Th/U and other geochemical evidence for the Réunion plume sampling a less differentiated mantle domain

Abstract

Like Mid-Ocean Ridge Basalts (MORB), most Ocean Island Basalts (OIB) record a Th/U decrease during the history of their source. Notable exceptions are lavas sampling EM1 mantle end-member, which have super-chondritic time-integrated Th/U ([Th/U] Pb). An intermediate situation is observed for the Réunion plume, where both present-day [Th/U] and [Th/U] Pb are similar to the bulk Earth value (weight ratio ∼ 3.9). If [Th/U] measured in basalts reliably reflects the source ratio, then the Réunion plume source could have evolved in closed system with respect to Th/U. Th/U decrease in the mantle has been ascribed either to early continental crust extraction, or to preferential recycling of U over Th since oxidizing conditions appeared at the surface of the planet (∼ 2.2 Ga). The primitive-like Th/U signature of Réunion is best explained by the absence of subduction influence since at least the Archean. This possibility is consistent with the 187Os/ 186Os signature, which is the less radiogenic of ocean island shield basalts. In addition, the difference in 3He/ 4He between Réunion (R/R A ∼ 12.5) and high- 3He/ 4He plumes (Hawaii, Iceland) most likely reflects a Réunion source less depleted in U and Th, rather than more degassed. In Sr–Nd–Pb–Os–He isotope space, Réunion signature plots in the region where OIB arrays converge, suggesting that the Réunion plume samples a component common to OIB, which is neither the source of MORB, nor the region where subducted plates are stored. It is suggested that the Réunion plume taps an early-depleted mantle domain subsequently influenced little, or not at all by recycling processes.