A MELT AND FLUID INCLUSION STUDY OF THE GAS-PHASE AT PITON DE LA FOURNAISE VOLCANO (REUNION ISLAND)

We have studied carbon, water, sulphur and chlorine contents in melt a nd fluid inclusions trapped in olivines from diverse feeding systems o f Piton de la Fournaise volcano (Reunion Island, Indian Ocean). As a w hole, water concentrations (0.59-1.10 wt%) are intermediate between th ose of tholeiites and alkali basalts, Sulphur concentrations (400-1700 ppm) are comparable to those observed in MORE whereas chlorine concen trations (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 wit h a fluid phase from at least 500 MPa total pressure to the surface. T he CO2 and H2O contents in the trapped melts, their major element comp ositions and the composition of the host olivines reveal a complex cry stallisation and entrapment history. In general, we observe that the l ess differentiated melt inclusions appear to have been trapped at grea ter 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 thei r 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 b efore venting. The range of concentrations of H2O and CO2 found in sam ples of a single eruption cannot be reconciled with continuous degassi ng of a single magma batch. We therefore propose that most olivines oc curring in the lavas have trapped melt batches at different stages of their volatile evolution and/or that olivines included melts of differ ing mantle provenances. This also implies that melt entrapment occurre d at different depths. Carbon and water evolution in parental magmas i s calculated to be from 1270 to 2770 ppm C and from 0.7 to 1.1 wt% wat er, respectively. The volatile contents of parental magmas allow compu tation of the following volatile fluxes for the average magma producti on rate of 0.1 m(3)/s observed in the last 50 years: CO2, 88 x 10(3) t ons a(-1); S, 123 x 10(2) tons a(-1); Cl, 2.6 x 10(2) tons a(-1), and (magmatic) H2O, 62 x 10(3) tons a(-1). With literature values of the d egree of melting, water and carbon contents in the mantle source would be between 350 and 1100 ppm and between 63 and 277 ppm C, respectivel y. Because no permanent emanations nor strong degassing during eruptio n is observed at the surface, it is thought that most gases are interc ept at an interface in the building system. (C) 1998 Elsevier Science B.V, All rights reserved.