Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes

• Published in: Lithos
• Main Authors: Petersson, Andreas, Bjärnborg, Karolina, Scherstén, Anders, Gerdes, Axel, Næraa, Tomas
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2017
• Subjects: Geology; Depleted mantle; Fennoscandia; Mantle enrichment; convergent margin; U–Pb–Hf
• Online Access: https://lup.lub.lu.se/record/3f90b676-48c4-4c0d-88d5-636abad3a951; https://doi.org/10.1016/j.lithos.2017.04.010

Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle. Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.