Carbonated eclogitic component beneath eastern China revealed by olivine phenocrysts in nephelinites

Abstract

Carbonates subducted into the mantle alongside mafic material play a critical role in the deep cycling of carbon. This recycled oxidized carbon strongly affects the physical properties of the mantle, changes the local mantle oxidation state, and enhances its ability to melt. Olivine phenocrysts, which are the first to crystallize from mantle-derived magma, record the lithology of the mantle source and provide a new perspective on CO2-rich mantle domains. Here we present data for olivine phenocrysts from Cenozoic nephelinites from eastern China, which have extremely high Ca (1108–6575 ppm) and Mn (1595–5065 ppm) and low Fo (71–85) contents and differ from olivine crystallized from CO2-poor mantle-derived melts (e.g., peridotitic or pyroxenitic mantle-derived melts; Ca < 3000 ppm, Mn < 2500 ppm). This can be explained if the olivine phenocrysts are in equilibrium with melts derived from a carbonated eclogite source, because melts produced experimentally from carbonated eclogite have high Ca and Mn and low MgO contents. The nephelinites have lower δ26Mg (−0.46‰ to −0.37‰) and higher δ66Zn (0.40‰ to 0.56‰) values compared with normal (MORB-source) mantle, lending further support to the influence of carbonated components during the formation of the nephelinites. Decreases in the Ca and Mn contents of the olivine phenocrysts with increasing Fo contents are due to mixing between the initial melt and a reacted melt produced by melt–peridotite interaction during ascent. Our findings demonstrate that the signature of carbonated eclogite in the mantle can be recorded by olivine compositions. This highlights the importance of olivine phenocrysts for tracing CO2-rich lithologies in the mantle and that high Ca and Mn contents of olivine phenocrysts suggest a carbonated eclogite source.