Mapping out the conduit of the Iceland mantle plume with helium isotopes

In order to investigate the lateral dimensions and the shallow dispersion of the Iceland mantle plume we have measured the ³He/ ⁴He ratios in closely spaced primitive (Mg#: 59-70) basaltic subglacial pillow lavas erupted along the Northern Rift Zone (NRZ) of Iceland. This detailed sampling has revealed for the first time a clear gradient in ³He/ ⁴He ratios along the NRZ, increasing from 8.0 times atmospheric ( R/ R_a) in the northernmost samples to 20.4 R/ R_a in central Iceland. In combination with previously published ³He/ ⁴He data, the new results from subglacial basaltic glasses from the neovolcanic rift zones of Iceland reveal a 'plateau' of high values ( ³He/ ⁴He ∼20 R/ R_a), which is approximately 100 km in diameter and located over the southeastern part of the island. The high ³He/ ⁴He plateau is well-defined and coincides with a regional gravity minimum and a seismic low velocity anomaly within the upper mantle beneath Iceland. We suggest that this zone of elevated ³He/ ⁴He ratios (>20 R/ R_a) outlines the width of the Iceland plume conduit at depth. ³He/ ⁴He ratios are well correlated with Nb/Th and Nb/Rb, but not well correlated with other isotope ratios (as indicated by Pb isotopes). The correlations between helium isotopes, the geophysical variations, and some extremely incompatible elements, such as Rb and Th, and the lack of correlation between He and Pb isotopes, are most likely due to preferential concentration of He into volatile-rich incipient melts, and the subsequent lateral dispersion of the plume at shallower levels.