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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.

Authors :
Chen, Shuo
Niu, Yaoling
Guo, Pengyuan
Gong, Hongmei
Sun, Pu
Xue, Qiqi
Duan, Meng
Wang, Xiaohong
Source :
Geochimica et Cosmochimica Acta. Dec2019, Vol. 267, p227-239. 13p.
Publication Year :
2019

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 (1.8–7.4 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 TiO 2 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. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
00167037
Volume :
267
Database :
Academic Search Index
Journal :
Geochimica et Cosmochimica Acta
Publication Type :
Academic Journal
Accession number :
139276870
Full Text :
https://doi.org/10.1016/j.gca.2019.09.031