The Chatham Islands: A window into the geochemical evolution of Zealandia

The temporal geochemical record of Zealandian alkaline intraplate volcanism reveals a significant geochemical evolution from Cretaceous HIMU (high time-integrated μ = ²³⁸U/²⁰⁴Pb Mantle) end member to Cenozoic HIMU-like volcanism with overall lower ²⁰⁷Pb/²⁰⁴Pb and variable ²⁰⁸Pb/²⁰⁴Pb ratios at a given ²⁰⁶Pb/²⁰⁴Pb ratio. In general, this temporal geochemical evolution has been reconstructed by piecing together volcanism taking place at many different localities covering different but limited age ranges. The Chatham Islands, covering an area of ~800km², represent the only known locality within Zealandia where volcanic activity has taken place nearly continuously over ~85 Ma and record the geochemical evolution from HIMU end member to HIMU-like volcanism. Therefore, the Chatham Islands are a key locality for reconstructing and understanding the geochemical evolution of Zealandia. Numerous models, such as an asthenospheric heritage (e.g. mantle plume) or metasomatic overprint of the SCLM by: 1) subduction zone fluids or 2) interaction with Hikurangi Plateau, have been proposed, but the style of geochemical change (abrupt or progressive), as well as the origin of the HIMU end member and HIMU-like sources remain enigmatic, as is the unique longevity of intraplate volcanism over ~85 Ma at a single locality. Geochemical analysis (major and trace elements, radiogenic Sr-Nd-Pb-Hf isotope ratios and mineral composition) of alkaline lavas and enclosed mantle xenoliths from the Chatham Islands, will be used to reconstruct their spatio-temporal geochemical evolution of HIMU to HIMU-like volcanism on the Chatham Islands and associated volcanism on Zealandia over the last ~85 Ma.