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Iron isotope fractionation during mid-ocean ridge basalt (MORB) evolution: Evidence from lavas on the East Pacific Rise at 10°30’N and its implications

Chen, Shuo; Niu, Yaoling; Guo, Pengyuan; Gong, Hongmei; Sun, Pu; Xue, Qiqi; Duan, Meng; Wang, Xiaohong

Iron isotope fractionation during mid-ocean ridge basalt (MORB) evolution: Evidence from lavas on the East Pacific Rise at 10°30’N and its implications Thumbnail


Authors

Shuo Chen

Yaoling Niu

Pengyuan Guo

Hongmei Gong

Pu Sun

Qiqi Xue

Meng Duan

Xiaohong Wang



Abstract

Whether the Earth’s mantle has a chondritic δ56Fe (deviation in 56Fe/54Fe from the IRMM-014 standard in parts per thousand) value or not remains under debate. The current view is that the observed average δ56Fe of mid-ocean ridge basalts (MORB) cannot be explained by partial melting of mantle source with chondritic value alone. Here, we report Fe isotope compositions on 29 MORB glasses sampled along a flowline traverse across the East Pacific Rise (EPR) axis at 10°30’N. These glasses show large MgO variation (1.8 to 7.4 wt.%) that forms a compositional continuum resulting from varying extent of fractional crystallization, which is accompanied by systematic Fe isotopic variation. Fractional crystallization modeling suggests that early crystallization of olivine, pyroxene and plagioclase gives rise to an iron enrichment trend and an increase in δ56Fe. Once Fe-Ti oxides appear on the liquidus and begin to crystallize, the FeOt and TiO2 contents of the residual melt decrease rapidly, which lead to a slight decrease in δ56Fe. These observations indicate that significant Fe isotope fractionation can indeed take place during MORB melt evolution. Hence, δ56Fe values of variably evolved MORB melts do not represent those of primary MORB melts and thus cannot be used to infer mantle source Fe isotope compositions. Importantly, δ56Fe values of primary MORB melts after correction for the effect of fractional crystallization can be well reproduced by mantle melting. Therefore, our study supports the idea that the Fe isotope composition of the accessible Earth is close to be chondritic. We note that conclusion would assume that the core, which takes up ∼ 90% of the Earth’s Fe, must have a chondritic Fe isotope composition.

Citation

Chen, S., Niu, Y., Guo, P., Gong, H., Sun, P., Xue, Q., …Wang, X. (2019). Iron isotope fractionation during mid-ocean ridge basalt (MORB) evolution: Evidence from lavas on the East Pacific Rise at 10°30’N and its implications. Geochimica et Cosmochimica Acta, 267, 227-239. https://doi.org/10.1016/j.gca.2019.09.031

Journal Article Type Article
Acceptance Date Sep 17, 2019
Online Publication Date Sep 25, 2019
Publication Date Sep 25, 2019
Deposit Date Sep 30, 2019
Publicly Available Date Sep 25, 2020
Journal Geochimica et Cosmochimica Acta
Print ISSN 0016-7037
Publisher Meteoritical Society
Peer Reviewed Peer Reviewed
Volume 267
Pages 227-239
DOI https://doi.org/10.1016/j.gca.2019.09.031
Public URL https://durham-repository.worktribe.com/output/1290344
Related Public URLs http://community.dur.ac.uk/yaoling.niu/MyReprints-pdf/2019ChenEtAl-GCA.pdf

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