Edwards, M A, Jackson, M G, Kylander-Clark, A R C, Harvey, J, Hagen-Peter, G A, Seward, G G E, Till, C B, Adams, J V, Cottle, J M, Hacker, B R & Spera, F J 2019, ' Extreme enriched and heterogeneous 87 Sr/ 86 Sr ratios recorded in magmatic plagioclase from the Samoan hotspot ', Earth and Planetary Science Letters, vol. 511, pp. 190-201 . https://doi.org/10.1016/j.epsl.2019.01.040Test
We report the major-element, trace-element, and 87 Sr/ 86 Sr compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock 87 Sr/ 86 Sr of 0.718592, and ALIA-115-21 with whole-rock 87 Sr/ 86 Sr of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to simultaneously measure 87 Sr/ 86 Sr ratios, major-element concentrations, and trace-element concentrations in the same plagioclase crystal volume. We find that two plagioclase crystals have extreme 87 Sr/ 86 Sr heterogeneity in excess of 5000 ppm (where ppm of 87 Sr/Sr variability86=10 6 ⋅[Sr/8687Sr max − 87 Sr/ 86 Sr min ]/ 87 Sr/ 86 Sr avg ). In two of the plagioclase crystals, we identify the highest 87 Sr/ 86 Sr ratios (0.7224) ever measured in any fresh, mantle-derived ocean island basalt (OIB) or OIB-hosted mineral phase. We find that in 87 Sr/ 86 Sr-versus-Sr concentration space, the six plagioclase crystals overlap in a “common component” region with higher 87 Sr/ 86 Sr 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- 87 Sr/ 86 Sr 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- 87 Sr/ 86 Sr plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower- 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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.
