Évaluation du pronostic vital des personnes âgées hospitalisées en urgence

Abstract

We have studied carbon, water, sulphur and chlorine contents in melt and fluid inclusions trapped in olivines from diverse feeding systems of Piton de la Fournaise volcano (Réunion Island, Indian Ocean). As a whole, water concentrations (0.59–1.10 wt%) are intermediate between those of tholeiites and alkali basalts. Sulphur concentrations (400–1700 ppm) are comparable to those observed in MORB whereas chlorine concentrations (200–300 ppm) are typical of alkali basalts. Carbon contents range from a few to 520 ppm C. We also observe sulphide globules that indicate saturation of the melts with sulphur compounds. The combined study of dissolved carbon contents in melt inclusions and of CO2-rich fluid inclusions indicates that the trapped melts may be saturated with a fluid phase from at least 500 MPa total pressure to the surface. The CO2 and H2O contents in the trapped melts, their major element compositions and the composition of the host olivines reveal a complex crystallisation and entrapment history. In general, we observe that the less differentiated melt inclusions appear to have been trapped at greater depth. Magmas fed to both the Summit Crater and the southeast rift zone must be stored at shallow levels before eruption to account for the observed distributions of water and carbon concentrations and their low values. These indicate that a large proportion of the volatiles that are present at greater depths are lost before venting. The North-West feeding system corresponds to magma that was much less degassed before venting. The range of concentrations of H2O and CO2 found in samples of a single eruption cannot be reconciled with continuous degassing of a single magma batch. We therefore propose that most olivines occurring in the lavas have trapped melt batches at different stages of their volatile evolution and/or that olivines included melts of differing mantle provenances. This also implies that melt entrapment occurred at different depths. Carbon and water evolution in parental magmas is calculated to be from 1270 to 2770 ppm C and from 0.7 to 1.1 wt% water, respectively. The volatile contents of parental magmas allow computation of the following volatile fluxes for the average magma production rate of 0.1 m3/s observed in the last 50 years: CO2, 88×103 tons a−1; S, 123×102 tons a−1; Cl, 2.6×102 tons a−1, and (magmatic) H2O, 62×103 tons a−1. With literature values of the degree of melting, water and carbon contents in the mantle source would be between 350 and 1100 ppm and between 63 and 277 ppm C, respectively. Because no permanent emanations nor strong degassing during eruption is observed at the surface, it is thought that most gases are intercept at an interface in the building system.