Magnesium and zinc isotopic anomaly of Cenozoic lavas in central Myanmar: Origins and implications for deep carbon recycling

Abstract

Carbon in the form of carbonate in sediments and altered oceanic crust can be transported into the mantle by subducted slabs, but how to identify recycled carbonate and decipher its storage depths in the mantle still requires to be investigated for typical subduction zones. Here we investigate possible carbonate recycling related to subduction of the Neo-Tethyan oceanic slab, by a combined study of Mg and Zn isotopes on three types of Cenozoic mantle-derived lavas from central Myanmar. The calc-alkaline lavas have arc-like geochemical characteristics and originated from the mantle wedge. These rocks have high δ26Mg (−0.20‰ to +0.03‰), interpreted to be caused by the input of Mg-rich fluids from dehydration of altered subducted slab. The alkali basalts of suite I are depleted in Nb and Ta and were derived from partial melting of the lithospheric mantle, which have normal δ26Mg and δ66Zn. By contrast, the alkali basalts of suite II display OIB-like trace element patterns and originated from the asthenospheric mantle. They possess extremely low δ26Mg (−0.66‰ to −0.48‰) and high δ66Zn (0.57‰ to 0.77‰) that is typically characterized by marine carbonates. No known processes (e.g., magmatic processes and diffusion effects) can account for the coupling of low δ26Mg and high δ66Zn except source heterogeneity caused by recycled magnesian carbonates (magnesite ± dolomite). This is supported by Mg-Zn-Sr isotopic mixing modelling that suggests a “magnesian carbonate + pyroxenite” source. Since calcic carbonate transforms to magnesite at depths of >300 km during subduction, carbonates carried by the Neo-Tethyan oceanic slab may have been recycled into the mantle transition zone as revealed by seismic tomography observations on central Myanmar.