The 1440AD eruption of the Soufrière of Guadeloupe (Lesser Antilles) : Petrological and experimental constraints on magma mixing and pre-eruptive zonation
Résumé
The 1440AD eruption of the Soufriere of Guadeloupe is characterized by a dramatic change of the chemical composition of erupted products, from andesitic to basaltic andesite. This compositional sequence has been interpreted to reflect the tapping of a silicic magma body remobilised following the arrival of a mafic magma batch. All erupted products comprise plagioclase, orthopyroxene, clinopyroxene, magnetite and rare ilmenite phenocrysts. Plagioclase shows complex oscillatory zoning, with rim compositions close to An65 but the average anorthite content seems to be slightly higher in the basaltic andesite. Orthopyroxene is unzoned to weakly zoned in most products except rare and small Mg-rich rims in the dark scoriae. Glass inclusions are rhyolitic. They show small variations in maximum water contents from 4.5 to 5.5wt% (from near-infrared spectroscopy). Pairs of coexisting Fe-Ti oxides yield equilibrium temperatures ranging between 800°C and 950°C for ∃NNO values between +0.4 and +1. A sample representative of the andesitic body was selected for the experiments. All were performed in an internally heated pressure vessel, at T between 800 and 950°C, P between 1 and 2 kbars, fO2 from ∃NNO-1 to ∃NNO+2, and for both water saturated and undersaturated conditions. Comparison between experimental and natural phase assemblage and chemical compositions allows the equilibrium conditions of the top of the andesitic body to be specified, i.e. 875-900°C, 1.5±0.25kbar, ∃NNO+0.7 and melt H2O content close to 5wt%. However, results suggest that the andesitic chamber was probably slightly chemically and thermally zoned before the mixing event. The erupted products seem to result from the mixing between a silicic phenocryst-rich magma and a hot basaltic phenocryst-poor magma. The trigger of the 1440AD eruption appears to be the injection of the basaltic magma because of the short time lapse between injection and eruption suggested by the petrological observations (preservation of banded pumices, rare overgrowths on phenocrysts), as already inferred by Semet et al..