Extreme isotopic heterogeneity in Samoan clinopyroxenes constrains sediment recycling

Lavas erupted at hotspot volcanoes provide evidence of mantle heterogeneity. Samoan Island lavas with high ⁸⁷Sr/⁸⁶Sr (>0.706) typify a mantle source incorporating ancient subducted sediments. To further characterize this source, we target a single high ⁸⁷Sr/⁸⁶Sr lava from Savai'i Island, Samoa for detailed analyses of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopes and major and trace elements on individual magmatic clinopyroxenes. We show the clinopyroxenes exhibit a remarkable range of ⁸⁷Sr/⁸⁶Sr-including the highest observed in an oceanic hotspot lava-encompassing ~30% of the oceanic mantle's total variability. These new isotopic data, data from other Samoan lavas, and magma mixing calculations are consistent with clinopyroxene ⁸⁷Sr/⁸⁶Sr variability resulting from magma mixing between a high silica, high ⁸⁷Sr/⁸⁶Sr (up to 0.7316) magma, and a low silica, low ⁸⁷Sr/⁸⁶Sr magma. Results provide insight into the composition of magmas derived from a sediment-infiltrated mantle source and document the fate of sediment recycled into Earth's mantle.

LA-ICP-MS Analysis of Clinopyroxenes in Basaltic Pyroclastic Rocks from the Xisha Islands, Northwestern South China Sea

Cenozoic volcanic rocks were recently discovered during full-coring kilometer-scale major scientific drilling in the Xisha Islands, northwestern South China Sea. A systematic mineralogical study of these samples was performed for this paper. The results show that the volcanic rock samples are basaltic pyroclastic. The major elements demonstrate that the clinopyroxenes are diopside and fassaite, which contain high Al2O3 (5.33–11.2 wt. %), TiO2 (2.13–4.78 wt. %) and CaO (22.5–23.7 wt. %). Clinopyroxenes have high REE abundances (104–215 ppm) and are strongly enriched in LREE (LREE/HREE = 3.56–5.14, La/YbN = 2.61–5.1). Large-ion lithophile elements show depleted characteristics. Nb/Ta shows obvious fractionation features: Nb is lightly enriched, relative to primitive mantle, but Ta is heavily depleted, relative to primitive mantle. The parental magma of the basaltic pyroclastic rocks belongs to a silica-undersaturated alkaline series, characterized by a high temperature, low pressure, and low oxygen fugacity. The AlIV content increases with decreasing Si concentration. The Si-unsaturated state causes Si-Al isomorphic replacement during the formation of clinopyroxene. The electric charge imbalance caused by the replacement of Si by Al is mainly compensated by Fe3+. The clinopyroxene discrimination diagrams show that the parental magma formed in an intraplate tectonic setting environment.