[SVC50-P11] 雌阿寒岳，9000年前の螺湾火砕流堆積物の不均質岩片から推定されるマグマ混合の進行過程
キーワード：雌阿寒岳、マグマ混合、縞状軽石
At Me-akan volcano, eastern Hokkaido, pyroclastic activity of plinian fall and pyroclastic flows including pumice and scoria have occurred at 13000-12000 years ago, forming Nakamachineshiri crater with 1.1 km diameter (Wada, 1989). At about 9000 years ago, pyroclastic flow containing pumice, scoria and heterogeneous juvenile ejecta such as banded pumice was flowed down along the Rawan river southwestern of Nakamachineshiri crater. We call this Rawan pyroclastic flow. We analyzed the chemical composition of groundmass glass and plagioclase phenocrysts and groundmass in two specimens of each pumice, scoria and banded pumice in detail.
The groundmass composition of scoria shows SiO2=61-70wt.% and has fixed chemical trend, whereas that of pumice concentrates to SiO2=77-79wt.%. The scoria part in the banded pumice varies from SiO2=61-76wt.% and shows wide compositional range connecting with scoria and pumice compositions. The pumice part in the banded pumice is slightly higher SiO2 composition (78-80wt.%) than pumice fragment.
Plagioclase phenocryst of scoria and pumice shows almost the same bimodal An content distribution of An58-60 peak and An72-92 wide peak. The lower An plagioclase phenocrysts of both scoria and pumice show the same texture, but the high An plagioclase phenocrysts are different origin between scoria and pumice; rapid crystrallization from mafic magma for scoria and long storage in magma chamber for pumice.
These results suggest that each magma produced scoria or pumice was already mixed in the single or plural magma chamber, and mafic magma produced scoria was injected into felsic magma produced pumice to mingle and mix in the conduit. Diffusion rate of mafic magma is faster than that of felsic magma, mixing proceeds inside of mafic magma incorporating felsic magma in central part in the conduit, producing the banded pumice.
The groundmass composition of scoria shows SiO2=61-70wt.% and has fixed chemical trend, whereas that of pumice concentrates to SiO2=77-79wt.%. The scoria part in the banded pumice varies from SiO2=61-76wt.% and shows wide compositional range connecting with scoria and pumice compositions. The pumice part in the banded pumice is slightly higher SiO2 composition (78-80wt.%) than pumice fragment.
Plagioclase phenocryst of scoria and pumice shows almost the same bimodal An content distribution of An58-60 peak and An72-92 wide peak. The lower An plagioclase phenocrysts of both scoria and pumice show the same texture, but the high An plagioclase phenocrysts are different origin between scoria and pumice; rapid crystrallization from mafic magma for scoria and long storage in magma chamber for pumice.
These results suggest that each magma produced scoria or pumice was already mixed in the single or plural magma chamber, and mafic magma produced scoria was injected into felsic magma produced pumice to mingle and mix in the conduit. Diffusion rate of mafic magma is faster than that of felsic magma, mixing proceeds inside of mafic magma incorporating felsic magma in central part in the conduit, producing the banded pumice.