Escape of unradiogenic osmium during sub-aerial lava degassing: Evidence from fumarolic deposits, Piton de la Fournaise, Réunion Island

Osmium isotopic compositions of samples collected at medium and low temperature (<350 °C) are very uniform and unfractionated (187Os/188Os between 0.130-0.135) and plot within the range of the April 2007 lava flow and the historical lavas of Piton de la Fournaise (i.e. 0.130-0.137). However the highest temperature condensates (Na-K sulfates with T of 384-400°C) yield lower 187Os/188Os ratios (i.e. 0.124-0.129) within the field of mantle signal. Such unradiogenic compositions are best explained if old mantle sulfides occur in lavas and contribute to volcanic gases. Within the general frame of osmium mantle geochemistry, loss of unradiogenic Os during magmas degassing could help to explain osmium isotope disequilibrium between lavas and melting residues.