Extreme enriched and heterogeneous ⁸⁷Sr/⁸⁶Sr ratios recorded in magmatic plagioclase from the Samoan hotspot

We report the major-element, trace-element, and 87Sr/86Sr compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock 87Sr/86Sr of 0.718592, and ALIA-115-21 with whole-rock 87Sr/86Sr of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to simultaneously measure 87Sr/86Sr ratios, major-element concentrations, and trace-element concentrations in the same plagioclase crystal volume. We find that two plagioclase crystals have extreme 87Sr/86Sr heterogeneity in excess of 5000 ppm (where ppm of 87Sr/Sr variability86=106⋅[Sr/8687Srmax−87Sr/86Srmin]/87Sr/86Sravg). In two of the plagioclase crystals, we identify the highest 87Sr/86Sr ratios (0.7224) ever measured in any fresh, mantle-derived ocean island basalt (OIB) or OIB-hosted mineral phase.We find that in 87Sr/86Sr-versus-Sr concentration space, the six plagioclase crystals overlap in a “common component” region with higher 87Sr/86Sr than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivine mineral inclusions (Fo=74.5±0.8, 2 SD) in the high-87Sr/86Sr zone of one plagioclase crystal to infer the bulk composition (Mg#=46.8±0.8, 2 SD) of the extreme EM2 magma from which the olivine and high-87Sr/86Sr plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower-87Sr/86Sr melt to generate the observed intra-crystal plagioclase isotopic heterogeneity.By inferring that subducted terrigenous sediment gives rise to EM2 signatures in Samoan lavas, we estimate that the quantity of sediment necessary to generate the most-elevated 87Sr/86Sr ratios observed in the Samoan plagioclase is ∼7% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment domain that constitutes 0.02% of the mass of the mantle, a much lower proportion than required in the EM2 mantle source. Even if subducted sediment is concentrated in large low-shear-velocity provinces (LLSVPs) at the base of the mantle (which constitute up to 7.7% of the mantle's mass), then only 0.25% of the LLSVPs are composed of sediment. This requires that the distribution of subducted sediment in the mantle is heterogeneous, and the high relative abundance of sediment in the Samoan EM2 mantle is an anomalous relic of ancient subduction that has survived convective attenuation