonated altered basalts (H2O: 2.63wt.% & CO2: 2.90wt.%), and carbonated peridotites (H2O: 1.98wt.% & CO2: 1.50wt.%). We include a CO2 solubility P–x[H2O wt.%] parameterization for sediment melts. We parameterize our model by varying two components: slab age (20, 40, 60, 80Ma) and convergence velocity (1, 2, 3, 4, 5, 6cmyear−1). 59 numerical models were run and show excellent agreement with the original code base. Three geodynamic regimes showed significant decarbonation. 1) Sedimentary diapirism acts as an efficient physical mechanism for CO2 removal from the slab as it advects into the hotter mantle wedge. 2) If subduction rates are slow, frictional coupling between the subducting and overriding plate occurs. Mafic crust is mechanically incorporated into a section of the lower crust and undergoes decarbonation. 3) During extension and slab rollback, interaction between hot asthenosphere and sediments at shallow depths result in a small window (~12.5Ma) of high integrated CO2 fluxes (205kgm−3Ma−1).