Is Iceland underlain by a plume in the lower mantle? Seismology and heliumisotopes

Tomographic images reveal an apparent fundamental disagreement in the inter pretations of seismic data pertaining to the depth of the source of lavas e rupted in the: Iceland region and the assumptions in helium geochemistry mo delling. Four recent independent tomography experiments image a major, stro ng, low-wave-speed anomaly in the upper mantle beneath Iceland that does no t continue down into the lower mantle, confirming earlier studies. On the o ther hand. some He-3/He-4 ratios measured in volcanic rocks from the Icelan d region are amongst the highest on Earth. Elevated He-3/He-4 ratios are co nventionally viewed as resulting from excess He-3 from a little-degassed, p rimitive reservoir, often assumed to be in the lower mantle, and a high He- 3/He-4 ratio is regarded as the most powerful geochemical indicator of a lo wer mantle plume. Suggested explanations for this disagreement include a mo del whereby material is transported up from the lower mantle by a structure that is too small to be detected by seismic tomography, and a model whereb y high He-3/He-4 ratios arise from the upper mantle. These results have sig nificant implications for models of plumes elsewhere.