1 0 0 0 OA 神鍋単成火山群の地質
- 著者
- 川本 竜彦
- 出版者
- 特定非営利活動法人 日本火山学会
- 雑誌
- 火山.第2集 (ISSN:24330590)
- 巻号頁・発行日
- vol.35, no.1, pp.41-56, 1990-04-14 (Released:2018-01-15)
The Kannabe monogenetic volcano group is situated in the back-arc region of southwest Japan and is composed of six scoria cones (Nishiki, Yamanomiya, Buri, Otsukue, Kiyotaki and Kannabe) and one scoria bed (Nishibashi-kita). Stratigraphic relationships between various groups of ejecta are inferred from the tephrochronology of fall scoriae and widespread tephra. With the exception of Yamanomiya and Kiyotaki, the volcanic activity began with the formation of scoriae and ended with a phase of lava flow extrusion. No such lava flows are developed in either Yamanomiya or Kiyotaki. The volcanic activity of the Kannabe volcano group mainly took place prior to 21,000-22,000 y. B. P. with the exception of the Kannabe scoria cone itself, which was active before 6,000-6,500 y. B. P. In the vicinity of the Yamanomiya scoria cone, a new debris avalanche deposit has been discovered. This consists mainly of blocks of the Yamanomiya scoria cone, which preserve their original stratification defined by preferential alignment of scoriae and spatters.
1 0 0 0 OA 高温高圧実験による沈み込むスラブ由来流体の化学組成の理解
1 0 0 0 OA マントルウェッジ流体の化学組成
- 著者
- 川本 竜彦
- 出版者
- 公益社団法人 東京地学協会
- 雑誌
- 地学雑誌 (ISSN:0022135X)
- 巻号頁・発行日
- vol.124, no.3, pp.473-501, 2015-06-25 (Released:2015-07-10)
- 参考文献数
- 165
- 被引用文献数
- 2 2
Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived flux: aqueous fluids, hydrous partial melts or supercritical fluids from the subducting slab through reactions. Whether the slab-derived flux is an aqueous fluid, a partial melt, or a supercritical fluid remains an open question. In general, with increasing pressure, aqueous fluids dissolve more silicate components and silicate melts dissolve more H2O. Under low-pressure conditions, those aqueous fluids and hydrous silicate melts remain isolated phases due to the miscibility gap. As pressure increases, the miscibility gap disappears and the two liquid phases becomes one phase. This vanishing point is regarded as critical end point or second critical end point. X-ray radiography experiments locate the pressure of the second critical end point at 2.5 GPa (83 km depth) and 700°C for sediment-H2O, and at 2.8 GPa (92 km depth) and 750°C for high-Mg andesite (HMA)-H2O. These depths correspond to the depth range of a subducted oceanic plate beneath volcanic arcs. Sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, may react with the mantle peridotite to form HMA supercritical fluids due to peritectic reaction between silica-rich fluids and olivine-rich mantle peridotite. Such HMA supercritical fluids may separate into aqueous fluids and HMA melts at 92 km depth during ascent. HMA magmas can be erupted as they are, if the HMA melts segregate without reacting to the overriding peridotite. Partitioning behaviors between aqueous fluids and melts are determined with and without (Na, K) Cl using synchrotron X-ray fluorescence. The data indicate that highly saline fluids effectively transfer large-ion lithophile elements. If the slab-derived supercritical fluids contain Cl and subsequently separate into aqueous fluids and melts in the mantle wedge, then such aqueous fluids inherit much more Cl and also more or less amounts of large ion lithophile elements than the coexisting melts. In contrast, Cl-free aqueous fluids can not effectively transfer Pb and alkali earth elements to the magma source. Enrichment of some large-ion lithophile elements in arc basalts relative to mid-oceanic ridge basalts has been attributed to mantle source fertilization by such aqueous fluids from a dehydrating oceanic plate. Such aqueous fluids are likely to contain Cl, although the amount remains to be quantified. If such silica-rich magmas survive as andesitic melts under a limited reaction with mantle minerals, they may erupt as HMA magmas having slab-derived signatures.