85 Ma volcanism on the Chatham Islands – A window into the geodynamic and geochemical evolution of Zealandia

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The Earth’s mantle is compositionally heterogeneous and exhibits geochemical heterogeneities at a variety of length scales, which were formed by continuous upper mantle depletion through melt extraction and re-enrichment by subducted and delaminated continental and oceanic materials. These ongoing processes form geochemical domains, but the distribution in the Earth is still poorly constrained. The Zealandian microcontinent, for example, is characterized by short-term intermittent HIMU-type volcanism over the last 100 Ma. HIMU (high time-integrated μ=238U/204Pb) is one of the four proposed mantle end members, which can describe the entire geochemical spectrum in multiple isotope space. There is a great debate as to whether the HIMU-type signature in Zealandia derives from the subcontinental lithospheric mantle (SCLM), from a shallow asthenospheric or deep (plume) source and consequently several models have been proposed, including subduction-zone-related metasomatism of the SCLM, decompression melting of the upper upwelling HIMU-type asthenosphere, or a mantle plume, which could have caused the break-up of Zealandia from West Antarctica. Recent studies of Zealandian intraplate lavas, reveal a more complex geochemical evolution. Using multiple isotope ratios (Sr-Nd-Pb-Hf) we could demonstrate a temporal shift from Cretaceous St. Helena HIMU end member composition to more “depleted HIMU-like” Cenozoic volcanism, which are most likely derived from distinct sources. To date, the temporal geochemical evolution of Zealandia has been reconstructed using numerous spatially-isolated localities, which are affected by distinct geochemical imprints from the highly heterogeneous lithosphere. The poorly sampled Chatham Islands represent the only known locality within Zealandia where volcanic activity has taken place over ≥80 Ma and additionally record the geochemical transition from Cretaceous St. Helena HIMU to Cenozoic HIMU-like volcanism. The Chatham Islands volcanism represents, therefore, an excellent opportunity to characterize the Zealandian volcanism from the Late Cretaceous Gondwana break-up and throughout the ~4000 km northwestward journey of the Zealandian microcontinent.The aim of the propose project is the generation of a geochemical (mineral chemistry, oxygen isotope and whole rock major/trace element and radiogenic Sr-Nd-Pb-Hf isotope) and geochronological (Ar-Ar ages) data set to determine the temporal and geochemical evolution of the previously poorly sampled Chatham Islands volcanism. In a large-scale geodynamic context, these results will provide crucial information about long-lived intraplate volcanism, the formation of both the HIMU mantle end member and HIMU-like compositions, the possible involvement of a mantle plume in the final break-up of Zealandia from Antarctica and the origin of post-breakup volcanism occurring on the Chatham Islands and Zealandia throughout the Cenozoic.