UC Santa Barbara

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 measure 87Sr/86Sr ratios, major-element, and trace-element concentrations in the same plagioclase crystal volume simultaneously. We find that two plagioclase crystals have extreme 87Sr/86Sr heterogeneity in excess of 5000 ppm. This range is over an order of magnitude greater than the reproducibility achieved for 87Sr/86Sr reference materials, including a homogeneous natural plagioclase crystal (± 350 ppm, 2 SD). 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.

When 87Sr/86Sr is plotted against Sr, the six plagioclase crystals form arrays that converge on a “common component” with a more extreme EM2 signature (i.e., with higher 87Sr/86Sr) than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivines (mean 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.

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 7.4 ± 0.2% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment reservoir that constitutes 0.02% of the mantle’s mass, a much lower concentration 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 an estimated 7.7% of the mantle’s mass), only 0.25% of the LLSVPs are composed of sediment. This suggests the distribution of 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.