Whether or not oceanic crust basalts are affected by plate subduction is a hot topic of debate. The South China Sea is one of the largest marginal basins in the western Pacific Ocean and has been surrounded by subduction of the Pacific plate and Indian plate, yet, to date, no study has clearly shown evidence of subduction in the geochemistry of volcanism in the basin due to a lack of sampling of igneous crust basalts on the seafloor. The International Ocean Discovery Program Expedition 349 cored seafloor basalts near the fossil spreading ridges of the eastern (Site U1431) and southwestern (Site U1433 and U1434) subbasins in the South China Sea. The recovered basalt samples indicated a pyroxenite-bearing peridotite mantle source. Here, we report Mg isotopic data from 14 of these oceanic crust basalt samples. The δ26Mg values of most basalts from the three drill holes were higher (up to −0.10‰) than that of the average mantle (−0.25‰). The lack of correlations of δ26Mg with geochemical indices of magmatic processes (e.g., MgO, CaO/Al2O3, La/Sm, Nb/Zr) suggests that crystal fractionation and partial melting had insignificant effects on the Mg isotopic compositions of the South China Sea basalts. Thus, the variations in Mg isotopes were inherited from their mantle sources. Considering the highly varied Ce/Pb ratios and elevated 87Sr/86Sr values but mantle-like 143Nd/144Nd values, we propose that the varied δ26Mg values were likely caused by metasomatism of subduction-released fluids. The coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-δ26Mg values of the South China Sea basalts resulted from mixing among pyroxenite-bearing peridotite mantle, the nearby Hainan plume materials, and subducting serpentinite-released fluids. Therefore, these Mg isotopes suggest that the mantle source of the South China Sea basalts was influenced by subducted materials, providing further evidence of the initial expansion, formation, and evolution of the South China Sea during plate subduction.
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Research Article|
June 07, 2022
Unusual δ26Mg values in oceanic crust basalts from the South China Sea
Renqiang Liao;
Renqiang Liao
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China
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Hongli Zhu;
Hongli Zhu
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
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Lipeng Zhang;
Lipeng Zhang
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
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He Li;
He Li
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
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Congying Li;
Congying Li
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
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Yongsheng He;
Yongsheng He
4
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
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Weidong Sun;
Weidong Sun
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China5
University of Chinese Academy of Sciences, Beijing 100049, China
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He Liu
He Liu
1
Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China2
Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
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GSA Bulletin (2022)
Article history
received:
04 Sep 2021
rev-recd:
03 Mar 2022
accepted:
01 Apr 2022
first online:
07 Jun 2022
Citation
Renqiang Liao, Hongli Zhu, Lipeng Zhang, He Li, Congying Li, Yongsheng He, Weidong Sun, He Liu; Unusual δ26Mg values in oceanic crust basalts from the South China Sea. GSA Bulletin 2022; doi: https://doi.org/10.1130/B36320.1
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