GEOCHEMISTRY OF GAS EMANATIONS - A CASE-STUDY OF THE REUNION HOT-SPOT, INDIAN-OCEAN

The results of analysis of natural emanations in Reunion Island show a clear magmatic origin for CO2 and He, while N2 and Ar arc predominantly derived from the atmosphere. The distribution of magmatic gases in the Piton des Neiges massif fits the local volcanotectonic context well and suggests that the areas concerned are still subject to volcanic activity at depth. A simple method is proposed for correcting gas concentration and isotope composition for water degassing. In doing so, the isotope and elemental (C, He) composition of gases is homogeneous for the two volcanoes. The isotope ratio of He (12.5 +/- 0.5 R/Ra) in the present discharges is in agreement with the results of previous studies on rocks of various ages from the two volcanoes. The isotope ratio of C (delta C-13 = -5 parts-per-thousand to -4 parts-per-thousand vs PDB) and the C/3 He ratio (approximately 4 x 10(9)) are similar to those found in other Hot Spot volcanic systems such as Kilauea (Hawaii) and Hengill (Iceland). These similarities suggest comparable volatile history for the respective mantle sources, the main differences being in the relative proportions of radiogenic He-4. In detail, Hot Spots appear enriched in C having a light composition with respect to MORB, possibly due to the addition of a C-rich (e.g. subducted) component, in addition to a relatively undegassed, He-3-rich, component.