MAGMA-CONDUIT INTERACTION AT PITON DE LA FOURNAISE VOLCANO (REUNION ISLAND) - A MELT AND FLUID INCLUSION STUDY

Major-element, Cl, S, F analyses have been performed on a wide selecti on of melt inclusions trapped in olivine (Fo(81-87)) from scoria and c rystal-rich lapilli samples of Piton de la Fournaise volcano. As a who le, they display a transitional basaltic composition. The melt inclusi ons (8-9 wt.% MgO, 0.62-0.73 wt.% K2O) are in equilibrium with olivine s (Fo(81-85)) in the samples from the Central Feeding Zone and the Sou th-East Feeding Zone and show a slight alkaline affinity. The melt inc lusions in olivines (Fo(85-87)) from the North-West Rift (NWR) contain 9.3-9.7 wt.% MgO and 0.54-0.58 wt.% K2O, with a more tholeiitic tende ncy. In oceanitic lavas and crystal-rich lapilli, the olivine xenocrys ts are recognisable by the presence of one or more secondary shear pla ne fracture(s) filled up with CO2 and alkali-rich basaltic melt inclus ions. In dunite nodules, olivines present also contain several seconda ry shear plane fracture(s) filled up with CO2 and high-SiO2 melt inclu sions. Secondary CO2-rich fluid inclusions in olivine (Fo(85-87)) from the NWR samples indicate PCO2 up to 500 MPa whereas, PCO2 ranges from 95 MPa to few tenths of bars in the other samples. Both the primary m elt inclusions and the secondary fluid inclusions strongly suggest tha t the olivine crystallises and accumulates over a wide depth range (15 km). It is envisioned that cumulative pockets with low residual poros ity are repeatedly percolated with a CO2-rich fluid phase, possibly as sociated with basaltic to SiO2-rich melts, and are finally disrupted a nd entrained to the surface when vigorous magma transfer occurs. The S iO2-rich residual melts in early-formed dunitic or gabbroic bodies may have acted as contaminant agents for the more alkali character of mag mas vented through the central feeding system, where a well-developed cumulative system is thought to exist. Finally, the existence of secon dary fluid and melt inclusions in olivines implies that the dunitic bo dies are weakened on the micrometric scale. (C) 1998 Elsevier Science B.V. All rights reserved.