| Popis: |
At Mt. Pelee (Martinique, F.W.I.), the 1902 and 1929 nuees ardentes occurred before the final emplacement of the dome, at an early phase during the eruption. The first 1902 nuees ardentes, on May 8 and May 20 were also the most destructive. Corresponding products present heterogeneous textures with the occurrence of mafic enclaves, banded and emulsified lavas. The terminal dome is andesitic and texturally homogeneous. These features are compatible with (1) co-eruption of two contrasted magmas (basalt and dacite), (2) interaction between the two magmas which led to intermediate andesitic compositions, (3) a predominantly mechanical magma mixing process, the evolution of which could be observed during the eruption. A similar evolution appeared in the 1929 sequence. Mineralogical evidence depends essentially on phenocryst transfers between basaltic and dacitic magmas, and their paragenetic implications, frequent disequilibrium features, wide compositional ranges, polymodal distribution of plagioclase and strong zoning of phenocrysts. All these features occur in the mingled hybrids as well as in blended mixtures. The chemical compositions of the juvenile magma of nuees ardentes range from basalt or basaltic andesite (mafic enclaves) to dacite (clear host lavas) with all intermediate (and predominant) andesite composition. The terminal dome is andesitic and from major- and trace-element data, it is interpreted as the result of a basalt-dacite mixing. Nevertheless a fractional crystallization process is necessary to generate the acid component from the basic one. We suggest a model comprising a double magma chamber, with a shallow reservoir, dacitic in composition, and a deep reservoir where the mafic magma is stored. The rising mafic magma intersects the superficial chamber, inducing magma mixing and rapidly triggering an explosive Pelean eruption. The early eruptive activity associated with a deep-seated mafic injection intersecting a differentiated shallow chamber immediately reaches a climax. We emphasize the necessity for quick detection of such a rising mafic magma, since the superficial chamber has a relatively short time-lag before an explosive and dangerous eruption. |
