Evidence for a homogeneous primary magma at Piton de la Fournaise (La Réunion): A geochemical study of matrix glass, melt inclusions and Pélé's hairs of the 1998–2008 eruptive activity

Magmas erupted at Piton de la Fournaise volcano since 0.5 Ma, display a large petrological and chemical range (picrites, 2 types of transitional basalts and differentiated magmas) and low amplitude isotopic heterogeneities. The recent activity (1998–2008) includes all magma types except evolved magmas. Matrix glass compositions from quenched lavas and Pélé's hairs of the whole 1998–2008 period define a single differentiation trend from a common basaltic melt (MgO ~ 9%) for the first time identified in the 2007 magmas. More primitive melt compositions (MgO ~ 12.5%) are only evidenced by olivine crystals with high Fo contents (Fo85–88.4). Evolutions of major and trace element of glass and mineral compositions are consistently modelled by a unique low pressure crystal fractionation process. The composition range of olivine melt inclusions is distinct from that of matrix glass and Pélé's hair and corresponds to equilibrium crystallisation in closed system of melts trapped from the main differentiation series at high temperature.The range of basaltic types at Piton de la Fournaise is the result of large variations in the differentiation degree (10 to 35% crystallisation) of a single primary basaltic melt and the addition in highly variable amounts (up to 50% in picrites) of co-genetic olivine or gabbroic cumulates. These cumulates may represent the shallow and dense bodies identified by seismic tomography and have likely been produced by the repetitive intrusion and differentiation of basalts along Piton de la Fournaise history. Depending on the shallow transfer paths, ascending magmas may disaggregate and incorporate various types of cumulates, explaining all particular features of basaltic magmas and picrites. These results emphasize the exceptional chemical homogeneity of the primary basaltic melt and of the differentiation process involved in volcanic activity of La Réunion hotspot since 0.5 Ma and the increasingly recognised role of melt–wall rock interactions in compositional and petrological diversity of erupted magmas.