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    Reliability of detrital marine sediments as proxy for continental crust composition: The effects of hydrodynamic sorting on Ti and Zr isotope systematics

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    Name:
    Klaver et al TiZr sediments_au ...
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    Description:
    Final Accepted Manuscript
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    Author
    Klaver, Martijn
    MacLennan, Scott A.
    Ibañez-Mejia, Mauricio
    Tissot, François L.H.
    Vroon, Pieter Z.
    Millet, Marc-Alban
    Affiliation
    Department of Geosciences, The University of Arizona
    Issue Date
    2021-05-21
    Keywords
    Detrital sediments
    Eastern Mediterranean Sea
    Hydrodynamic sorting
    Ti isotopes
    Zr isotopes
    
    Metadata
    Show full item record
    Publisher
    Elsevier BV
    Citation
    Klaver, M., Maclennan, S. A., Ibañez-Mejia, M., Tissot, F. L. H., Vroon, P. Z., & Millet, M.-A. (2021). Reliability of detrital marine sediments as proxy for continental crust composition: The effects of hydrodynamic sorting on Ti and Zr isotope systematics. Geochimica Et Cosmochimica Acta, 1–19. http://doi.org/10.1016/j.gca.2021.05.030
    Journal
    Geochimica Et Cosmochimica Acta
    Rights
    © 2021 Elsevier Ltd. All rights reserved.
    Collection Information
    This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu.
    Abstract
    The isotopic composition of the detrital sediment record harbours a valuable proxy for estimating the composition of the erodible upper crust since the Archaean. Refractory elements such as titanium (Ti) and zirconium (Zr) can display systematic variations in their isotopic composition as a result of magmatic differentiation. Hence, for such elements, the isotope composition of detrital sediments could potentially be used to infer the average composition (e.g., SiO2 content) of their source region, even when elemental systematics are obfuscated by weathering and diagenetic processes. A key premise of this approach is that the isotopic composition of sediments remains unbiased relative to their protolith. To what extent isotopic fractionation can occur during sedimentary processes, notably the hydrodynamic sorting of heavy mineral assemblages with contrasting isotopic compositions, remains poorly understood. We investigate the effects of such processes on the Ti and Zr isotope composition of a suite of detrital sediments from the Eastern Mediterranean Sea (EMS). These sediments are binary mixtures of two main provenance components, Saharan dust and Nile sediment, with strongly contrasting mineralogical and geochemical signatures. The EMS sediments display clear evidence for hydrodynamic sorting of zircon, expressed as a large variation in Zr/Al2O3 and deviation of εHf relative to the terrestrial εNd-εHf array. Our new data, however, do not show pronounced Zr isotope variation resulting from either hydrodynamic sorting of zircon or sediment provenance. Although this agrees with theoretical models that predict negligible equilibrium zircon-melt Zr isotope fractionation, it contrasts with recent observations suggesting that kinetic Zr isotope fractionation might be a common feature in igneous rocks. For the EMS sediments, the negligible shift in Zr isotope composition through hydrodynamic sorting means that fine-grained samples accurately reflect the composition of their source. The nearly overlapping Zr isotope compositions of Sahara- and Nile-derived sediment, however, means that Zr isotopes, in this case, have insufficient resolution to be a useful provenance proxy. Titanium behaves differently. A small but resolvable, systematic difference in Ti isotope composition is observed between the Sahara and Nile provenance components. Samples with a strong Saharan dust signature show some Ti isotope evidence for hydrodynamic sorting of oxides in tandem with zircon, but a much stronger effect is inferred for Nile sediment. Regression of the EMS sediment samples shows that the Ti isotope composition of the Nile-derived component is strongly fractionated compared to its protolith, the Ethiopian flood basalts. Whereas Ti in Nile sediment is carried in essentially unmodified concentration, and by inference isotope composition, from its sources to the delta, large-scale hydrodynamic sorting of Fe-Ti oxides occurs in the delta. This process decreases TiO2/Al2O3 of the residual fine-grained sediment fraction and shifts its Ti isotope composition to heavier compositions. The potential of such an “oxide effect” in detrital sediments has implications for crustal evolution models that use Ti isotopes as a proxy for the proportion of felsic crust and can account for the observed scatter in the shale record.
    Note
    24 month embargo; available online 21 May 2021
    ISSN
    0016-7037
    DOI
    10.1016/j.gca.2021.05.030
    Version
    Final accepted manuscript
    Sponsors
    National Science Foundation EAR-2131632
    Additional Links
    https://www.sciencedirect.com/science/article/abs/pii/S0016703721003033
    ae974a485f413a2113503eed53cd6c53
    10.1016/j.gca.2021.05.030
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