Gannoun A., Vlastelic I., Schiano Pierre. (2015). Escape of unradiogenic osmium during sub-aerial lava degassing : evidence from fumarolic deposits, Piton de la Fournaise, Reunion Island. Geochimica et Cosmochimica Acta, 166, p. 312-326. ISSN 0016-7037.
Titre du document
Escape of unradiogenic osmium during sub-aerial lava degassing : evidence from fumarolic deposits, Piton de la Fournaise, Reunion Island
Geochimica et Cosmochimica Acta, 2015,
166, p. 312-326 ISSN 0016-7037
This study presents new Re-Os isotope and elemental data in gas condensates and corresponding lavas in order to examine the geochemical behavior of these two elements during magma degassing at Piton de la Fournaise, Reunion Island. Gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca. 100 degrees C include Na-K sulfate (aphthitalite), Na sulfate (thenardite), Ca-Cu sulfate (e.g. gypsum), Ca-Mg-Al-Fe fluoride (e.g. ralstonite) and native sulfur. The high temperature deposits show trace element typical of volcanic gas with high enrichment in Re (24 to 79 ppb), almost two order of magnitude higher than the corresponding lavas but with Os abundances similar to those of the lavas (14-132 ppt). In contrast the Os contents of the low temperature fluoride deposits (13-77 ppb) are higher than any of the other condensates. The fluorides are also enriched in Re, albeit to lesser extent than Os (2.9-15.3 ppb). Based on high-temperature samples, the fluid/melt partition coefficients estimated for Re and Os are 100 +/- 80 and 1 +/- 2, respectively. Considering 1% of fluid loss, these partition coefficients translate into emanation coefficients of 0.50 (0.17-0.65) for Re and 0.01 (0-0.03) for Os. These results indicate that Re, unlike Os, is highly volatile at Piton de la Fournaise. Osmium isotopic compositions of samples collected at medium and low temperature (<350 degrees C) are very uniform and unfractionated (Os-187/Os-188 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 degrees C) yield lower Os-187/Os-188 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.
