High ~3He/~4He ratios in picritic basalts from Baffin Island and the role of a mixed reservoir in mantle plumes

摘要

The high ~3He/~4He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic ~3He/~4He ratios yet recorded. A strong correlation between ~3He/~4He and ~(87)Sr/~(86)Sr, ~(143)Nd/~(144)Nd and trace element ratios demonstrate that the ~3He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest ~3He/~4He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a ~3He-recharged depleted mantle (HRDM) reservoir may be the principal source of high ~3He/~4He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.