Calcium isotopic signatures of depleted mid-ocean ridge basalts from the northeastern Pacific
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Calcium isotopic signatures of depleted mid-ocean ridge basalts from the northeastern Pacific* ZHU Hongli1, 2, 3, DU Long4, **, ZHANG Zhaofeng5, SUN Weidong1, 2, 3, 6 1
Center of Deep Sea Research, Institute of Oceanography, Chinese Academy of Sciences, Qingdao 266071, China
2
Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237,
3
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
China 4
College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
5
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou
6
University of Chinese Academy of Sciences, Beijing 100049, China
510640, China
Received Jan. 19, 2020; accepted in principle Mar. 9, 2020; accepted for publication Mar. 25, 2020 © Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract A number of high-temperature processes (e.g., melt-rock reactions, metasomatism, partial melting) can produce significant Ca isotopic fractionation and heterogeneity in the mantle, but the mechanism for such fractionation remains obscure. To investigate the effect of mantle partial melting on Ca isotopic fractionation, we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts (MORBs) from the East Pacific Rise and Ecuador Rift in the northeastern Pacific. The δ44/40Ca of these MORB samples exhibit a narrow variation from 0.84‰ to 0.88‰ with an average of 0.85‰±0.03‰, which are similar to those of reported MORBs (0.83‰±0.11‰) and back-arc basin basalts (BABBs, 0.80‰±0.08‰) in literature, but are lower than the estimate value for the bulk silicate Earth (BSE, 0.94‰±0.05‰). The low δ44/40Ca signatures of MORB samples in this study cannot be caused by fractional crystallization, since intermediate-mafic differentiation has been demonstrated having only limited effects on Ca isotopic fractionation. Instead, the offset of δ44/40Ca between MORBs and the BSE is most likely produced by mantle partial melting. During this process, the light Ca isotopes are preferentially transferred to the melt, while the heavy ones tend to stay in the residue, which is consistent with the fact that δ44/40Ca of melt-depleted peridotites increases with partial melting in literature. The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral (Cpx and Opx) Ca isotopic fractionation and melting mineral modes. Mantle partial melting is one of the common processes that can induce lower δ44/40Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle. Keyword: Ca isotopes; mid-ocean ridge basalts (MORBs); mantle partial melting; magma differentiation
1 INTRODUCTION Calcium is one of the major elements in the Earth and has six stable isotopes (40Ca, 42Ca, 43Ca, 44Ca, 46 Ca, and 48Ca) in nature (DePaolo, 2004). Except H and He, Ca has the largest re
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