Melt and fluid inclusions in olivines from the last 1998 eruption of Piton de la Fournaise (PdF), have recorded a range of volatile partial pressures (350--420 MPa), the highest, so far for this volcano, are found in Fo-rich olivines from a vent remote from the main eruptive activity. Such pressures indicate that fractionation of olivine (and other crystals) occur below the crust-mantle boundary in a CO2-rich volatile saturated environment. Together with previously published data, the model that emerges for PdF is one where olivines which have crystallized from multiple past magma injection events [see Albar¿de et al., 1997>, may be picked up by newly intruding magma, anywhere from upper-mantle depths to the surface. This model is likely to apply to other shield volcanoes. Magma production and transport to the surface is accompanied by continuous open-system degassing through the permeable volcanic pile. The calculated H2O content of primary basalts (MgO≈12--14 wt.%) from PdF may reach 0.7--1 wt.% implying a rather H2O-rich hot spot mantle source. ¿ 1999 American Geophysical Union