| Budget Amount *help |
¥3,900,000 (Direct Cost: ¥3,900,000)
Fiscal Year 2001: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 2000: ¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 1999: ¥2,100,000 (Direct Cost: ¥2,100,000)
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| Research Abstract |
1. Sample collection
168 lavas, predominantly basalts and basaltic andesites, were collected from volcanoes of seventeen regions (Fuji - Aogashima) in the Izu arc, Japan. These samples cover the depth of the subducting slab between 150 km and 210 km.
2. Analyzes of major and trace elements
Fresh, relatively primitive lavas (56 samples) were selected for chemical analyzes. Major element compositions were analyzed by XRF. Ba, Rb, Sr, Pb, Be and Li concentrations and B, Nb, Zr and Y concentrations were determined by GF-AAS and ICP-AES, respectively. For comparison, lavas from the Kurile, Kamchatka and Mariana arcs were also analyzed. A technique for measuring rare earth elements using ion chromatography was developed.
3. Results and discussion
B/Nb, Pb/Nb and Ba/Nb ratios of the Izu arc lavas show clear across-arc variations, in which the ratios all systematically decrease with increasing the slab depth. The extent of mantle metasomatism by the slab-derived aqueous fluid is B【similar or equal】Ba>Pb【similar or equal】Rb>K>Li【similar or equal】Be at the trench side and Ba>Rb>Pb【similar or equal】B【similar or equal】K>Li【similar or equal】Be at the back-arc side. Similar trends are observed in the Kurile-Kamchatka arc, suggesting that this may represent the general trend in island arcs. These data indicate that the trace element composition of the fluid liberated from the slab changes with increasing slab depth. This change in fluid composition implies that at least two hydrous minerals dehydrating at different temperatures and pressures are involved, and the dehydration takes place continuously rather than episodically.
An anomalous across-arc variation is observed near the subduction-transform boundary in the northernmost Kamchatka arc, suggesting particularly large fluid influx beneath this region.
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