5:15 PM - 6:30 PM
[SVC31-P06] Conduit ascent process of mafic subplinian eruption: Constraints from plagioclase microlites in the 1235 scoria of Kirishima-Ohachi volcano, Japan.
Keywords:plagioclase, mafic subplinian eruption, Kirishima, Ohachi volcano, shallow conduit process, microlite crystallization
Mildly explosive eruption is common styles of high-temperature, less viscous mafic magma. However, many cases of Plinian eruptions of mafic magmas have been reported in this decade. Recent studies suggest that microlite crystallization at the shallow conduit depth significantly increases viscosity and causes the viscous-brittle transition of mafic magma, which induces sustained fragmentation and explosion. Therefore, it is important to clarify the processes of microlite crystallization during conduit ascent for understanding the eruption mechanism of mafic magmas. The 1235 subplinian eruption is the largest eruption of Ohachi volcano, which is one of the active volcanoes in Kirishima, Japan. During the eruption, basaltic andesite magma with bulk SiO2 content ~51-55wt.% erupted and formed three thick fall scoria layers (ThT-a, ThT-b, and ThT-c) called Takaharu scoria (Tsutsui et al., 2007). Because the groundmass crystallization occurred at the shallow conduit depth, the groundmass texture is expected to have information about the conduit ascent process of the magma. In this study, we performed textural analyses of the plagioclase microlites in Takaharu scoria to clarify the conduit ascent process of mafic magma during a subplinian eruption.
In this study, we investigated glassy scoria grains (2-8 mm in size) in ThT-a, ThT-b, and ThT-c. FE-EPMA (JEOL JCXA-8530FPlus) at Earthquake Research Institute (ERI) of the University of Tokyo, Japan, was used to acquire backscatter electron (BSE) images of the groundmass of the samples. The BSE images were quantitatively analyzed to determine crystal volume fractions, crystal number densities, and crystal size distributions (CSDs) of plagioclase microlites.
Plagioclase, olivine, orthopyroxene, and clinopyroxene were found as phenocrysts in the Takaharu scoria, with a total phenocryst contents of ~19-22 vol.%. The groundmass is composed of plagioclase, mafic minerals and glass. Because olivine and pyroxene in the groundmass are difficult to distinguish on BSE images, we treated them together as mafic minerals. The microlite contents (Φ) of plagioclase are ~10-30 vol.% for ThT-a, ~10-37 vol.% for ThT-b, and ~6-33 vol.% for ThT-c, respectively. On the other hand, the microlite contents were less than 5 vol.% for mafic minerals.
Measured CSDs for plagioclase microlites are often inflected. Least-squares regression into the combination of two distinct log-linear CSD relations was done to determine the intercept, log n0, and the slope, 1/L0, of each CSD relation. The n0 for the finer crystals ranges ~108.6-9.4 mm-4 for ThT-a, 108.7-9.7 mm-4 for ThT-b, and 109.1-10.2 mm-4 for ThT-c, respectively; it increases from ThT-A to C. On the other hand, the 1/ L0 varies in the ranges of 78-169 mm-1 for ThT-a, 94-166 mm-1 for ThT-b, and 116-261 mm-1 for ThT-c, respectively; it also increases from ThT-A to C. The number densities of plagioclase microlite (N) were determined from the values of n0 and 1/L0. The Ns of plagioclase microlites were ~3.8×1015-1.3×1016m-3 for ThT-a, ~7.6×1015-3.6×1016m-3 for ThT-b, and ~1.0×1016-6.8×1016m-3 for ThT-c, respectively. N was almost constant regardless of Φ for ThT-a, whereas positive relations between Φ and N are observed for ThT-B and C.
The microlite number density decompression-rate meter (Toramaru et al., 2008) was applied to the plagioclase microlites to estimate the conduit ascent rate of the magma. The decompression rates were estimated to be in the range of ~0.31-0.73 MPa/s for ThT-a, ~0.45-1.38 MPa/s for ThT-b, and ~0.34-1.24 MPa/s for ThT-c, respectively; these are comparable with those estimated for other mafic subplinian eruptions. The estimated decompression rate increases from ThT-a to ThT-b, and then decreases to ThT-c. These results are consistent with the relation of the dispersal areas between three scoria units (Tsutsui et al., 2007). Our results suggest that the eruption intensity is controlled by the conduit ascent rate of the mafic magma.
In this study, we investigated glassy scoria grains (2-8 mm in size) in ThT-a, ThT-b, and ThT-c. FE-EPMA (JEOL JCXA-8530FPlus) at Earthquake Research Institute (ERI) of the University of Tokyo, Japan, was used to acquire backscatter electron (BSE) images of the groundmass of the samples. The BSE images were quantitatively analyzed to determine crystal volume fractions, crystal number densities, and crystal size distributions (CSDs) of plagioclase microlites.
Plagioclase, olivine, orthopyroxene, and clinopyroxene were found as phenocrysts in the Takaharu scoria, with a total phenocryst contents of ~19-22 vol.%. The groundmass is composed of plagioclase, mafic minerals and glass. Because olivine and pyroxene in the groundmass are difficult to distinguish on BSE images, we treated them together as mafic minerals. The microlite contents (Φ) of plagioclase are ~10-30 vol.% for ThT-a, ~10-37 vol.% for ThT-b, and ~6-33 vol.% for ThT-c, respectively. On the other hand, the microlite contents were less than 5 vol.% for mafic minerals.
Measured CSDs for plagioclase microlites are often inflected. Least-squares regression into the combination of two distinct log-linear CSD relations was done to determine the intercept, log n0, and the slope, 1/L0, of each CSD relation. The n0 for the finer crystals ranges ~108.6-9.4 mm-4 for ThT-a, 108.7-9.7 mm-4 for ThT-b, and 109.1-10.2 mm-4 for ThT-c, respectively; it increases from ThT-A to C. On the other hand, the 1/ L0 varies in the ranges of 78-169 mm-1 for ThT-a, 94-166 mm-1 for ThT-b, and 116-261 mm-1 for ThT-c, respectively; it also increases from ThT-A to C. The number densities of plagioclase microlite (N) were determined from the values of n0 and 1/L0. The Ns of plagioclase microlites were ~3.8×1015-1.3×1016m-3 for ThT-a, ~7.6×1015-3.6×1016m-3 for ThT-b, and ~1.0×1016-6.8×1016m-3 for ThT-c, respectively. N was almost constant regardless of Φ for ThT-a, whereas positive relations between Φ and N are observed for ThT-B and C.
The microlite number density decompression-rate meter (Toramaru et al., 2008) was applied to the plagioclase microlites to estimate the conduit ascent rate of the magma. The decompression rates were estimated to be in the range of ~0.31-0.73 MPa/s for ThT-a, ~0.45-1.38 MPa/s for ThT-b, and ~0.34-1.24 MPa/s for ThT-c, respectively; these are comparable with those estimated for other mafic subplinian eruptions. The estimated decompression rate increases from ThT-a to ThT-b, and then decreases to ThT-c. These results are consistent with the relation of the dispersal areas between three scoria units (Tsutsui et al., 2007). Our results suggest that the eruption intensity is controlled by the conduit ascent rate of the mafic magma.