Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs

Abstract

Abstract Oxygen isotopes are useful tracers of silicate melt generation processes because of the relatively constant abundance of oxygen in silicate reservoirs and the large isotopic fractionation that can occur between 18O and 16O during low ( 350 °C) temperature alteration processes at Earth's surface. Studies of oceanic island basalts (OIB) have demonstrated the important role of assimilation of hydrothermal altered crust on 18O/16O ratios, as well as evidence that some OIB mantle sources contain recycled oceanic or continental crust and lithosphere based on correlations between oxygen and radiogenic isotopes. To further investigate how oxygen isotope signatures may be used as tracers in intraplate volcanic rocks, we report olivine compositions from South African olivine melilitites. Olivine melilitites are considered to be related to Group 1 kimberlites and form from asthenospheric melting beneath mature oceanic islands or under off-craton continental lithosphere. South African olivine melilitites also exhibit radiogenic isotopic signatures similar to high-μ (HIMU; high-238U/204Pb) OIB, suggesting sources containing subducted oceanic lithosphere. Olivine from South African melilitites has trace element compositions that are consistent with a magmatic origin from a HIMU-type mantle melt and have a remarkably restricted range in primary 18O/16O ratios (δ18O = 4.99–5.26‰; Average = 5.14 ± 0.17‰, 2σ) that are within the mantle olivine range (δ18O = 5.2 ± 0.3‰). These compositions indicate that South African olivine melilitites require a HIMU mantle source with the oxygen isotope characteristics of ambient peridotite mantle and can be explain through either: (1) intra-mantle differentiation processes that fractionate U(and Th) from Pb, but not 18O/16O ratios, or (2) a dominantly peridotitic source with HIMU-like trace-element and radiogenic isotope characteristics inherited from equilibration and remixing of ancient recycled oceanic lithosphere. In contrast, some HIMU ocean island basalts require mantle sources with low-δ18O, indicating that they originate from distinct recycled mantle lithologies (e.g., pyroxenite/eclogite).