Isotopic fractionation of zirconium during magmatic differentiation and the stable isotope composition of the silicate Earth

Edward C. Inglis*, Frédéric Moynier, John Creech, Zhengbin Deng, James M. D. Day, Fang-Zhen Teng, Martin Bizzarro, Matthew Jackson, Paul Savage

*Corresponding author for this work

    Research output: Contribution to journalArticle

    9 Citations (Scopus)
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    Abstract

    High-precision double-spike Zr stable isotope measurements (expressed as δ 94/90 Zr IPGP-Zr , the permil deviation of the 94 Zr/ 90 Zr ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in δ 94/90 Zr IPGP-Zr (0.046 ± 0.037‰; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of >0.7 show a limited range in δ 94/90 Zr IPGP-Zr (0.053 ± 0.040‰; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in δ 94/90 Zr IPGP-Zr , from −0.045 ± 0.018 to 0.074 ± 0.023‰. Highly evolved igneous differentiates (>65 wt% SiO 2 ) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53‰. These results suggest that both mantle melt depletion and extreme magmatic differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a α zircon-melt of 0.9995 based on the whole rock δ 94/90 Zr IPGP-Zr values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depleted mantle source (N-MORB) or that underwent zircon saturation (SiO 2 > 65 wt%) are removed from the dataset to give an estimate of the primitive mantle Zr isotope composition of 0.048 ± 0.032‰; 2sd, n = 48. These data show that major controls on Zr fractionation in the Earth result from partial melt extraction in the mantle and by zircon fractionation in differentiated melts. Conversely, fertile mantle is homogenous with respect to Zr isotopes. Zirconium mass-dependent fractionation effects can therefore be used to trace large-scale mantle melt depletion events and the effects of felsic crust formation.

    Original languageEnglish
    Pages (from-to)311-323
    Number of pages13
    JournalGeochimica et Cosmochimica Acta
    Volume250
    DOIs
    Publication statusPublished - 1 Apr 2019

    Bibliographical note

    Copyright the Author(s) 2019. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

    Keywords

    • Non-traditional stable isotope
    • Zr isotopes
    • Magmatic differentiation
    • MORB
    • OIB

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