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Mineral and chemical variations within an ash-flow sheet from Aso caldera, Southwestern Japan

Contributions to Mineralogy and Petrology

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DOI: 10.1007/BF00371528

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Abstract

Although products of individual volcanic eruptions, especially voluminous ash-flow eruptions, have been considered among the best available samples of natural magmas, detailed petrographic and chemical study indicates that bulk compositions of unaltered Pleistocene ash-flow tuffs from Aso caldera, Japan, deviate significantly from original magmatic compositions. The last major ash-flow sheet from Aso caldera is as much as 150 meters thick and shows a general vertical compositional change from phenocryst-poor rhyodacite upward into phenocryst-rich trachyandesite; this change apparently reflects in inverse order a compositionally zoned magma chamber in which more silicic magma overlay more mafic magma. Details of these magmatic variations were obscured, however, by: (1) mixing of compositionally distinct batches of magma during upwelling in the vent, as indicated by layering and other heterogeneities within single pumice lumps; (2) mixing of particulate fragments-pumice lumps, ash, and phenocrysts-of varied compositions during emplacement, with the result that separate pumice lenses from a single small outcrop may have a compositional range nearly as great as the bulk-rook variation of the entire sheet; (3) density sorting of phenocrysts and ash during eruption and emplacement, resulting in systematic modal variations with distance from the caldera; (4) addition of xenocrysts, resulting in significant contamination and modification of proportions of crystals in the tuffs; and (5) ground-water leaching of glassy fractions during hydration after cooling. Similar complexities characterize ash-flow tuffs under study in southwestern Nevada and in the San Juan Mountains, Colorado, and probably are widespread in other ash-flow fields as well. Caution and careful planning are required in study of the magmatic chemistry and phenocryst mineralogy of these rocks. ?? 1967 Springer-Verlag.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Mineral and chemical variations within an ash-flow sheet from Aso caldera, Southwestern Japan
Series title:
Contributions to Mineralogy and Petrology
DOI:
10.1007/BF00371528
Volume
16
Issue:
4
Year Published:
1967
Language:
English
Publisher location:
Springer-Verlag
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
300
Last page:
327
Number of Pages:
28