Magnesium-carbon isotopes trace carbon recycling in continental subduction zone

Abstract

Subduction of the oceanic crust is an important mechanism for transporting the surface carbon into the deep mantle. Subduction of the continental crust is generally preceded by subduction of the oceanic slab, which may carry different sources of crustal carbon into the same subduction zone with enhanced carbon recycling at the convergent plate boundary. Thus, it is significant to identify whether crustal carbon was recycled into the mantle along continental subduction zones. In this contribution, we report for the first time the Mg and C isotope compositions of post-collisional mafic igneous rocks from the Hong'an and Dabie orogens in east-central China. There are relatively low δ26Mg values of −0.77‰ to −0.24‰ and high δ13C values of −4.0‰ to −1.8‰ in association with very high C contents of 9712–10,177 ppm for the Hong'an rocks, indicating recycling of the early subducted, carbonated oceanic crust into the mantle. In contrast, there are relatively high δ26Mg values of −0.23‰ to −0.18‰ and low δ13C values of −5.9‰ to −4.1‰ in association with relatively low C contents of 711–2313 ppm for the Dabie rocks, demonstrating recycling of the crustal carbon from the subsequently subducted continental lithosphere. These interpretations are verified by quantitative modeling of the geochemical data for the two orogens of mafic igneous rocks. The results suggest that addition of 30–40% carbonated oceanic crust-derived materials and 10–20% continental crust-derived materials to the mantle sources can produce the Mg–C–Sr–Nd–Hf isotope compositions of mafic igneous rocks in the Hong'an and Dabie orogens, respectively. Therefore, the different types of crustal carbon were incorporated into the mantle wedge at different depths by the successive subduction of oceanic and continental slabs. In particular, the carbon recycling is evident in the continental subduction zone. As such, carbon fluxes in and out of the mantle in continental subduction zones also need to be taken into account when constraining the global C cycle.