Une paragenèse inhabituelle à fassaïte, mélilite et rhönite dans un basalte alcalin contaminé au contact d'un récif corallien (Saint-Leu, Ile de la Réunion)

Abstract

The amount of calcium assimilated by the basaltic lavas, the oxygen fugacity and thermal gradient controlled the distribution of secondary mineral assemblages in these basalts depending on proximity to contaminating coral rocks. Within corallites, melilite has crystallized before fassaïte and rhönite, while a few centimeters away from coral skeletons, fassaïte was first generated, replacing basalt pyroxenes partly or completely. Rhönite is absent in this zone. The chemical compositions of fassaïte and rhönite can easily be distinguished from previous data presented in literature on earth [= onshore] rocks : undersaturation in silica, higher Fe-Ti contents. The associated minerals have crystallized at high temperature (1 000 °C), total fluid pressure being close to the atmospheric one and high CO₂ fugacity.