Extreme isotopic heterogeneity in Samoan clinopyroxenes constrains sediment recycling

Abstract

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.

Trachytic Melt Inclusions Hosted in Clinopyroxene Offer a Glimpse Into Samoan EM2‐Endmember Melts

“…One clinopyroxene-hosted MI has two partially resorbed Fo 73 olivine crystals, suggesting interaction of the trachytic melt with a mafic magma as the olivines and the surrounding trachyte melt are not in equilibrium. This observation is consistent with two-component mixing between a mafic (olivine-bearing) magma and a much more evolved magma that has been inferred from relationships between major and trace element compositions and radiogenic isotopic ratios in the Samoan magmatic suite alluded to above (Adams et al, 2021;Edwards et al, 2019;Jackson et al, 2007). This raises several important questions: what is the origin of these high-silica MIs?…”

“…The work presented here uses the clinopyroxene-hosted melt inclusions in the ALIA-D115-18 sample to place further constraints on the composition and petrogenetic history of EM2. We emphasize that, remarkably, the range of calculated trachytic EM2 endmember model melts from Adams et al (2021) bear a striking resemblance to the trachytic melt inclusions of this study.…”

“…This lava, ALIA-D115-18, was dredged off the submarine flanks of Savai'i Island, western Samoa, and similar to other lavas from the same dredge haul-(ALIA115 dredge haul) showing the most geochemically enriched 87 Sr/ 86 Sr values ever measured in relatively fresh whole rock ocean island basalts (OIB)-this lava exhibits the "enriched mantle 2" (EM2) geochemical signature (Jackson et al, 2007). The EM2 signature is believed to be the result of addition of ancient subducted terrigenous sediment to the mantle source region (Adams et al, 2021;Edwards et al, 2019;Jackson et al, 2007;White & Hofmann, 1982). However, important questions remain regarding the composition of EM2 or its source, including the precise composition of the EM2 endmember, the petrogenetic history of EM2-derived OIB melts, and how these melts mix with non-EM2 melts in magma chambers.…”

“…Of course, this is dependent on the fact that the MIs have not reached equilibrium among all inclusions, which in turn is dependent on magmatic residence times. Magmatic residence time calculations, based on Fe/Mg inter-diffusion within clinopyroxenes from the ALIA-D115-18 lava, show residence times about on the order of <100 years from clinopyroxene crystallization to eruption (Adams et al, 2021). This rather short residence time is insufficient to allow incompatible and most major elements in the MIs to fully re-equilibrate with their host based on diffusion calculations.…”

“…However, important questions remain regarding the composition of EM2 or its source, including the precise composition of the EM2 endmember, the petrogenetic history of EM2-derived OIB melts, and how these melts mix with non-EM2 melts in magma chambers. A recent study by Adams et al (2021) uses binary mixing theory, high precision 87 Sr/ 86 Sr and 143 Nd/ 144 Nd analyses of single-crystal magmatic clinopyroxene, and major and trace element data of clinopyroxenes within the ALIA-D115-18 sample to calculate the EM2 magma mixing endmember as being trachytic in composition. Further, this trachytic composition is shown to represent a magma mixing endmember, that when mixed with a much more mafic endmember, can reproduce the chemical signatures observed in the entire ALIA-D115 lava suite from which ALIA-D115-18 is derived.…”