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Etude de la phase gazeuse occluse dans les laves de «l’Ardoukôba», volcan éphémère d’une dorsale émergée (Afar) |
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| Authors: | J L Zimmermann J L Cheminee M Chaigneau |
| Institution: | 1. Centre de Recherches Pétrographiques et Géochimiques, B.P. 20, 54501, Vandaeuvre-lès-Nancy Cedex 2. Laboratoire de Géologie, Ecole Normale Supérieure, 46 rue d’Ulm, 75005, Paris 3. Laboratoire des Gaz, Université René Descartes, 4 Avenue de l’Observatoire, 75006, Paris |
| Abstract: | Gases trapped in lavas of three main flows of the Ardoukôba eruption (8 to 15 November, 1978) have been analysed by mass spectrometry. These analyses concern both plagioclase phenocrysts and microcrystalline mesostasis. Fluids are released between 500°C and 1200°C, and consist of H2O, CO2, CO, N2, SO2, HCl, H2, CH4 with traces of hydrocarbons and H2S. The total content is less than 0.3–0.4 wt. % of samples with about 0.1–0.15 wt % of H2O. No significant variation among the three flows is observed. Plagioclase phenocrysts are less abundant in fluids than the mesostasis (～2/3). The gases trapped in these phenocrysts are richer in CO and organic compounds, whereas mesostasis contain more H2O, CO2 and SO2. CO is likely produced by reduction of CO2 and H2O with carbon during either analyses or eruption itself, or is of primary origin. In the latter case, gas composition suggests an entrapment temperature of about 1200°C ± 75°C. Kinetic study of the water and carbon dioxide release allows to calculate the diffusion characteristics of these fluids. Water and carbon dioxide behave rather similarly. Plagioclase gives a single activation energy value (8 Kcal/mole), while mesostasis gives two values (8 Kcal/mole, 15 Kcal/mole). Diffusion coefficients at 20°C are estimated to fall in the range 10?13 · 10?12 cm2 · sec?1. |
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