| 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. |
| Genre: | Article |
| ProdID: | 70010518 |
| Citation Author: | Lipman, P. W. |
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| Citation End Page: | 327 |
| Citation Issue: | 4 |
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| Citation Language: | English |
| Citation Larger Work Title: | Contributions to Mineralogy and Petrology |
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| Citation Number Of Pages: | 28 |
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| Citation Search Results Text: | Mineral and chemical variations within an ash-flow sheet from Aso caldera, Southwestern Japan; 1967; Article; Journal; Contributions to Mineralogy and Petrology; Lipman, P. W. |
| Citation Start Page: | 300 |
| Citation Volume: | 16 |
| Citation Year: | 1967 |
| Type: | citation/reference |
| Text: | Mineral and chemical variations within an ash-flow sheet from Aso caldera, Southwestern Japan; 1967; Article; Journal; Contributions to Mineralogy and Petrology; Lipman, P. W. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| URL (DIGITAL OBJECT IDENTIFIER): | http://dx.doi.org/10.1007/BF00371528 |
| Date Other: | Sun, 1 Jan 1967 00:00 -0600 |
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