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Les pyroxènes et leurs inclusions, marqueurs privilégiés des nuées ardentes (Saint-Vincent, Antilles, 1979)

1984; Volume: 107; Issue: 1 Linguagem: Inglês

10.3406/bulmi.1984.7792

0180-9210

Jacques-Marie Bardintzeff,

earthquake and tectonic studies

Pyroxenes of St. Vincent Soufrière volcano andesito-basaltic nuées ardentes (1979) show clearly three stages of crystallization. 1) Cr-rich diopsides (with chromites and Fo 89 olivines) crystallize early (P > 10 kbar) from primary basaltic magma. 2) Salites characterize a second high P H2O stage (P H2O = 5 kbar) and occur in cumulate with Al-magnetites, Fo 75 olivines, Ca-plagioclases and amphiboles. 3) Augites and orthopyroxenes appear in the last stage with Na-plagioclases. Primary melt inclusions do not occur in diopsides ; their compositions are andesito-basaltic in salites and andesito-dacitic in augites. The different crystallization-stages are displayed in some products of the eruption (cumulates, blocks and ash-matrix of nuées ardentes, domes). The proposed volcanological interpretation is that of mechanical mixing of residual dacitic magma (in small amount) produced by fractional crystallization with reinjected andesitic magma. This mixing produces sudden variations of thermodynamic conditions, triggering nuées ardentes.