EXTREME TEMPORAL HOMOGENEITY OF HELIUM-ISOTOPES AT PITON-DE-LA-FOURNAISE, REUNION ISLAND

OCEAN island basalts (OIBs) have strontium, neodymium and lead isotopic compositions that are different from those of mid-ocean-ridge basalts, (MORBs), reflecting long-term differences in the chemical characteristics of the respective mantle source reservoirs. The high 3He/4He ratios at some islands such as Hawaii and Iceland1-7 indicate that these basalts come from sources that are less degassed than the source of MORB. Many islands exhibit considerable variability in Sr, Nd and Pb isotopes8-10, but detailed studies of temporal variations in helium isotopes have been restricted to Hawaiian volcanoes - at Mauna Loa, for example, significant variations in 3He/4He have been found for the past 30,000 years11. Here we report on 3He/4He ratios from Piton de la Fournaise volcano on Reunion Island. No variations are found over the long time of 360,000 years, indicating a remarkable uniformity of 3He/4He for the (large) mantle source region over this timescale. The He-Sr-Pb systematics at this island may reflect the simultaneous contribution of both recycled materials (perhaps subducted crust) and primitive components to the Réunion source. © 1990 Nature Publishing Group.