10:45 AM - 12:15 PM
[SVC29-P10] Petrological constraints on magma ascent processes during the 1977 eruption of Usu volcano, Japan.
Keywords:explosive eruption, magma discharge rate, decompression rate, groundmass crystal texture
In a simple model, a magma discharge rate is expected to decrease with overpressure in a magma reservoir. Indeed, the exponential decay of the discharge rate with time was observed in some cases (e.g., Wadge, 1981; Hreinsdóttir et al, 2014). In contrast, the discharge rate during explosive eruptions shows complicated behavior. For instance, three sub-plinian eruptions and a vulcanian eruption were followed by an additional sub-plinian eruption, and then a vulcanian eruption occurred again during the 1977 eruption of Usu volcano. During the four sub-plinian eruptions, the discharge rate is higher than that during the two vulcanian eruptions; hence, the discharge rate once decreased and then increased, and finally decreased again in the case of the Usu volcano. In order to understand the mechanism of these variations in the discharge rate, we investigated the magma ascent processes of the Usu 1977 eruption based on petrological studies on pumices.
We measured the bulk density of pumices and selected 2–4 pumices with the mode value of the density distribution. From these pumices, we obtained back scattered electron images to investigate the number density and crystallinity of plagioclase microlite. We also performed FT-IR micro reflectance measurements to determine water contents in melt inclusions. Bulk chemical compositions of some pumices were measured by XRF.
Our analytical results are summarized below: (1) bulk chemical compositions show no clear variation during eruptions, and the water contents in melt inclusions slightly decrease with time, (2) the bulk density distribution of pumices indicates that the vesicularity of pumices is high in the third and fourth sub-plinian eruptions, (3) the number density of plagioclase microlite increased with time, but decreased in the final vulcanian eruptions, and (4) crystallinity slightly increased with time. In summary, we infer that the conditions in the magma reservoir did not change so much during the 1977 eruption, but the number density of plagioclase microlite increased and then decreased, indicating that the decompression rate increased and then decreased. No clear decreases in the decompression rate and water content in melt inclusions was found in the vulcanian eruption between the third and fourth sub-plinian eruptions, which implies that the transition from the third sub-plinian to the following vulcanian eruption was not induced by the decrease in overpressure in the magma reservoir. Therefore, it is inferred that not only the overpressure of magma reservoir but also other mechanisms may influence the temporal evolution of the discharge rates and eruption styles.
We measured the bulk density of pumices and selected 2–4 pumices with the mode value of the density distribution. From these pumices, we obtained back scattered electron images to investigate the number density and crystallinity of plagioclase microlite. We also performed FT-IR micro reflectance measurements to determine water contents in melt inclusions. Bulk chemical compositions of some pumices were measured by XRF.
Our analytical results are summarized below: (1) bulk chemical compositions show no clear variation during eruptions, and the water contents in melt inclusions slightly decrease with time, (2) the bulk density distribution of pumices indicates that the vesicularity of pumices is high in the third and fourth sub-plinian eruptions, (3) the number density of plagioclase microlite increased with time, but decreased in the final vulcanian eruptions, and (4) crystallinity slightly increased with time. In summary, we infer that the conditions in the magma reservoir did not change so much during the 1977 eruption, but the number density of plagioclase microlite increased and then decreased, indicating that the decompression rate increased and then decreased. No clear decreases in the decompression rate and water content in melt inclusions was found in the vulcanian eruption between the third and fourth sub-plinian eruptions, which implies that the transition from the third sub-plinian to the following vulcanian eruption was not induced by the decrease in overpressure in the magma reservoir. Therefore, it is inferred that not only the overpressure of magma reservoir but also other mechanisms may influence the temporal evolution of the discharge rates and eruption styles.