Like Mid-Ocean Ridge Basalts (MORB), most Ocean Island Basalts (0113) 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 Reunion plume, where both present-day [Th/U] and [Th/U](Pb) are similar to the bulk Earth value (weight ratio similar to 3.9). If [Th/U] measured in basalts reliably reflects the source ratio, then the Reunion 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 (similar to 2.2 Ga). The primitive-like Th/U signature of Reunion is best explained by the absence of subduction influence since at least the Archean. This possibility is consistent with the Os-187/Os-186 signature, which is the less radiogenic of ocean island shield basalts. In addition, the difference in He-3/He-4 between Reunion (R/R-A similar to 12.5) and high-He-3/He-4 plumes (Hawaii, Iceland) most likely reflects a Reunion source less depleted in U and Th, rather than more degassed. In Sr-Nd-Pb-Os-He isotope space, Reunion signature plots in the region where OIB arrays converge, suggesting that the Reunion plume samples a component common to 0113, which is neither the source of MORB, nor the region where subducted plates are stored. It is suggested that the Reunion plume taps an early-depleted mantle domain subsequently influenced little, or not at all by recycling processes. (c) 2006 Elsevier B.V. All rights reserved.
Observ Phys Globe Clermont Ferrand, UMR 6524, F-63038 Clermont Ferrand, France; Observ Phys Globe Clermont Ferrand, UMR 6524, Lab Magmas & Volcans, F-63038 Clermont Ferrand, France; Observ Phys Globe Grenoble, UMR 5025, Lab Geodynam Chaines Alpines, F-38041 Grenoble, France; Observ Volcanol Piton Fournaise, Inst Phys Globe Paris, UMR 7154, F-97418 La Reunion, FranceArticleEnglish