Marine Carbonate Component in the Mantle Beneath the Southeastern Tibetan Plateau: Evidence From Magnesium and Calcium Isotopes

Tracing and identifying recycled carbonates is a key issue to reconstruct the deep carbon cycle. To better understand carbonate subduction and recycling beneath the southeastern Tibetan Plateau, high-K cal-alkaline volcanic rocks including trachy-basalts and trachy-andesites from Tengchong were studied using Mg and Ca isotopes. The low Mg-26 (-0.310.03 to -0.380.03) and Ca-44/40 (0.67 +/- 0.07 parts per thousand to 0.80 +/- 0.04 parts per thousand) values of these volcanic rocks compared to those of the mantle (-0.25 +/- 0.07 parts per thousand and 0.94 +/- 0.05 parts per thousand, respectively) indicate the incorporation of isotopically light materials into the mantle source, which may be carbonate-bearing sediments with low Mg-26 and Ca-44/40 values. In addition, no correlations of Mg-26 and Ca-44/40 with either SiO2 contents or trace element abundance ratios (e.g., Sm/Yb and Ba/Y) were observed, suggesting that limited Mg and Ca isotopic fractionation occurred during cal-alkaline magmatic differentiation. A binary mixing model using Mg-Ca isotopes shows that 5-8% carbonates dominated primarily by dolostone were recycled back into the mantle. Since Tengchong volcanism is still active and probably related to ongoing plate tectonic movement, we propose that the recycled carbonates are derived from oceanic crust related to the ongoing subduction of the Indian plate.