5:15 PM - 7:15 PM
[SVC33-P04] Decompression rate-dependence of groundmass texture of basaltic andesite magma: Comparison between Nishinoshima Ep. 4 and Izu-Oshima 1986B eruptions
Keywords:Nishinoshima, Izu-Oshima, Basaltic andesite, Groundmass texture, Decompression rate
The style of mafic explosive eruption varies significantly from Strombolian to Plinian. The main factor controlling the eruption style is the decompression rate of magma during conduit ascent. Texture of the groundmass minerals formed during conduit ascent are expected to record information on the decompression rate of magma (e.g., Toramaru et al., 2008). However, groundmass crystallization also depends on the chemical composition, water content, and temperature. Therefore, what are the textural parameters most suitable as indicators of decompression rate and their quantitative relationships are unclear. To overcome this situation, it would be effective to compare the groundmass textures of pyroclasts from eruptions with different eruption styles but the same chemical composition and temperature, among those whose magma eruption rates are known through observation. Pyroclasts from the Izu-Oshima 1986B eruption (hereafter, 86B) and the Nishinoshima Ep. 4 (hereafter, N4) satisfy this condition. These have almost the same chemical composition and temperature, but the eruption style is different; sub-Plinian for 86B and violent Strombolian for N4. In this study, we compared the results of the textural analysis of the groundmass of scoria from these two eruptions and examined the relationship between groundmass texture, decompression rate, and eruption style.
The 86B samples were TB2 scoria deposits collected at loc. 52 (Mannen, 2006), 3 km east of the crater. The N4 samples were collected by the Japan Meteorological Agency's observation vessel Ryofu-Maru on July 11, 2020, 18.5 km north-northwest of Nishinoshima. We focused on glassy particles that make up the majority of both samples. BSE images and elemental maps were obtained using the FE-EPMA at the Earthquake Research Institute, University of Tokyo (ERI), and the FE-SEM-EDS at Shizuoka University, and textural analysis was performed on the obtained BSE images. In addition, chemical analyses of minerals and glasses were performed using the FE-EPMA at ERI.
The groundmass of both scoriae consists mainly of plagioclase, clinopyroxene, and small amounts of Fe-Ti oxides, and only N4 scoria contains a small amount of olivine. The groundmass crystal contents of plagioclase (pl) and clinopyroxene (cpx), VPl and VCpx, were 12.1-32.9 vol% and 0.9-21.4 vol% for 86B, and 20.8-61.9 vol% and 6.2-18.7 vol% for N4, respectively. The crystal number densities of pl and cpx, NPl and NCpx, were 1.1-16.3×105 mm-2 and 0.6-11.7×105 mm-2 for 86B, and 1.3-6.0×105 mm-2 and 1.0-9.6×105 mm-2 for N4, respectively. The mean crystal sizes of pl and cpx, SPl, SCpx (Si=√Vi /√Ni), were 0.01-0.08 mm and 0.01-0.06 mm for 86B, and 0.03-0.05 mm and 0.01-0.03 mm for N4, respectively. VPl and SPl are smaller and NPl is larger for 86B compared to N4. On the other hand, VCpx, NCpx, and SCpx are almost identical between both eruptions.
The magma discharge rate is larger for the Izu-Oshima 1986B eruption of ~8.4×105 kg/s (~336 DRE m3/s; Mannen, 2006) than for the Nishinoshima Ep.4 eruption of ~6.06 DRE m3/s (Kaneko et al., 2022), suggesting that N4 scoria experienced a lower decompression rate than 86B scoria. Therefore, the clear difference in the texture of groundmass plagioclase seen in N4 scoria and 86B are likely to reflect the difference in decompression rate. In contrast, neither the crystal amount nor the number density of clinopyroxene changes significantly between N4 scoria and 86B scoria. This indicates that the texture of groundmass clinopyroxene does not reflect decompression rate. This may be because the gradient of clinopyroxene liquidus against water content is smaller than that of plagioclase (Toramaru et al., 2008), making it insensitive to the decompression rate. Our results suggest that the texture of groundmass plagioclase (particularly mean size) is useful as an indicator of decompression rate.
The 86B samples were TB2 scoria deposits collected at loc. 52 (Mannen, 2006), 3 km east of the crater. The N4 samples were collected by the Japan Meteorological Agency's observation vessel Ryofu-Maru on July 11, 2020, 18.5 km north-northwest of Nishinoshima. We focused on glassy particles that make up the majority of both samples. BSE images and elemental maps were obtained using the FE-EPMA at the Earthquake Research Institute, University of Tokyo (ERI), and the FE-SEM-EDS at Shizuoka University, and textural analysis was performed on the obtained BSE images. In addition, chemical analyses of minerals and glasses were performed using the FE-EPMA at ERI.
The groundmass of both scoriae consists mainly of plagioclase, clinopyroxene, and small amounts of Fe-Ti oxides, and only N4 scoria contains a small amount of olivine. The groundmass crystal contents of plagioclase (pl) and clinopyroxene (cpx), VPl and VCpx, were 12.1-32.9 vol% and 0.9-21.4 vol% for 86B, and 20.8-61.9 vol% and 6.2-18.7 vol% for N4, respectively. The crystal number densities of pl and cpx, NPl and NCpx, were 1.1-16.3×105 mm-2 and 0.6-11.7×105 mm-2 for 86B, and 1.3-6.0×105 mm-2 and 1.0-9.6×105 mm-2 for N4, respectively. The mean crystal sizes of pl and cpx, SPl, SCpx (Si=√Vi /√Ni), were 0.01-0.08 mm and 0.01-0.06 mm for 86B, and 0.03-0.05 mm and 0.01-0.03 mm for N4, respectively. VPl and SPl are smaller and NPl is larger for 86B compared to N4. On the other hand, VCpx, NCpx, and SCpx are almost identical between both eruptions.
The magma discharge rate is larger for the Izu-Oshima 1986B eruption of ~8.4×105 kg/s (~336 DRE m3/s; Mannen, 2006) than for the Nishinoshima Ep.4 eruption of ~6.06 DRE m3/s (Kaneko et al., 2022), suggesting that N4 scoria experienced a lower decompression rate than 86B scoria. Therefore, the clear difference in the texture of groundmass plagioclase seen in N4 scoria and 86B are likely to reflect the difference in decompression rate. In contrast, neither the crystal amount nor the number density of clinopyroxene changes significantly between N4 scoria and 86B scoria. This indicates that the texture of groundmass clinopyroxene does not reflect decompression rate. This may be because the gradient of clinopyroxene liquidus against water content is smaller than that of plagioclase (Toramaru et al., 2008), making it insensitive to the decompression rate. Our results suggest that the texture of groundmass plagioclase (particularly mean size) is useful as an indicator of decompression rate.