Fluid-magma decoupling in a hot-spot volcano

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 Albarède 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.