[SVC39-P03] Microanalytical evidence for Quaternary magma dynamics of crystal-rich and crystal-poor dacitic magma beneath Yotei Volcano, Hokkaido
Keywords:Yotei Volcano, Dacite, Magma dynamics, Explosive eruption
The Y37 fall deposit (~47 ka) is composed of white pumice, grey pumice, banded pumice, and scoria. The dominant white pumice is a crystal rich (34 wt%) dacite (SiO2 = 66.5 ± 0.7wt%) containing plagioclase, hornblende, quartz and Fe-Ti oxides. The Y25 tephra (35.8 ± 0.4 ka) is also composed of white pumice, grey pumice, banded pumice and scoria, but the dominant Y25 white dacite pumice (SiO2 = 66.9 ± 0.8 wt%) is crystal poor (3.9 wt%) with a mineral assemblage of plagioclase, orthopyroxene, clinopyroxene and Fe-Ti oxides. Like Y25, Y26 (36.4 ± 0.1 ka) is composed of poorly-phyric white dacite pumice (SiO2 = 65.1 ± 0.2 wt%), grey pumice, banded pumice and scoria, and the white dacitic pumice contains the same minerals as Y25. Y27 (36.5 ± 0.1) is composed of basaltic andesitic scoria (SiO2 = 55.3 ± 0.2 wt%) and include the same minerals as Y25 and Y26. Glomerophric clots of pyroxene and plagioclase are found in all eruptions.
EMP analysis reveals their difference of mineral compositions among these eruptive products. Plagioclase in the Y37 dacite has a bimodal population of cores; one group An = 38–61 and the second group with An 70–91; whereas placioclase core in Y25, 26, and 27 reveal narrow ranges of An = 50–60, 50–68 and 72–90 respectively. Calculated P–T conditions of pre-eruptive magma of Y37 shows shallower and cooler conditions (828–852 ℃ and 99–175 MPa using Hb thermo-barometer; Ridolfi et al., 2010) than that of Y25, Y26 and Y27 (959–968 ℃, 610–730 MPa; 978–996℃, 390–580 MPa; 1048–1062 ℃, 470–480 MPa, respectively based on two-pyroxene pairs; Putirka, 2008). Calculations using Fe-Ti oxides (Ghiorso and Evans, 2008) provides lower temperature than that of other calculation methods such as 727 ℃ (Y37), 917 ℃ (Y27), 957 ℃ (Y26). Pyroxene pairs in clots show Mg#(cpx) < 0.75 and some of these have KD <0.95, suggesting sub-solidus re-equilibrium. U-Pb dating of zircon from the Y37 dacite reveals a trimodal population distribution (20–25 Ma, 6–15 Ma and younger than 3 Ma).
These data suggest systematic magma dynamics beneath Yotei volcano. The crystal-rich, large magnitude explosive Stage I represents upper crustal (~5 km) pre-eruptive conditions. The textural maturity, complex zircon U–Pb age populations and bimodal An contents of Y37 dacite suggest remobilization of older granitic precursors by recharging andesitic magma. On the other hand, Stage II dacites represent deeper condition (12 to 15 km) as well as Y27 andesite. Crystal clots appear to contain antecrysts and xenocrysts liberated from deep mafic mush zones and distributed throughout dacitic and andesitic melt extracted from parental source magmas.