15:00 〜 15:15
[SGC37-11] かんらん石メルト包有物を用いたマグマ生成過程の解明: 中国地方，阿武単成火山群を例として
キーワード：阿武単成火山群、メルト包有物
In the Chugoku region of Japan, the subduction of the Philippine Sea (PHS) Plate has led to sporadic magmatic activity, characterized by the eruption of various rock series. The petrogenesis of this magmatism has been a subject of debate, with previous studies proposing different mechanisms. Iwamori (1992) suggested that magmatic activity is driven by the upwelling of a volatile-rich wet plume, whereas Kimura et al. (2014) and Nguyen et al. (2020) argued that it results from the influx of mildly high-temperature asthenospheric mantle injection from the back-arc side. The primary controlling factor-whether water or high temperature-remains unresolved, as prior studies have only indirectly estimated or even ignored water content of the primary magma.
This study aims to directly quantify volatile components in primary magma by analyzing melt inclusions (MIs) in olivine phenocrysts and to constrain mantle melting conditions. In general, during magma ascent, volatile elements degas, resulting in their depletion in erupted lavas. However, MIs, trapped within phenocrysts, preserve the original melt composition and volatile content at the time of mineral crystallization (Wallace et al., 2021). Olivine, as one of the earliest crystallizing minerals from the basaltic magma, is particularly valuable for retaining primary melt compositions.
This study focuses on high-vesicularity scoria samples from Mt. Takamureyama (0.18±0.01 Ma; Kakubuchi et al., 2000) in the Abu Monogenetic Volcano Group, located in northern Yamaguchi Prefecture, Chugoku region. MIs within olivine phenocrysts were analyzed to determine volatile (H2O, CO2, S, F and Cl) and major element compositions. Petrographic observations were conducted using a polarized light microscope, and whole-rock chemical compositions of the scoria samples were measured by X-ray fluorescence (XRF) analysis. Volatile components in melt inclusions were quantified using secondary ion mass spectrometry (SIMS), while major element compositions were analyzed with an electron probe microanalyzer (EPMA).
Two types of MIs were identified: "spiny" and "smooth". Spiny MIs are generally richer in H2O, ranging from 1.4 to 5.3 wt%, compared to smooth MIs(0.4-4.53 wt%), while no significant differences were observed in other volatile components between spiny and smooth MIs. This suggests that only water has selectively diffused out from the smooth MIs. Therefore, following discussion will focus on the more primitive spiny MIs. SiO2 and Na2O+K2O contents of the spiny MIs ranged from 44.5 to 48.5 wt% and 3.8 to 5.5 wt%, respectively, classifying them as alkaline basalts. As SiO2 increases, Na2O, K2O, CaO, and P2O5 decrease monotonically, while Al2O3, FeO, and MgO remain nearly constant, forming a linear trend. Additionally, low FeO*/MgO ratio (0.77 in average) and high forsterite content (88 in average) of the host olivine phenocrysts indicate that the primary magma composition has been well preserved as spiny MIs.
The major element trends in the melt inclusions cannot be explained by crystal fractionation or assimilation-fractional crystallization (AFC) processes, as incompatible elements such as K2O and P2O2 do not follow typical FC and AFC trends. Instead, these trends suggest mixing between two distinct magmas: a volatile-rich, low-SiO2 magma and a volatile-poor, high-SiO2 magma.
To constrain mantle melting conditions, we applied the method of Lee et al. (2009), utilizing the directly measured water content (3.0-5.3 wt%) of the melt inclusions. The estimated mantle melting pressure and temperature were 1.3-2.2 GPa and 1,277-1,288 °C, respectively. Compared to previous studies, the estimated pressures and temperatures are lower by 0.1-1.5 GPa and 60-200 °C, respectively, due to higher water content in the primary melt. These findings suggest that the magmatism at Abu Volcano does not necessarily require an influx of high-temperature asthenosphere, supporting wet plume or flux-melting hypothesis.
This study aims to directly quantify volatile components in primary magma by analyzing melt inclusions (MIs) in olivine phenocrysts and to constrain mantle melting conditions. In general, during magma ascent, volatile elements degas, resulting in their depletion in erupted lavas. However, MIs, trapped within phenocrysts, preserve the original melt composition and volatile content at the time of mineral crystallization (Wallace et al., 2021). Olivine, as one of the earliest crystallizing minerals from the basaltic magma, is particularly valuable for retaining primary melt compositions.
This study focuses on high-vesicularity scoria samples from Mt. Takamureyama (0.18±0.01 Ma; Kakubuchi et al., 2000) in the Abu Monogenetic Volcano Group, located in northern Yamaguchi Prefecture, Chugoku region. MIs within olivine phenocrysts were analyzed to determine volatile (H2O, CO2, S, F and Cl) and major element compositions. Petrographic observations were conducted using a polarized light microscope, and whole-rock chemical compositions of the scoria samples were measured by X-ray fluorescence (XRF) analysis. Volatile components in melt inclusions were quantified using secondary ion mass spectrometry (SIMS), while major element compositions were analyzed with an electron probe microanalyzer (EPMA).
Two types of MIs were identified: "spiny" and "smooth". Spiny MIs are generally richer in H2O, ranging from 1.4 to 5.3 wt%, compared to smooth MIs(0.4-4.53 wt%), while no significant differences were observed in other volatile components between spiny and smooth MIs. This suggests that only water has selectively diffused out from the smooth MIs. Therefore, following discussion will focus on the more primitive spiny MIs. SiO2 and Na2O+K2O contents of the spiny MIs ranged from 44.5 to 48.5 wt% and 3.8 to 5.5 wt%, respectively, classifying them as alkaline basalts. As SiO2 increases, Na2O, K2O, CaO, and P2O5 decrease monotonically, while Al2O3, FeO, and MgO remain nearly constant, forming a linear trend. Additionally, low FeO*/MgO ratio (0.77 in average) and high forsterite content (88 in average) of the host olivine phenocrysts indicate that the primary magma composition has been well preserved as spiny MIs.
The major element trends in the melt inclusions cannot be explained by crystal fractionation or assimilation-fractional crystallization (AFC) processes, as incompatible elements such as K2O and P2O2 do not follow typical FC and AFC trends. Instead, these trends suggest mixing between two distinct magmas: a volatile-rich, low-SiO2 magma and a volatile-poor, high-SiO2 magma.
To constrain mantle melting conditions, we applied the method of Lee et al. (2009), utilizing the directly measured water content (3.0-5.3 wt%) of the melt inclusions. The estimated mantle melting pressure and temperature were 1.3-2.2 GPa and 1,277-1,288 °C, respectively. Compared to previous studies, the estimated pressures and temperatures are lower by 0.1-1.5 GPa and 60-200 °C, respectively, due to higher water content in the primary melt. These findings suggest that the magmatism at Abu Volcano does not necessarily require an influx of high-temperature asthenosphere, supporting wet plume or flux-melting hypothesis.