Geochemistry

Helium Tracks the Heart of Iceland's Plume

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Science  10 Mar 2000:
Vol. 287, Issue 5459, pp. 1713
DOI: 10.1126/science.287.5459.1713b

Beneath Iceland's snow and ice are many active volcanoes created by the Mid-Atlantic Ridge rift zone and a hot spot plume. Geophysical evidence, such as gravity and seismic tomography, has distinguished the hot spot plume as a cylinder of upwelling, hot mantle material centered beneath southeastern Iceland. Because the basalts that erupt from the volcanoes can derive from the rift zone, the plume, or a mixture of the two, geochemical evidence can be used to refine or confirm the geophysical models.

Breddam et al. used the ratio of 3He/4He to distinguish basalts derived solely from the plume. This ratio should be higher for plume basalts because the plume samples the deeper, more primitive and undegassed part of the mantle where more 3He is trapped. Indeed, the helium ratio increased in a systematic way as they sampled basalts nearer to the center of the geophysically defined plume, indicating that the plume is about 100 kilometers in diameter and centered under southeastern Iceland. The high helium ratios represent melt formed in the deeper parts of the plume (below 125 kilometers) and brought directly up to the surface through the center of the plume cylinder. Moderately high helium ratios that have been measured outside of the plume center by other research groups represent denser, less 3He-enriched melts formed in the shallower parts of the plume that have been deflected laterally away from the heart of the hot spot and that may also have mixed with the rift zone melts.—LR

Earth Planet. Sci. Lett.176, 45 (2000).

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