The contribution of subducted carbonate sediments to the genesis of the Southwestern Colombian arc magmas was investigated using a comprehensive petrography and geochemical analysis, including determination of major and trace element contents and Sr, Nd, Hf and Pb isotope compositions. These data have been used to constrain the depth of decarbonation in the subducted slab, indicating that the decarbonation process continues into the sub-arc region, and ultimately becomes negligible in the rear arc. We propose on the basis of multi-isotope approach and mass balance calculations, that the most important mechanism to induce the slab decarbonation is the infiltration of chemically reactive aqueous fluids from the altered oceanic crust, which decreasingly metasomatize the mantle wedge, triggering the formation of isotopically different primary magmas from the volcanic front (VF) with relatively high ¹⁷⁶Hf/¹⁷⁷Hf, high ⁸⁷Sr/⁸⁶Sr, negative values of εNd and lower Pb isotopes compared to the rear arc (RA). The presence of more aqueous fluids at the volcanic front may increase the degree of decarbonation into carbonate-bearing lithologies. Moreover, with increasing pressure and temperature in the subduction system, the decrease in dehydration of the slab, leads to cessation of fluid-induced decarbonation reactions at the rear arc. This development allows the remaining carbonate materials to be recycled into the deep mantle.

One of the most important implications of this study in the deep carbon-cycle is that even if some amount of carbonate-rich sediments are removed at the fore-arc and sub-arc regions, carbonate-rich sediments can potentially deliver carbon to the mantle via subducted carbonate eclogite. If the subducted slab is recycled into the sources of ocean-island basalts (OIB), the presence of residual carbonate which have a relatively high ¹⁷⁶Hf/¹⁷⁷Hf ratio may also be recycled (Chauvel et al., 2008) and it may explain the isotopic signatures found in some Hawaiian basalts (Blichert-Toft et al., 1999) and may contribute to the C-fluxes estimated for OIB (Marty and Tolstikhin, 1998).