The apparently innocuous observation that the Piton de la Fournaise lavas can be divided into a dominant basaltic type (MgO= 7%) and two subordinated picritic and differentiated series with fairly variable compositional patterns poses the difficult question of the relationships between the geochemical diversity of the lavas and the differentiation regime of the volcano. The Rayleigh equation can be written as
dCi /Ci = (Di - 1)dF/F
(i: element, C: concentration in the residual melt, F: fraction of residual melt, D: solid-liquid partition coefficient) which shows that the dispersion of compatible element concentrations in residual melts critically depends on how much olivine-bearing cumulate is removed. From the Mg, Cr, and Ni contents in basalts, we found a surprisingly small and consistent value of dF/F × 5 %. A mechanism therefore has to be found that allows the Piton de la Fournaise to erupt substantially differentiated lavas for which F is nearly constant over × 500 ky.
Buffering compatible elements in basalts requires that the liquid equilibrated with their cumulate in a solid-dominated environment, a situation reminiscent of the conditions of melting. The FeO/MgO ratio and the Ni concentration of the basalts indicate that the dominant solid phase cannot be mantle material. This suggests that, just before eruption, the steady-state basalts must have equilibrated with an olivine ± clinopyroxene assemblage which imposes its compatible element characteristics to the interstitial magmatic liquid. This can be achieved in a number of ways, but percolation of liquids through a crystal mush is certainly efficient. Solitary porosity waves, known to form in wet porous media, travel unmodified over long distances (Scott and Stevenson, 1986). Albarède (1995) adapted Pfann's (1952) equation of zone refining to the case of a melt travelling as a zone of crystal mush. We assume that the plumbing system of the volcano is filled with unconsolidated cumulates from previous eruptions. Packets of magmas percolate through the mush, forming a column of solid-liquid suspension of finite heigth L. In the melt-rich zone, volume porosity is F. The liquid leaves behind a volume porosity j of trapped melt. A characteristic distance Zi over which the concentration of element i vary by a factor e is found as