Major element, trace element, and Sr and Nd isotopic compositions were determined on a suite of lavas and intrusives from Piton des Neiges and Piton de la Foumaise volcanoes of Reunion Island. The temperatures of appearance of mineral phases in six basalt samples were determined by melting and recrystallization experiments at pressures from 1--atm to 2000 MPa. The compositions of minerals, both in the starting basalts and in the experimental run products, were analyzed by electron microprobe and compared to determine the temperature and depth of their formation in the basalts. The Sr and Nd isotopes and ratios of highly incompatible trace elements of basalt samples from both volcanoes are virtually identical, indicating that the mantle source for the magmas is chemically homogeneous, both in time and space. The lavas of the active volcano (Piton de la Fournaise) and the late stage Differentiated Series of Piton des Neiges are slightly more alkali-rich than the primitive shield lavas of Piton des Neiges. The alkali enrichment could be the result of lower degrees of partial melting of the mantle for the young eruptions, but Zr and the rare earths do not fit this model. An alternative explanation is that the alkali enrichment resulted from contamination of a Piton des Neiges-type magma with the low-melting component of the lava pile. The melting experiments and least squares modeling indicate that augite was an important fractionating phase and that the fractionation of olivine plus augite occurred at depths of at least 5 km. The temperature of the magma chamber at this depth was between 1200¿ and 1300 ¿C. The parent magma composition for Reunion had between 15 and 17% MgO, and all Piton des Neiges magmas can be derived from it by fractional crystallization at 5 to 15 km depth. Piton de la Foumaise magmas can be derived from this parent magma if they are also enriched with alkali elements by an independent process. ¿ American Geophysical Union 1988