NOBLE-GAS SYSTEMATICS OF REUNION ISLAND, INDIAN-OCEAN

Noble gas concentrations and isotopic ratios have been analyzed in a suite of 40 samples from Reunion Island by crushing and stepwise heating. Olivine phenocrysts came from recently erupted oceanites, collected from Piton de la Fournaise as well as from 0.5- to approximately 2-Ma-old eruptions from Piton des Neiges; xenolith nodules came from the Chisny crater, close to the Piton de la Fournaise; glass samples came from small seamounts from the Fournaise II region, east of Reunion. The Reunion isotopic noble gas signature "Re" is best preserved in the olivine phenocrysts and is characterized as follows: He-4/He-3 = 55,400 +/- 3,100, Ne-20/Ne-22 = 9.90 +/- 0.11, Ne-21/Ne-22 = 0.0292 +/- 0.0002, Ar-40/Ar-36 = 450 +/- 70 and air-type Kr and Xe. These ratios and the systematics of the data are consistent with an origin of the noble gases by mixing from lower-and upper-mantle material and/or old subducted crustal material. There is no measurable change in the He-4/He-3 ratio between 2 Ma and present. The xenolith data are somewhat confusing since they have noble gas ratios that are consistent neither with a simple upper-mantle origin nor an origin in the source region of Reunion magma. Xenolith and olivine phenocryst He-4/He-3 ratios are identical but xenolith Ar-40/Ar-36 ratios of 1600-7970 are higher than phenocryst ratios, and would seem to imply an origin from the depleted upper mantle. The xenolith Ne isotopic ratios Ne-20/Ne-22 and Ne-21/Ne-22 are high and fall between those of typical MORB and the mass discrimination line. To resolve this contradiction, we propose a combination of mixing of upper-mantle-derived material and Reunion magma, and a diffusion process, with fractionation of the noble gases due to different diffusion coefficients.