Evolution comparée de deux séries alcalines du Pacifique Central: Rôle de la fugacité d’oxygène et de la pression d’eau

A comparative mineralogical and geochemical study of two alkali-basalt series from French Polynesia (Rapa and Nuku-Hiva islands) reveals several important differences, particularly in the degree of Fe enrichment in ferromagnesian minerals and the behaviour of Ti, V and Zn. These differences appear to be connected with the onset of magnetite crystallization, which is early in the Nuku-Hiva suite but later in that of Rapa. The variable influence of this phase is ascribed to differences in IO₂, which is probably ultimately dependent on variations in P_{H 2 O}. In both cases, observed chemical variations may be explained in terms of fractional crystallization, with the occurrence of some oversaturated residual liquids resulting from the separation of mica.