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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 and Niu, Yaoling and Guo, Pengyuan and Gong, Hongmei and Sun, Pu and Xue, Qiqi and Duan, Meng and Wang, Xiaohong (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 . pp. 227-239.

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.

Item Type:Article
Full text:Publisher-imposed embargo until 25 September 2020.
(AM) Accepted Manuscript
Available under License - Creative Commons Attribution Non-commercial No Derivatives.
File format - PDF
(1598Kb)
Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1016/j.gca.2019.09.031
Publisher statement:© 2019 This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Date accepted:17 September 2019
Date deposited:22 October 2019
Date of first online publication:25 September 2019
Date first made open access:25 September 2020

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