Magnesium and calcium isotopic characteristics of Tengchong volcanics: Recycling of marine carbonates into the SE Tibetan mantle

Abstract:

Post-collisional high-K calc-alkaline volcanic rocks are widely distributed in Tengchong in the southeastern margin of Tibetan Plateau. Previous considerable petrological and Sr-Nd-Pb isotopic researches undoubtedly indicate that the mantle beneath Tengchong is heterogeneous and enriched. However, the genesis of such a kind of mantle is still poor understood and needs more constrains. One of the key points lead to the answer to this question is that, is there any recycled carbonate involved? Therefore, Magnesium and calcium isotopic compositions of mantle-derived volcanics should be investigated because they are good candidates to be potentially used to trace recycling of ancient marine carbonates into the mantle.

In this study, we report high-precision Mg and Ca isotopic compositions for calc-alkaline volcanic rocks in Tengchong. These volcanic rocks show significantly lighter δ²⁶Mg values (-0.44 to -0.36‰) than the mantle value (-0.25±0.07‰). Similarly, they display lighter δ⁴⁴Ca values (0.65-0.80‰) than the mantle value (1.05±0.04‰). Because neither δ²⁶Mg nor δ⁴⁴Ca are correlated with SiO₂ (50.8-61.6 wt.%) contents, and there is no relationships between δ²⁶Mg or δ⁴⁴Ca and typical trace element abundance ratios (e.g. Sm/Yb, Ba/Y), we conclude that magma differentiation or partial melting would lead to limited Mg and Ca isotopes fractionation. Thus, low δ²⁶Mg and δ⁴⁴Ca signatures of Tengchong volcanic rocks probably reflect that the δ²⁶Mg and δ⁴⁴Ca characteristics of the underneath mantle source, and are resulted from adding ancient marine carbonates into the primitive mantle which has low Mg and Ca isotopic compositions. Our model simulation using a two end-member mixing between Mg-Ca isotopic compositions of primitive mantle and ancient marine carbonate indicates that carbonates involved in the mantle source is mainly dolostone with minor limestone. Combined with the geotectonic evolution history in Tengchong, we propose that the enriched mantle is caused by sediments delivered by the subducted Tethyan oceanic crust. Our results also provides a good case study of using Mg-Ca isotopic characteristics as a useful tool to trace carbonates recycled into the mantle.

This work was supported by Natural Science Foundation of China (No. 41373007, No. 41490632 and 91328204).