Timing of water plume eruptions on Enceladus explained by interior viscosity structure

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Helium and lead isotopes reveal the geochemical geometry of the Samoan plume

Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume4-10. Here we present a unique relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. In Pb-isotopic space, the Samoan data form several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has signatures associated with one of four mantle endmembers with low 3He/4He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high μ=238U/204Pb) and DM (depleted mantle). Critically, the four isotopic-geographic groups converge on a common region of Pb-isotopic space with high 3He/4He. This observation is consistent with several low 3He/4He components in the plume mixing with a common high 3He/4He component, but not significantly with each other, otherwise the four isotopic groups would be obscured by mixing. The mixing relationships inferred from the new He and Pb isotopic data paint the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high 3He/4He plume matrix that hosts, and mixes with, several distinct low 3He/4He components

Combined iron and magnesium isotope geochemistry of pyroxenite xenoliths from Hannuoba, North China Craton: implications for mantle metasomatism

© 2017, Springer-Verlag Berlin Heidelberg. We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ 57 Fe = −0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ 26 Mg = − 0.28 to −0.25 with an average of −0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ 57 Fe ranging from 0.12 to 0.30. The δ 57 Fe values of minerals in these garnet-bearing pyroxenites also vary widely (−0.25 to 0.08 in olivines, −0.04 to 0.25 in orthopyroxenes, −0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ 26 Mg (−0.43) relative to the mantle. The δ 26 Mg of minerals in the garnet-bearing pyroxenite range from −0.35 for olivine and orthopyroxene, to −0.34 for clinopyroxene, 0.04 for spinel and −0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ 57 Fe (as high as 1.00) but the lightest δ 26 Mg (as low as −1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ 57 Fe and δ 26 Mg in the Hannuoba garnet-bearing pyroxenite and in the phlogopite clinopyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle