Fayal dans l’Atlantique et Rapa dans le Pacifique: deux séries faiblement alcalines évoluant sous conditions anhydres

Though in two different geodynamic sites, the Isles of Fayal (Azores, North Atlantic) and Rapa (Australes, Pacific) show large similitudes. The two series differentiated under dry conditions by a process of crystal fractionation with low oxygen fugacity. Olivine is present in both series and evolves from Fo₈₃ to Fo₅ (Rapa) and from Fo₇₇ to Fo₂₂ (Fayal). The two series present some features of the tholeiitic series, even though their high contents in incompatible elements (K₂O, TiO₂, etc.) and normative Ne (0.30 in basalt) would assimilate them to alkaline series. These apparent conflicting datas are explained by an anhydrous melting (<15%) under high pressure (>15kb) of an undepleted pyrolitic mantle, according to a recent proposal byJaques andGreen (1980).