3.2 LI Shuguang

Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China

Shu-Guang Li1,2,∗, Wei Yang3,∗, Shan Ke1, Xunan Meng1, Hengci Tian3, Lijuan Xu1, Yongsheng He1, Jian Huang2, Xuan-Ce Wang4, Qunke Xia5, Weidong Sun6, Xiaoyong Yang2, Zhong-Yuan Ren6, Haiquan Wei7, Yongsheng Liu8, Fancong Meng9 and Jun Yan10

1, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China; 2, CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 3, Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 4, The Institute for Geoscience Research, Department of Applied Geology, Curtin University, Perth, WA 6845, Australia; 5, School of Earth Sciences, Zhejiang University, Hangzhou 310027, China; 6, Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 7, Institute of Geology, China Earthquake Administration, Beijing 100029, China; 8, State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; 9, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China and 10, School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China.

Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Especially, the fate of subducted carbon and its store position in the mantle is an important issue. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated. This Mg isotopic anomaly can be subdivided into two domains: (1) the mainland anomaly, that completely overlaps the stagnant Pacific slab in the mantle transition zone beneath eastern China documenting a causal relationship between the large-scale Mg isotopic anomaly in the mainland and the Pacific slab subduction. The trace element features of the basalts suggest that this low δ26Mg anomaly is caused by recycled sedimentary carbonates; (2) the Hainan anomaly, that overlaps a young mantle plume in South China Sea. The trace element features of the basalts suggest that this low δ26Mg anomaly is caused by recycled carbonetd eclogite. These results suggest that sedimentary carbonate can be subducted into convective upper mantle but limited into lower mantle. However, the mantle-like or heavy Mg isotopic compositions of the island arc basalts suggest that Mg isotopes can not trace recycled carbon in island arc volcanism. This problem may be caused by selective dissolution process of carbonates during subduction process that may result in the removal of most of the Ca-rich carbonates from, and leaving Mg-rich carbonates in, the subducting slabs. Combining the all observations, deep carbon recycling can be subdivided into two cycles: cycle 1, the partially dissolved Ca-rich carbonates during the slab dehydration process were injected into the mantle wedge, and then released by arc volcanism; and cycle 2, the undissolved Mg-rich carbonates during slab dehydration were carried into the mantle transition zone by subducted slabs, melted and metasomatized the upper mantle, and were released by intraplate volcanism. The residence time of cycle 1 is approximately 5–10 Myr and the minimum resident time for cycle 2 is approximately 60 Myr, which is considerably shorter than previous estimations.