17:15 〜 19:15
[SVC30-P02] Time-dependent changes in the magma plumbing system revealed by zircon dating and geochemistry; the case of the Unzen volcano
キーワード：マグマだまり、ジルコン、年代測定
Understanding how the magma plumbing system is maintained under the volcano for long periods of time, and how its geochemical changes lead to differences in surface eruptive phenomena, is essential for long-term prediction of the explosiveness of future volcanic eruptions. In this study, we used the Unzen volcano, Japan as an example to clarify what kind of geochemical changes the magmatic system has undergone from the earliest erupted magmas (˜500 ka) to the present day. In this presentation, we will mainly discuss the results of U-Th and U-Pb dating, trace element composition and Hf isotope analysis of zircon found in multiple eruptions at Unzen volcano.
The past 500 ky of activity at Unzen volcano is divided into two main periods. The older period has started at 600 ka and ended around 300 ka. After that, the latter period has started from about 300 ka to the present. The older products are distributed mainly outside the Unzen Graben on the Shimabara Peninsula, and this activity began with basaltic eruptions. After that, the composition of the products gradually changed to more SiO2-rich andesites and dacites. The volcanic activity during this period generated pyroclastic-flows, debris-avalanches, lava domes and lava flows. On the other hand, the distribution of the younger deposits is limited within the Unzen Graben. The volcanics are mostly crystal-rich and of intermediate compositions (andesites and dacites) and typically contain mafic enclaves. The younger activity is characterized by lava dome effusions, block and ash flows and phreatomagmatic products.
In this study, the following samples were analyzed at the Swiss Federal Institute of Technology Zürich (ETH Zürich), using LA-ICP-MS and MC-ICP-MS: the Tonosaka, Takaiwa, Futtsu and Ayugaerino-taki lavas from the older period, and the lavas from the younger period that erupted during the Heisei eruption, the Shichimensan, and the Fukkoshi. For the zircon erupted in the younger period, U-Th disequilibrium dating was first carried out, and for some zircon found to be in secular equilibrium, U-Pb dating was subsequently performed to determine the crystallization ages. For the older Unzen products, only U-Pb dating was applied (too old for the U-Th method).
The crystallization ages of the individual zircon spread over several tens to hundreds of thousands of years in all analyzed samples. Some units (e,g, Tounosaka andesite) contain zircon xenocrysts (8–250 Ma) while other mostly contain antecrysts (zircons crystals that grew during the active magmatic phase of Unzen volcano, but significantly before the age of the eruption). To estimate the eruption ages, we determined the youngest population in the data set (phenocrystic zircon) using a statistical methods (Iterative-MSWD). The ages obtained were generally consistent with the ages estimated in previous studies, except for the Heisei, Mayuyama. We think that the two youngest samples, Heisei and Mayuyama did not crystallize zircon close to the eruption ages (little to no phenocrystic zircon), perhaps due to a strong reheating prior to eruption. All other units have U-Th or U-Pb ages that appear consistent with previously determined eruptions ages, using other dating techniques. Finally, we compiled all of the crystallization ages of individual zircon to obtain a frequency of zircon age distributions during the evolution of the Unzen magmatic system. The compiled age variation shows a bimodal age distribution, with a clear lower crystallization rate between 350 and 450 ka. This roughly coincides with the period showing fewer eruptions. The paucity of zircon at that time is possibly related to a slower magma production rate, anticipating the shift of the magmatic plumbing system more to the East (Period 2). Interestingly, the average value of εHf (t) in zircon over a period of 500,000 years is extremely constant (between +6 and +8, with a standard error of 0.9). This result indicates that the state of the mantle source was stable during this period, and no significant crustal contamination occurred.
The past 500 ky of activity at Unzen volcano is divided into two main periods. The older period has started at 600 ka and ended around 300 ka. After that, the latter period has started from about 300 ka to the present. The older products are distributed mainly outside the Unzen Graben on the Shimabara Peninsula, and this activity began with basaltic eruptions. After that, the composition of the products gradually changed to more SiO2-rich andesites and dacites. The volcanic activity during this period generated pyroclastic-flows, debris-avalanches, lava domes and lava flows. On the other hand, the distribution of the younger deposits is limited within the Unzen Graben. The volcanics are mostly crystal-rich and of intermediate compositions (andesites and dacites) and typically contain mafic enclaves. The younger activity is characterized by lava dome effusions, block and ash flows and phreatomagmatic products.
In this study, the following samples were analyzed at the Swiss Federal Institute of Technology Zürich (ETH Zürich), using LA-ICP-MS and MC-ICP-MS: the Tonosaka, Takaiwa, Futtsu and Ayugaerino-taki lavas from the older period, and the lavas from the younger period that erupted during the Heisei eruption, the Shichimensan, and the Fukkoshi. For the zircon erupted in the younger period, U-Th disequilibrium dating was first carried out, and for some zircon found to be in secular equilibrium, U-Pb dating was subsequently performed to determine the crystallization ages. For the older Unzen products, only U-Pb dating was applied (too old for the U-Th method).
The crystallization ages of the individual zircon spread over several tens to hundreds of thousands of years in all analyzed samples. Some units (e,g, Tounosaka andesite) contain zircon xenocrysts (8–250 Ma) while other mostly contain antecrysts (zircons crystals that grew during the active magmatic phase of Unzen volcano, but significantly before the age of the eruption). To estimate the eruption ages, we determined the youngest population in the data set (phenocrystic zircon) using a statistical methods (Iterative-MSWD). The ages obtained were generally consistent with the ages estimated in previous studies, except for the Heisei, Mayuyama. We think that the two youngest samples, Heisei and Mayuyama did not crystallize zircon close to the eruption ages (little to no phenocrystic zircon), perhaps due to a strong reheating prior to eruption. All other units have U-Th or U-Pb ages that appear consistent with previously determined eruptions ages, using other dating techniques. Finally, we compiled all of the crystallization ages of individual zircon to obtain a frequency of zircon age distributions during the evolution of the Unzen magmatic system. The compiled age variation shows a bimodal age distribution, with a clear lower crystallization rate between 350 and 450 ka. This roughly coincides with the period showing fewer eruptions. The paucity of zircon at that time is possibly related to a slower magma production rate, anticipating the shift of the magmatic plumbing system more to the East (Period 2). Interestingly, the average value of εHf (t) in zircon over a period of 500,000 years is extremely constant (between +6 and +8, with a standard error of 0.9). This result indicates that the state of the mantle source was stable during this period, and no significant crustal contamination occurred.