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 selection of melt inclusions trapped in olivine (Fo(81-87)) from scoria and crystal-rich lapilli samples of Piton de la Fournaise volcano. As a whole, they display a transitional basaltic composition. The melt inclusions (8-9 wt.% MgO, 0.62-0.73 wt.% K2O) are in equilibrium with olivines (Fo(81-85)) in the samples from the Central Feeding Zone and the South-East Feeding Zone and show a slight alkaline affinity. The melt inclusions 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 tendency. In oceanitic lavas and crystal-rich lapilli, the olivine xenocrysts are recognisable by the presence of one or more secondary shear plane fracture(s) filled up with CO2 and alkali-rich basaltic melt inclusions. In dunite nodules, olivines present also contain several secondary shear plane fracture(s) filled up with CO2 and high-SiO2 melt inclusions. 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 melt inclusions and the secondary fluid inclusions strongly suggest that the olivine crystallises and accumulates over a wide depth range (15 km). It is envisioned that cumulative pockets with low residual porosity are repeatedly percolated with a CO2-rich fluid phase, possibly associated with basaltic to SiO2-rich melts, and are finally disrupted and entrained to the surface when vigorous magma transfer occurs. The SiO2-rich residual melts in early-formed dunitic or gabbroic bodies may have acted as contaminant agents for the more alkali character of magmas vented through the central feeding system, where a well-developed cumulative system is thought to exist. Finally, the existence of secondary fluid and melt inclusions in olivines implies that the dunitic bodies are weakened on the micrometric scale. (C) 1998 Elsevier Science B.V. All rights reserved.