Iron isotopic compositions of HIMU Ocean island basalts: Implications for the mantle source lithology

Abstract

Basalts from the islands of Mangaia, Tubuai, Rurutu (old stage), and Raivavae (Rairua stage) in the South Pacific, and from St. Helena Island in the South Atlantic, are characterized by extremely radiogenic Pb isotopic compositions (e.g., 206Pb/204Pb > 20.5). These ocean island basalts (OIBs) define the famous HIMU (high μ, where μ = 238U/204Pb) component in the mantle ‘zoo’, which has traditionally been thought to represent ancient recycled oceanic crust with highly fractionated U/Pb and Th/Pb ratios. However, recent high-precision analyses of minor and trace elements in olivine phenocrysts from HIMU OIBs suggest a peridotitic mantle source rather than pyroxenitic/eclogitic remnants of subducted oceanic crust. To better constrain the lithology of the HIMU source is crucial for further understanding the nature of the HIMU component. Here we utilize stable iron (Fe) isotopes, a novel tool for identifying the mantle source lithology of basalts, to further investigate the lithology of the HIMU source by examining classic HIMU OIBs from the Cook-Austral volcanic chain in the South Pacific. The results show that these OIBs have δ57Fe values varying from 0.09‰ to 0.18‰, which are similar to those of normal mid-ocean ridge basalts (N-MORBs, δ57Fe = 0.15 ± 0.05‰, 2SD). Quantitative calculations indicate that the fractional-crystallization-corrected δ57Fecorr values (0.06–0.15‰) of these basalts can well be explained by partial melting of garnet peridotite with δ57Fe values ranging from 0.05‰ to 0.09‰. Such low δ57Fecorr values, however, are difficult to be reproduced by partial melting of eclogite with a MORB-like δ57Fe value (δ57Fe = 0.15‰). Our new Fe isotope data, combined with previously reported heavy whole-rock zinc isotopes (δ66Zn = 0.38 ± 0.03‰) as well as high Mn/Fe (100Mn/Fe = 1.5–1.7) and Ca/Al (mostly >15) ratios of olivine phenocrysts in the HIMU OIBs, further confirm that the lithology of the HIMU mantle source is carbonated peridotite.