The Piton de la Fournaise basaltic shield volcano (Reunion Island, Indian Ocean) is the latest superficial expression of the hot spot that produced the Deccan Traps 65 ma ago.
Its products consist in part of oceanite lava flows with 30%vol average olivine. No permanent outgassing occurs.
The study of melt and fluid inclusions trapped in minerals allows to determine magma compositions at different stages of their evolution. They are also, in this case, the only tool to precisely characterize the volatile phase associated to those magmas and to follow its evolution. The major element composition of melt inclusions trapped in pyroclastic olivines (Mg#82 to 87) of oceanites evidence the transition between alkali and tholeiitic basalts clearly illustrated by the negative correlation between alkalis and SiO2. Those variations cannot be accounted for by only crystal fractionnation, but strongly suggest a mixing process.
Two groups of samples can be distinguished, corresponding to two different feeding styles:
- The Mg#85-87 olivines from the most primitive samples, emplaced along the NW rift zone, crystallised at pressures from 1.8 to 4 kilobars, their melt inclusions contain 9.3-9.7 wt% MgO and 0.54-0.58 wt% K2O.
- The Mg#82-85 olivines from others samples, emplaced along the SE and NE rift zones and at the summit of the volcano, crystallised at pressure from 1.3 to 0.1 kilobars, their melt inclusions contain 8-9 wt% MgO and 0.62-0.73 wt% K2O.
The primitive samples contain much more carbon (220-550 ppmC) than late evolved samples (0-140 ppmC).
The transitional character of these basalts is confirmed by volatile element concentrations. Water concentrations (0.56-1.26 wt%) are intermediate between those of tholeiites and alkali basalts. Chlorine concentrations (200-300 ppm) are closer to those of alkali basalts whereas sulphur concentrations (400-1500 ppm) are comparable to those of MORBs. Fluorine contents vary from 440 to 560 ppm. Those variations can be partly explained by degassing processes.
Indeed, the coupled study of melt and rich-CO2 fluid inclusions witnesses continuous CO2 outgassing from 5 kilobars depth (PCO2~Ptot) up to the surface (PH2O~Ptot). Sulphur globules also indicate sulphur saturation. Water and sulfide degas just before the eruption but chlorine and fluorine apparently do not.
Water, carbon, chlorine, sulfur contents suggest that the mantle source of Reunion transitional basalts falls between the depleted mantle source of tholeiitic basalts and the deeper mantle source of alkali basalts.
The different parent magmas can be distinguished on the basis of K2O/H2O (0.6 to 1.5), CO2/(CO2+H2O) (0.4 to 0.5), F/H2O (0.1 to 0.15), F/P2O5 (0.11 to 0.2), F/K2O (0.05 to 0.08), F/Cl (1.5 to 2) ratios.
The water and carbon results help to calculate initial contents of the mantle sources between 0.8 and 1.5 wt% H2O and 1400 to 4100 ppmC respectively. These values agree well with estimates for MORBs (Pineau and Javoy, 1994).
We propose that the Reunion island magmas result from a mixing between a tholeiitic and an alkali mantle sources.
The study of secondary melt and fluid inclusions trapped in olivines indicates olivine accumulation and then percolation by a CO2-rich magmatic phase. These olivines may be carried away by ascending magmas.
These secondary features prove the existence of several cumulative bodies at different levels below Piton de la Fournaise volcano (from 5 kilobars to a few hundred bars). This CO2-rich magmatic phase which infiltrate cumulative bodies would play an important role in the rheology of the system. It has been proposed that cumulative bodies with intercumulus liquid may initiate flank destabilisations in Kilauea volcano (clague and Denlinger, 1994).
All these observations based on primary and secondary melt inclusions study probably may extend to Kilauea volcano (picrites and different sorts of olivines crystals), and also, to other hot spot basaltic shield volcanoes.
Clague, D.A. & Denlinger, R.P., Bull Volcanol. 56, 425-434 (1994).
Pineau, F. & Javoy, M., Earth Planet Sci Lett. 123, 179-198 (1994).