Low-δ18O zircon xenocrysts in alkaline basalts; a window into the complex carbonatite-metasomatic history of the Zealandia lithospheric mantle

Abstract

Abstract Megacrystic zircon grains from alkaline basaltic fields are rare but can provide fundamental insights into mantle metasomatic processes. Here, we report in-situ U-Pb ages, trace element concentrations and hafnium and oxygen isotopes for fourteen zircon megacrysts from two intraplate alkaline basalt locations in New Zealand. U-Pb ages indicate the zircons crystallised between 12.1 and 19.8\u202fMa. Zircon oxygen isotopic compositions range from low to mantle-like compositions (grain average δ18O\u202f=\u202f3.8–5.1‰). Hafnium isotopes (eHf(t)\u202f=\u202f+3.3 to +10.4) mostly overlap with intraplate mafic rocks and clinopyroxene in metasomatized peridotitic mantle xenoliths but show no correlation with most trace element parameters or oxygen isotopes. The zircons are interpreted to have formed by the reaction between low-degree melts derived from pre-existing mantle metasomes and the depleted mantle lithosphere prior to eruption and transport to the surface. The low Hf concentration, an absence of Eu anomalies, and elevated U/Yb compared to Nb/Yb in the megacrystic zircons are interpreted to show that the source metasomes comprised subduction- and carbonatite-metasomatised lithospheric mantle. As these trace element characteristics are common for megacrystic zircon in intra-plate basaltic fields globally, they suggest the prevalence of subduction- and carbonatite-metsasomatised mantle under these intraplate volcanic regions. The unusually low δ18O was likely present prior to metasomatic enrichment and may have resulted from high-temperature hydrothermal alteration during initial mantle lithosphere formation at a mid ocean ridge or, possibly, during subduction-related processes associated with continent formation. The combination of proportionally varied contributions from carbonatite- and subduction-metasomatised lithospheric melts with asthenospheric melts may explain the variety of primitive intraplate basalt compositions, including low δ18O reported for some local intraplate lavas.