THE GEOCHEMICAL REGIMES OF PITON-DE-LA-FOURNAISE VOLCANO (REUNION) DURING THE LAST 530,000 YEARS

Piton de la Fournaise (Reunion, Indian Ocean) is a large active volcan o which shares many features with the Hawaiian volcanoes. Its particul arly simple elemental and isotopic geochemistry suggests, however, a r ather homogeneous mantle source and makes this volcano ideally suited for petrogenetic studies. We report mineralogical descriptions, major element, trace element and Sr isotope compositions of four volcanic se quences spanning the 530 ka of the known lifetime of the volcano. The lavas change with time from mildly alkalic to mildly tholeiitic. This change is due to crystal fractionation becoming shallower with time as olivine replaces clinopyroxene as the liquidus phase at lower pressur e. Fractionation of an olivine-clinopyroxene assemblage at lithospheri c-mantle pressure drives tholeiitic basalts into the field of alkali b asalts and puts some limitation on trace-element modelling of the melt ing process. Most Fournaise lavas are basalts with a narrow compositio nal range (steady-state basalts) and picrites containing common plasti cally deformed phenocrysts. The buffering of the compatible element co mpositions indicates that the lavas last equilibrated in solid-dominan t conditions which are incompatible with magma chamber processes and m ay have resulted from the ascent of liquid-rich zones through dykes fi lled with a slurry, olivine +/- clinopyroxene crystals. A quantitative model describing mass balance during the displacement of a solitary p orosity wave leaving behind cumulates with residual porosity accounts for the major and trace element distributions. The dykes are flushed d uring major magma surges forming picritic slurries (= oceanites). The transient alkalic lavas are products of dynamic crystallization on col d dyke walls during times of lower magma fluxes. The total lifetime of a Reunion volcano (greater than or equal to 2.1 my) includes early an d waning stages with alkalic activity bracketing a steady-state stage of tholeiitic activity. We estimate that Reunion volcanism results fro m the impingement on the base of lithosphere of a solitary wave of hot spot material with a radius of 100-130 km and a velocity of 5-9 cm/yr.