A global review of Hf-Nd isotopes: New perspectives on the chicken-and-egg problem of ancient mantle signatures

We present the first global review on the Sm-Nd and Lu-Hf isotope systematics of the mantle; it includes all published data on peridotites and pyroxenites from all tectonic settings (>1100 combined Hf-Nd analyses), as well as previous compilations for oceanic basalts and material such as oceanic and continental sediments. We first provide a comprehensive overview of the main reservoirs and mechanisms accounting for the contrasting variability of radiogenic isotope systematics in the sub-oceanic mantle and the relative homogeneity of its volcanic products, highlighting the paradigm change promoted by the use of Hf isotopes. Secondly, we summarize the different models invoked to explain the decoupling/(re-)coupling of Hf and Nd isotopes. Decoupling above the mantle array is often related to melt-peridotite interaction involving ancient protoliths, whereas coupled Hf-Nd or decoupling below the array are shown to be insufficient criteria to exclude the involvement of such protoliths. The Hf-Nd isotope variability of the SCLM is then addressed using a tectono-thermal classification based on the Global Lithospheric Architecture Mapping (GLAM) project. The extreme variability that characterizes the cratonic SCLM reflects the long-term preservation of depleted signatures overprinted by ancient and recent metasomatic episodes. Refertilized SCLM domains fingerprinted by variably decoupled Hf-Nd isotope systematics record subduction-related processes, which also appears to be instrumental in the recycling of continental material into the convective mantle. We show that there is a critical “chicken-and-egg” question underpinning debates on the spatio-temporal evolution of the SCLM: whether ancient signatures are pre-existing in the lithosphere (e.g. “lithospheric memory” during refertilization) or introduced into the convective mantle (i.e. recycling). Importantly, our compilation shows that fertile lithologies such as pyroxenites can also carry extremely depleted isotopic signatures. In particular, delamination of gravitationally unstable, pyroxenite-rich arc roots represents a volumetrically significant flux of material characterized by ancient radiogenic Hf and basalt-like Nd-isotope compositions that can, once recycled, account for the Hf variability observed between MORB suites. In this context, the characteristic HIMU-like or coupled Hf-Nd signatures observed in garnet-pyroxenite layers from orogenic peridotite massifs probably reflects long-term processing (re-coupling) of recycled lithospheric material in the convective mantle. In contrast, continental dispersal during rifting (± plume-related processes) appears to be mostly limited to buoyant SCLM remnants in the oceanic lithosphere, and these are unlikely to be recycled unless previously refertilized. This work brings a new geodynamic perspective to the ancient signatures identified as chemical and isotopic heterogeneities in the oceanic lithosphere and convective mantle. These conclusions imply that (1) subduction is the main driver of mass transfer between lithosphere and asthenosphere and (2) the long-term evolution of the Earth's mantle and crust are directly linked to convergent plate-tectonic processes, at least since the Archean.