Modelling the Recent Geochemical Evolution of the Piton de la Fournaise Volcano, Reunion Island, 1931-1986

The Piton de la Fournaise is the only active volcano on theisland of R?union. Samples of the 1931–1986 eruptionshave been analysed for their major and trace element contentswith emphasis on high accuracy measurement of REE, Th, and Zrby isotope dilution. The lavas include transitional basalts and picrites with 40–50%olivine. Examination of the dislocation structures in the picriticolivines indicates a xenocrystic origin which is confirmed bysome mineral-melt Fe/Mg disequilibria and anomalous incompatibleelement ratios. The picrites are therefore interpreted as dueto pulses of basalt magmas which disrupted preexisting, geneticallyunrelated, olivine-rich cumulates. The combination of whole rock, mineral and groundmass analysesenabled precise calculation of the modal proportions. The phenocrystassemblages obtained thereby are consistent with the chemicalevolution of the lava series suggested by the mass balance equations,which is interpreted as reflecting little mineral sorting duringfractionation. The whole range of basaltic compositions canbe generated by up to 15% fractional crystallization of clinopyroxene-rich(20 Ol-80 Cpx) to plagioclase-bearing (20 Ol-50 Cpx-30 Plag)cumulates. By cqmparison with experimental liquid lines of descent,the order of crystallization (olivine-clinopyroxene-plagioclase)suggests a pressure of crystallization in excess of 500 MPa,i.e. much deeper than the c. 2km deep magma chamber inferredfrom magnetic measurements and seismic activity. The Ni-MgOsystem indicates that the Fe-rich primitive magma (9% MgO, 225ppm Ni) crystallized 12 ? 5% cumulate to yield the least differentiatedbasalt of the series. The historic basalts erupted prior to1931 and analyzed in this work were extracted from a more Mg-richsource. The inversion of batch melting equations for incompatible elementsis improved relative to previous attempts and a complete errorhandling is incorporated. It is suggested that the Fournaiselavas were derived by melting of a source enriched in compatibleelements leaving a harzburgitic residue without significantfractionation of the incompatible elements. Coupled variations of Ce/Yb and major element ratios over c.17-y cycles suggest that lower degrees of melting produce smallerbatches of magma which, upon ascent and cooling, undergo moresevere fractionation.