Petrology of 1977 to 1998 eruptions of Piton de la Fournaise, La Réunion Island

Geochemical and petrological variations in 35 historical eruptions from the April 1977 eruption to the March–September 1998 eruption at Piton de la Fournaise have been analyzed for their major elements abundances, mineralogical compositions and oxygen isotopes. During these 21 years, samplings were repeated several times during eruptions as well as temperature measurements.Since the end of 1977 to 1998, all erupted lavas, except 1998-Hudson basalt, belong to the group of the “steady state basalts”. However, evidences for small degrees of differentiation can be observed. During a single eruption, the earliest lava emitted contains less phenocrysts and has almost always a lower MgO⁎ (= MgO/[MgO + FeOt]) than the lavas emitted later, suggesting that it is more differentiated. From one eruption to the next, between 1977 and 1998, bulk compositions are more and more differentiated towards the first lava of 1998. A process of low-pressure fractional crystallisation, involving small proportions of olivine, clinopyroxene and plagioclase, allows to explain this evolution.A major result of our study is to show that this evolution does not correspond to a unique fractionation trend. Over the considered period, different types of magmas have been distinguished. The first magma type, erupted from October 1977 to November 1987, was richer in potassium (0.82 wt.% in average) than the second one, and apparently derived from the liquid of the 1977 oceanite. The second magma type, erupted from February 1988 to September 1998, was poorer in K2O (0.69 wt.% in average). In March 1998, a third magma type (Hudson basalt) erupted simultaneously with the second one. Hudson basalt has undergone a wehrlitic fractionation and is supposed to come from a deeper magma reservoir than other erupted magmas for this period.From 1977 to 1998, the ∂18O of lavas progressively decreases with time, together with the increasing chemical differentiation of the magmas by low-pressure fractional crystallisation. This variation agrees with the hypothesis of magma contamination at shallow level in the altered volcanic edifice (Vlastélic, I., Deniel, C., Bosq, C., Télouk, P., Boivin, P., Bachèlery, P., Famin, V., Staudacher, T., 2009-this issue. Pb isotope geochemistry of Piton de la Fournaise historical lavas. Journal of Volcanology and Geothermal Research, 184, 63−78 (this issue)).Overall, petrological variations are in good agreement with a model of a shallow reservoir discontinuously supplied between 1977 and 1998. The speed of the variations, the coexistence of different liquids, the chemical evolution of the lavas during the same eruption, lead us to favour a model of chemically stratified reservoir, constituted by a complex of dykes, sills and laccoliths close to the surface and located above larger magma bodies.