Zinc Isotope Fractionation in Mid-ocean Ridge Basalt Differentiation

Chinese Academy of Sciences

Zinc (Zn) isotope systematics is a useful tool to study planetary evolution and trace carbon cycling in the Earth’s mantle. However, zinc isotope data for mid-ocean ridge basalts (MORB), the most widespread magmatic rocks on Earth, are scarce. The possible effects of MORB differentiation and mantle compositional heterogeneity on MORB Zn isotope compositions remain unclear.

Recently, a research team led by Prof. XIAO Yuanyuan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has revealed the systematic zinc isotope fractionation during MORB differentiation.

The study was published in Geochimica et Cosmochimica Acta on Feb. 21.

The researchers reported Zn isotope data of MORB lavas from the East Pacific Rise (EPR) at 10°30’N. The geochemical variations of these samples are mainly controlled by varying extents of magma differentiation (MgO = 7.38-1.76 wt.%), instead of by varying extents of mantle melting or mantle compositional variations.

A small δ66Zn range of 0.21-0.32‰, with an average of 0.26 ± 0.07‰, was observed in these lavas. δ66Zn values increased with increasing extent of magma differentiation (decreasing MgO and CaO abundances), which demonstrated unequivocal Zn isotope fractionation (~0.1‰) during MORB differentiation by fractional crystallization of olivine, clinopyroxene and Fe-Ti oxides.

The δ66Zn values for primitive normal-type MORB (N-MORB) and depleted MORB mantle (DMM) were estimated to be ~0.22‰ and ~0.17‰, respectively. Depleted MORB mantle (DMM) shared similar δ66Zn values with the bulk silicate Earth, suggesting that extraction of the continental crust from the initially primitive mantle, leaving behind a residual DMM, causes insignificant Zn isotope fractionation.

“Although magma differentiation and other igneous processes can cause limited Zn isotope fractionation, the existence of possible Zn isotope heterogeneity in the oceanic upper mantle is expected and awaits testing,” said Dr. SUN Pu, first author of the study.

Bathymetric map of the northern East Pacific Rise (EPR) and the vicinity and the sampling locations. (Image by IOCAS)

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