14:45 〜 15:00
[SGC33-05] 強親鉄性元素及びRe-Os同位体系から探るSredinny Range北部（カムチャッカ）のマグマ生成プロセス
キーワード：カムチャッカ、強親鉄性元素、Os同位体、同化分別結晶作用、島弧火成活動
The subduction zone is a relatively "cold place" on Earth. Observational and experimental studies in petrology, geochemistry, and geophysics suggest that magma is generated associated with the circulation of H2O-rich fluids. A standard quantitative model has also been proposed that incorporates the following processes; generation and migration of the fluid released from the subducted plate, transportation into the mantle wedge, the melting point depression of rock, and melting to supply magma to the volcanoes. Within this framework, the structure and processes of the subducted side (i.e., arc side), such as the composition and temperature of the mantle wedge, have been extensively studied to understand the formation of arc lava. In addition to these studies, researches on the ocean area have progressed in recent years, and are being developed from the viewpoint that the material and thermal structure of the subducting ocean plate, that is, the condition of the "subducting side" has a great influence on geological processes on the arc.
For studying such a subduction zone-scale material cycle, the elemental concentrations and isotopic ratios of the island arc lava are useful as a tracer of the subducted material. In this study, we focused on highly siderophile elements (HSE) and Re-Os isotope systems, which are considered to be indicators of subduction substances and melting degree owing to the large variation in elemental partitioning of the HSE between melt/fluid and solid.
Kamchatka is one of the largest volcanic arcs in the world that has unique tectonic settings such as being adjacent to the triple junction of the plates, the significant along-arc variation of both arc magmatism and the subducting plate, and subduction of the prominent seamount chain. It is a suitable place for studying how the variable subduction conditions affects the arc magmatism. We studied North Sredinny Range (N-SR) area, which is located at the northernmost part of the long volcanic chain on the backarc side of Kamchatka and is located further north of the subducted Pacific Plate. We analyzed 30 Quaternary lava samples from the N-SR region for HSE and Re-Os isotopic ratios and discuss the volcanic processes in this region.
The Os concentrations in the N-SR lavas range from 0.0001 to 0.0814 ppb and 187Os/188Os ratios range from 0.1276 to 0.8421. The FeO*/MgO of N-SR lavas suggests fractional crystallization from a relatively undifferentiated lava. A positive correlation between Os, Ir, Ru concentrations and FeO*/MgO, and a model calculation of fractional crystallization show that the fractional crystallization of phenocryst minerals such as olivine has removed highly compatible elements including Os from the lavas. The correlation between 1/Os and 187Os/188Os ratio shows a linear relationship with different slopes for the individual sub-areas within the studied area, suggesting that each sub-area has undergone a different amount of crustal assimilation during the AFC process in the N-SR area. In addition, the Os concentration and 187Os/188Os in the mantle source were estimated from element and isotope correlations, and the fluid contribution to the source mantle estimated independently from other trace element data show that a seamount subducted in the past could exist under the N-SR area and may contribute to the generation of magma.
For studying such a subduction zone-scale material cycle, the elemental concentrations and isotopic ratios of the island arc lava are useful as a tracer of the subducted material. In this study, we focused on highly siderophile elements (HSE) and Re-Os isotope systems, which are considered to be indicators of subduction substances and melting degree owing to the large variation in elemental partitioning of the HSE between melt/fluid and solid.
Kamchatka is one of the largest volcanic arcs in the world that has unique tectonic settings such as being adjacent to the triple junction of the plates, the significant along-arc variation of both arc magmatism and the subducting plate, and subduction of the prominent seamount chain. It is a suitable place for studying how the variable subduction conditions affects the arc magmatism. We studied North Sredinny Range (N-SR) area, which is located at the northernmost part of the long volcanic chain on the backarc side of Kamchatka and is located further north of the subducted Pacific Plate. We analyzed 30 Quaternary lava samples from the N-SR region for HSE and Re-Os isotopic ratios and discuss the volcanic processes in this region.
The Os concentrations in the N-SR lavas range from 0.0001 to 0.0814 ppb and 187Os/188Os ratios range from 0.1276 to 0.8421. The FeO*/MgO of N-SR lavas suggests fractional crystallization from a relatively undifferentiated lava. A positive correlation between Os, Ir, Ru concentrations and FeO*/MgO, and a model calculation of fractional crystallization show that the fractional crystallization of phenocryst minerals such as olivine has removed highly compatible elements including Os from the lavas. The correlation between 1/Os and 187Os/188Os ratio shows a linear relationship with different slopes for the individual sub-areas within the studied area, suggesting that each sub-area has undergone a different amount of crustal assimilation during the AFC process in the N-SR area. In addition, the Os concentration and 187Os/188Os in the mantle source were estimated from element and isotope correlations, and the fluid contribution to the source mantle estimated independently from other trace element data show that a seamount subducted in the past could exist under the N-SR area and may contribute to the generation of magma.