1989 Fiscal Year Final Research Report Summary
Homogeneity of magmas estimated from volcanic glasses
| Project/Area Number |
63540617
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
地質学一般
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| Research Institution | National Science Museum |
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Principal Investigator |
TIBA Tokiko National Science Museum, Department of Geology senior research officer, 地学研究部, 主任研究官 (20000127)
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| Project Period (FY) |
1988 – 1989
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| Keywords | volcanic glass / pyroclastic flow deposit / (volcanic) ash / rhyolite / obsidian |
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| Research Abstract |
Homogeneity of the magma that produced voluminous pyroclastic flows and ash fall deposits (AT) was investigated by major element analyses on volcanic glasses in the pyroclastic flow deposit exposed at Iwato, Kokubu City, Kagoshima Prefecture. Volcanic glasses included in the pyroclastic flow deposit have a considerably uniform chemistry, in spite of their differences in color, shape and size. Their compositional ranges are: SiO_2 77-78wt%, Al_2O_3 12.5-13.0wt%, FeO 1.5-2.0wt%, CaO 1.0wt%, Na_2O 3.5-4.0wt%, K_2O 3.0-3.5wt%.
Volcanic glasses in ash fall deposits around Daisen and Hiruzen are classified into the following three groups based on their major element chemistry: 1) glasses with SiO_2 73-74wt%, Na_2O 3-4wt%, K_2O 2.5wt%, 2) those with SiO_2 75.5-76.5wt%, Na_2O 3-4wt%, K_2O 3wt%, 3) those with SiO_2 77-78wt%, Na_2O 3.5-4.5wt%, K_2O 3.0-3.5wt%. Group 2) is characterized by slightly high Cl contents. Group 3) shows similarity to the volcanic glasses in AT.
The glassy rhyolite JR-2 (obsidian) , used as a standard in energy dispersive X-ray analysis, gives compositional variation of SiO_2 76.2-77.5wt%, Al_2O_3 12.4-13.lwt%, Na_2O 4.0-4.6wt%, K_2O 4.3-4.8wt%.
Both volcanic glasses from the pyroclastic flow and JR-2 show distinct correlations between element. ratios Si/K-Al/K and -Na/K. In these felsic melts, the major variation of compositions is presented by relative K-enrichment and Al- and Na-depletions.
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