Mantle Plumes and Entrainment: Isotopic Evidence

Many oceanic island basalts show sublinear subparallel arrays in Sr-Nd-Pb isotopic space. The depleted upper mantle is rarely a mixing end-member of these arrays, as would be expected if mantle plumes originated at a 670-kilometer boundary layer and entrained upper mantle during ascent. Instead, the arrays are fan-shaped and appear to converge on a volume in isotopic space characterized by low $^{87}$Sr/$^{86}$Sr and high $^{143}$Nd/$^{144}$Nd, $^{206}$Pb/$^{204}$Pb, and $^3$He/$^4$He ratios. This new isotopic component may be the lower mantle, entrained into plumes originating from the core-mantle boundary layer.