Tectonic controls on the isotopic juvenility of mid-Cretaceous continental arcs: Global compilations and implications for chemical weathering

Abstract

Continental arcs influence the global carbon cycle via degassing and chemical weathering. Several studies also suggested that the isotopic composition of continental arcs plays an important role in controlling the strontium (Sr) isotopic composition of seawater through chemical weathering. Yet, geological or tectonic drivers of isotopic variations of continental arcs on a global scale are missing in previous studies. Here, we compiled two independent global databases for mid-Cretaceous (125-85 Ma) continental arcs. The first database maps the location, geometry, and tectonic nature of the basements upon which the arcs were built. The second compiles Sr-Nd-Hf isotopes data for these continental arcs. We find that isotopically-juvenile arcs form when their basements are generated by tectonic processes such as oceanic terrane accretion, upper plate extension, slab breakoff, and ridge subduction. During the mid-Cretaceous, most continental arcs were fully or partially built on juvenile basements and yielded juvenile isotopes on a global scale. Our compilation reinforces the importance of isotopically-juvenile continental arcs observed in the global Hf isotopes in detrital zircon for the mid-Cretaceous, but also provides the geological context for these trends. Since many of these isotopically-juvenile arcs were located in tropical or warm-temperate regions during the mid-Cretaceous, these uplifted rocks were preferentially eroded and weathered providing juvenile Sr isotopes to the oceans. This study highlights the importance of tectonic processes in controlling isotopic compositions of continental arc, which has implications for ocean chemistry, global chemical weathering feedback, and the long-term carbon cycle.