Extremely high He isotope ratios in MORB-source mantle from the proto-Iceland plume

• Published in: Nature
• Main Authors: Stuart, F.M., Lass-Evans, S., Fitton, J.G., Ellam, R.M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Nature Publishing Group 2003
• Subjects: QE Geology; Baffin Island; Canada; Baffin; Iceland; Ocean Island
• Online Access: http://eprints.gla.ac.uk/284/; http://eprints.gla.ac.uk/284/1/Stuart%5B1%5D.pdf; https://doi.org/10.1038/nature01711

The high 3 He/ 4 He 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 3 He/ 4 He ratios yet recorded. A strong correlation between 3 He/ 4 He and 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and trace element ratios demonstrate that the 3 He-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 3 He/ 4 He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a 3 He-recharged depleted mantle (HRDM) reservoir may be the principal source of high 3 He/ 4 He 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.