Extinct radiogenic isotope constraints on mantle dynamics

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Over the past years, mantle geochemistry has been spurred by new discoveries of small but widespread isotope anomalies of radiogenic 182W and 142Nd in rocks of Archean and Cenozoic age that are decay products of long extinct 182Hf and 146Sm (half lifes of 9 Myrs and 103 Myrs, respectively). In a geodynamic context, these new findings imply that vestiges of Hadean (> 4.0 Ga) differentiation processes on Earth have survived in the mantle throughout Earth history, requiring a change of the long accepted paradigm that Earth’s mantle has been efficiently stirred through geologic time by plate tectonics. Despite these new findings, however, the exact origins of the 182W and 142Nd isotope anomalies and their evolution through Earth history have remained ambiguous. Popular explanations include incomplete addition of the early late veneer, continuous core-mantle interaction, magma ocean relics or vestiges of early crust-mantle differentiation. There is also a large time gap in the available 182W-142Nd record ranging from the late Archean to the Cenozoic and combined 182W-142Nd datasets are scarce. We propose a combined high precision 182W and 142Nd study on selected mafic rock suites from Archean to Phanerozoic age to close this time gap and to evaluate the contrasting hypotheses above. Such measurements are at the analytical limits, and these will be pushed further by developing a new protocol for 142Nd measurements by Multicollector ICPMS. The measurements will be complemented by major/trace element, radiogenic isotope, highly siderophile element (mafic samples) and noble gas data (OIBs). These data will particularly serve to elucidate the influences of mantle lithosphere, crustal contamination and missing late veneer contributions on the 182W inventory. Many of these complementary data are already available for our samples from precursor or companion projects. Our combined 182W-142Nd measurements will cover hitherto unexplored key Archean to early Proterozoic units from various cratons as well as Late Proterozoic to Cenozoic mafic rocks from various settings, focusing on OIBS and LIPs that are all tapping various mantle domains. The results will not only provide new clues on the origin of these isotope anomalies, but will also have general significance for understanding mantle dynamics through geologic time, including the origin and evolution of lower mantle heterogeneities such as LLSVPs and ULVZs and the origin of plume-derived magmatism.