Deep-Mantle Water Transport into the Continental Crust: Insights from Zircon Water and Hf-O Isotopes in Granulite Xenoliths

Earth’s deep water cycle plays a fundamental role in plate tectonics, magmatism, and crustal differentiation. This study investigates the pathways and mechanisms of water transport from Earth’s interior to the continental crust by analyzing zircon water contents, U-Pb-Hf-O isotopes, and whole-rock geochemistry from granulite xenoliths in the Tuoyun Basin, Western Tianshan. We identified three distinct zircon generations: Neoproterozoic protolith zircons (~800-600 Ma) with high water contents (median: ~404 ppm), Paleozoic-Mesozoic metamorphic zircons (~600-100 Ma) showing pronounced water depletion (median: ~55 ppm), and Cretaceous-Paleogene host-basalt zircons (median: ~166 ppm). The elevated water contents and mantle-like δ¹⁸O values in the protolith and host-basalt zircons, combined with positive Nb-Ta anomalies and enriched Hf isotopes, indicate that the parental magmas were likely derived from a hydrous mantle transition zone rather than supracrustal sources. We propose a novel two-stage transport model where hydrous mantle-derived magmas first underplated the lower crust to form the protolith; subsequent granulite-facies metamorphism then dehydrated these rocks with a calculated efficiency of ~86%, releasing fluids that ascended to trigger mid-crustal water-fluxed melting and granitic magmatism. These findings provide direct geochemical evidence that granulite dehydration in the lower crust is a critical link in the deep-water cycle, facilitating the transport of mantle-derived water to the surface and driving the progressive maturation of the continental crust.

Acknowledgement: This study was financially supported by funding from the National Natural Science Foundation of China Major Project (41890831), the University of Hong Kong (HKU) Internal Grants for Member of Chinese Academy of Sciences (102009906) and for Distinguished Research Achievement Award (102010100), the Hong Kong RGC grants (JLFS/P-702/24 and 17308023), and the National Key Research and Development Program of China (2023YFF0803804).