Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks
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Indirect evidence for the presence of a felsic continental crust, such as the elevated 49Ti/47Ti ratios in Archean shales, has been used to argue for ongoing subduction at that time and therefore plate tectonics. However, rocks of intermediate to felsic compositions can be produced in both plume and island arc settings. The fact that Ti behaves differently during magma differentiation in these two geological settings might result in contrasting isotopic signatures. Here, we demonstrate that, at a given SiO2 content, evolved plume rocks (tholeiitic) are more isotopically fractionated in Ti than differentiated island arc rocks (mainly calc-alkaline). We also show that the erosion of crustal rocks from whether plumes (mafic in average) or island arcs (intermediate in average) can all produce sediments having quite constant 49Ti/47Ti ratios being 0.1–0.3 per mille heavier than that of the mantle. This suggests that Ti isotopes are not a direct tracer for the SiO2 contents of crustal rocks. Ti isotopes in crustal sediments are still a potential proxy to identify the geodynamical settings for the formation of the crust but only if combined with additional SiO2 information.
Deng , Z , Chaussidon , M , Savage , P , Robert , F , Pik , R & Moynier , F 2019 , ' Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks ' Proceedings of the National Academy of Sciences of the United States of America , vol. 116 , no. 4 , pp. 1132-1135 . https://doi.org/10.1073/pnas.1809164116
Proceedings of the National Academy of Sciences of the United States of America
Copyright © 2018 Deng et al. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1073/pnas.1809164116
DescriptionF.M. acknowledges funding from the European Research Council (ERC) under Horizon 2020 Framework Programme/ERC Grant Agreement 637503 (Pristine). F.M. and M.C. acknowledge the financial support of the UnivEarthS Labex Program at Sorbonne Paris Cité (Grants ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP Plateau d’Analyse haute Résolution (PARI) and by Region Île-de-France Sesame Grant 12015908.
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