A Discussion on volcanism and the structure of the Earth - Aspects of magmatic evolution of Réunion Island

As with the Hawaiian islands, the volcanic construction of Reunion can be related to two main phases of activity-a shield-forming stage of predominantly olivine-basalt composition, and a declining stage comprising more varied products (basalt-trachyte). The older of the two Reunion volcanoes (Piton des Neiges) appears to have completed both these stages, but the younger volcano (Piton de la Fournaise) is still in the shield-forming stage, and is lagging approximately 1.5 Ma$^{\ast}$ behind Piton des Neiges in its evolution. The chemical data indicate a considerable degree of coherence between the various rock types produced during the different stages of development, and it is concluded that they all stem from essentially the same hypersthene-normative, picritic, primitive magma, generated by partial melting of peridotite in the low-velocity layer of the upper mantle. The shield-forming lavas are believed to represent relatively rapid ascent of this magma, only modified by olivine fractionation, but the declining stage seems to require intermediate-depth fractionation (olivine+pyroxene) to account for its initially nepheline-normative character, followed by high level fractionation (olivine+plagioclase+pyroxene+magnetite, etc.) to produce the hawaiite-mugearite-benmoreite-trachyte sequence. Volume considerations appear to favour an open system of basalt extraction, involving a relatively modest (7 to 15%) degree of partial melting and continuous replenishment of the mantle source beneath Reunion, rather than a closed system with its restricted basalt potential even if as much as 50% partial melting is postulated. $^{\ast}$ 1 Ma = 1 million years.