Rubidium isotopic compositions of the mantle sources of ocean island basalts

Abstract

Ocean island basalts (OIB) offer valuable insights into the chemical composition of Earth’s mantle. Distinct radiogenic isotope compositions recorded in OIB have been used to characterize diverse mantle sources, indicating the presence of recycled crustal components within plume sources. Rubidium isotopes have the potential to trace crustal recycling, given the significant enrichment of Rb in the crust relative to the mantle, and the inherent variability of Rb isotopes among crustal materials. Here we present Rb isotope data of twenty-eight OIB and two peridotite xenoliths representing EM1, EM2, HIMU, and FOZO mantle, with locations spanning the Pacific, Atlantic, and Indian Oceans. Except for one sample from Mangaia, which was affected by low-temperature alteration, the Rb isotopic compositions of remaining samples reflect the characteristics of their mantle source, exhibiting a δ⁸⁷Rb range of variations from −0.19 to +0.01 ‰. To a first order, the Rb isotopic similarity between OIB, peridotite xenoliths, and previously reported mid-ocean ridge basalts (MORB) suggests a relatively homogeneous Rb isotope composition of the mantle, displaying an average δ⁸⁷Rb value of −0.12 ± 0.09 ‰ (2SD, n = 52). Nevertheless, a discernible difference exists in primary lava compositions from Mangaia, with their Rb isotopic compositions (−0.02 ± 0.06 ‰, 2SD, n = 3) being 0.1 ‰ heavier than other OIB samples and the Bulk Silicate Earth (BSE) estimate. The heavy Rb isotope enrichment in Mangaia, the most extreme (highest ²⁰⁶Pb/²⁰⁴Pb) HIMU-type OIB examined in this study, is attributed to incorporation of high δ⁸⁷Rb signal within the mantle source, potentially due to the deep recycling of ancient altered oceanic crust.