TARDI-MAGMATIC CLAYS IN THE MOST DIFFERENTIATED TERMS OF THE OCEANITE SERIES OF THE PITON DES NEIGES (REUNION ISLAND)

Hydrous magmas are very common in the most differentiated term of calc-alkaline series where the presence of micas, amphiboles, and the associated mesothermal alteration, testify of the importance of the juvenile fluids. Recent studies [1-3] even suggested that clays minerals may form during the last stage of the crystallization of basaltic rocks. We focused on the formation of clay minerals at high temperature during the latest crystallization stage of syenite intrusions which are considered as the ultimate term of the alkaline basaltic series of the Piton des Neiges (Réunion island, western India Ocean, France) [4].

The syenitic rocks are characterized by a phaneritic texture with an exceptionally high intergranular primary macroporosity and the absence of perthitic exsolution in the feldspars. Such petrographic features are indicative of a rapid cooling which precluded the developments of subsolidus reactions usually observed in such intrusive rocks. Chlorite-like minerals crystallized during the latest stage of the fractional crystallization with quartz, carbonates and accessory minerals (zircon, monazite, rutile, apatite, pyrite). This observation is supported by isotopic microanalyses of O and C (ionic probe, CRPG, Nancy) of quartz, opaline, calcite and ankerite associated with these differentiated term. The low d13C values found in the carbonate crystals (-49 to -53‰) associated with d18O values ranging from 3 to 8 ‰, are indicative of juvenile origin of the mineralizing fluid.

We propose that a chemical transfer occurred at the end of the crystallization process, from the magmatic phases affected by post-magmatic reaction to the primary porosity, in a CO2-rich juvenile fluid and leading to the crystallization of tardi-magmatic clays. The proposed process is likely not specific of the particular case presented here and can be generalize to all of the alkaline series, so to the primitive crust of the telluric planets.

[1] Meunier et al. (2008), Clays Clay Min. 56, 730-750; [2] Meunier et al. (2012), Nature Geoscience 5, 739–743; [3] Berger et al. (2014), Planet. Space Sci. 95, 25-32; [4] Upton & Wadsworth (1967), Amer. Miner. 52, 1475-1492.