EFFECT OF SHALLOW-LEVEL CONTAMINATION ON THE HELIUM ISOTOPE SYSTEMATICS OF OCEAN-ISLAND LAVAS

THE bimodal distribution of helium isotope ratios in ocean-island lava s has provided powerful constraints on the composition and evolution o f the Earth's mantle(1-3). 'High-He-3 hotspot' ratios (He-3/He-4 great er than in mid-ocean-ridge basalt, MORB) found on Hawaii(4) and Icelan d(5) are thought to trace pristine plumes from the deep(er) mantle, wh ereas 'low-He-3 hotspot' values (He-3/ He-4 less than or equal to MORB value) at Tristan da Cunha(6) and St Helena(7) are considered to char acterize plumes composed (in part) of recycled oceanic or continental crust. Here we report the observation of both 'high-He-3' and 'low-He- 3' characteristics(8) in lavas from a single ocean island-Heard Island , in the Indian Ocean. Whereas the high-He-3 lavas provide unambiguous evidence for the involvement of a deep-seated plume in their genesis, we argue that the low He-3/He-4 ratios in other lavas result from sha llow-level contamination by radiogenic helium before eruption. These o bservations call into question the presumed association between low-He -3 ratios (at Heard Island and elsewhere) and ancient crustal material recycled back into the mantle.