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

In order to investigate the lateral dimensions and the shallow dispersion o f the Iceland mantle plume we have measured the He-3/He-4 ratios in closely spaced primitive (Mg#: 59-70) basaltic subglacial pillow lavas erupted alo ng the Northern Rift Zone (NRZ) of Iceland. This detailed sampling has reve aled for the first time a dear gradient in He-3/He-4 ratios along the NRZ, increasing from 8.0 times atmospheric (R/R-a) in the northernmost samples t o 20.4 R/R-a in central Iceland. In combination with previously published H e-3/He-4 data, the new results from subglacial basaltic glasses from the ne ovolcanic rift zones of Iceland reveal a 'plateau' of high values (He-3/He- 4 similar to 20 R/R-a), which is approximately 100 km in diameter and locat ed over the southeastern part of the island. The high He-3/He-4 plateau is well-defined and coincides with a regional gravity minimum and a seismic lo w velocity anomaly within the upper mantle beneath Iceland. We suggest that this zone of elevated He-3/He-4 ratios (> 20 R/R-a) outlines the width of the Iceland plume conduit at depth. He-3/He-4 ratios are well correlated wi th Nb/Th and Nb/Rb, but not well correlated with other isotope ratios (as i ndicated by Pb isotopes). The correlations between helium isotopes, the geo physical variations, and some extremely incompatible elements, such as Rb a nd Th, and the lack of correlation between He and Pb isotopes, are most lik ely due to preferential concentration of He into volatile-rich incipient me lts, and the subsequent lateral dispersion of the plume at shallower levels . (C) 2000 Elsevier Science B.V. All rights reserved.