Evaluation of CO2 concentration in a carbonated eclogite mantle source: A new attempt based on the compositions of olivine phenocrysts

Carbon can be transported into the deep mantle via subduction, thereby affecting the physical properties of the mantle, such as lowering the solidus, reducing the seismic velocity, and enhancing the electrical conductivity. However, constraining the quantity of recycled carbon in the mantle is challenging. Here, we present a method for quantitatively estimating the source CO2 of Cenozoic nephelinites from eastern China. Experimental studies have shown that the CO2 concentration of the basalt source is a function of the bulk partition coefficient of Ca between melt and its source. Based on the extremely high Ca contents of olivine phenocrysts from those nephelinites, our calculations indicate that those nephelinites from eastern China, which are thought to originate from a carbonated eclogite, were derived from a mantle source containing at least 0.2–5.2 wt% CO2. This quantity is 1–2 orders of magnitude higher than the inferred CO2 concentration in the MORB mantle. Our findings provide insights into estimating carbon concentration in the mantle and suggest that some parts of the mantle, most likely the mantle transition zone (MTZ), are extremely carbon-rich. This also offers an alternative explanation for the observed high electrical conductivity in the MTZ.