13:45 〜 15:15
[SVC34-P05] The pre-eruptive process of the Fukutoku-Oka-no-Ba 2021 eruption: constraints from the crystals in dark enclaves
キーワード：福徳岡ノ場、トラカイト、プレ噴火プロセス、マグマ供給系、集斑晶
Fukutoku-Oka-no-Ba (FOB) is one of the active submarine volcanoes of the Izu-Bonin arc. The latest and largest eruption of the volcano occurred on 13 August 2021, which was a phreatomagmatic eruption with an erupted magma volume of ~0.1 km3. The erupted magma has a trachytic composition and most pumice is gray with dark enclaves containing crystal clots. In this study, we performed textural and chemical analyses for minerals and glass in the dark enclaves to investigate the pre-eruptive process of the FOB 2021 eruption.
We used the gray pumice of the FOB 2021 eruption collected at Uzahama beach, Okinawa Island and Okibara beach, Miyake-Jima. The dark enclaves were picked up from crushed pumice and were processed into polished thin sections. We used FE-EPMA at Earthquake Research Institute (ERI) for textural observations and element concentration mapping, and EPMA at ERI for chemical analyses, respectively.
Most of the crystal clots were composed of An-poor plagioclase (An#~33-48), Fe-rich olivine (Fo~65-70) and clinopyroxene (cpx) (Mg#~70-78), accompanied by magnetite and apatite (Group-A). These minerals are chemically homogeneous, and they have the brown-colored interstitial glass and melt inclusions. The interstitial glass and the melt inclusions are almost homogeneous with a trachytic composition similar to the gray pumice-forming glass. On the other hand, An-rich plagioclase (An#~93-94), Mg-rich olivine (Fo~87-92) and cpx (Mg#~80-88, Cr#~0.8-30) were rarely found as isolated phenocrysts (Group-B). These minerals have homogeneous cores with normally zoned rims (up to several of tens microns of thickness). The olivine is enriched in CaO (~0.25-0.35 wt%) and depleted in NiO (~0.09-0.28 wt%) compared to lithospheric mantle olivine, and it has inclusions of Cr-rich spinel (Mg#~56-60, Cr#~78-80). The cpx has melt inclusions of basaltic to trachyandesitic composition.
Our results indicate that the trachytic melt was in equilibrium with Group-A minerals. The absence of chemical zoning in Group-A minerals as well as the chemical homogeneity of the co-existing melt suggest that thermal-chemical fluctuation was negligible right before the eruption and/or that the duration between the thermal-chemical instability to the eruption was short; the diffusion time of Ti in magnetite to form detectable chemical zoning of 5 micron thickness is estimated to be ~7hr. Using the plagioclase-melt and the cpx-melt geothermo-hygrometers calibrated for trachytic melt, the equilibrium temperature-melt H2O content conditions are estimated to be ~935 ºC and ~3.7 wt%, respectively. Assuming the crustal density of 2390 kg/m3, the pressure of H2O-saturation is calculated to be ~69 MPa, which corresponds to a depth of ~3 km. The estimated depth is consistent with the previous geophysical observations that the low-seismic velocity, high-attenuation, and non-magnetic zone exist at a depth > 2km beneath the volcano.
The CaO-MnO-Fo content relation of Group-B olivine and Cr# of the spinel inclusions suggest that they crystallized from a primitive melt derived from a highly depleted mantle. The low Ni content of the olivine may be attributed to the coexistence of the sulfide phase, implying the contribution of subduction-related sediment component. Cr# of Group-B cpx positively correlates with Mg#, and the highest Cr# cpx may have crystallized with Group-B olivine and spinel inclusions. The continuous variation of core composition and chemically zoned rim of the cpx suggest the existence of another magma chamber where Group-B minerals crystallized, located at a larger depth than the chamber of the trachytic melt. The less-evolved, mafic magma containing Group-B minerals intruded into the shallower trachytic magma chamber right before the eruption, which may have increased the overpressure to trigger the eruption, but mixing between the trachytic and the mafic magmas was minor.
We used the gray pumice of the FOB 2021 eruption collected at Uzahama beach, Okinawa Island and Okibara beach, Miyake-Jima. The dark enclaves were picked up from crushed pumice and were processed into polished thin sections. We used FE-EPMA at Earthquake Research Institute (ERI) for textural observations and element concentration mapping, and EPMA at ERI for chemical analyses, respectively.
Most of the crystal clots were composed of An-poor plagioclase (An#~33-48), Fe-rich olivine (Fo~65-70) and clinopyroxene (cpx) (Mg#~70-78), accompanied by magnetite and apatite (Group-A). These minerals are chemically homogeneous, and they have the brown-colored interstitial glass and melt inclusions. The interstitial glass and the melt inclusions are almost homogeneous with a trachytic composition similar to the gray pumice-forming glass. On the other hand, An-rich plagioclase (An#~93-94), Mg-rich olivine (Fo~87-92) and cpx (Mg#~80-88, Cr#~0.8-30) were rarely found as isolated phenocrysts (Group-B). These minerals have homogeneous cores with normally zoned rims (up to several of tens microns of thickness). The olivine is enriched in CaO (~0.25-0.35 wt%) and depleted in NiO (~0.09-0.28 wt%) compared to lithospheric mantle olivine, and it has inclusions of Cr-rich spinel (Mg#~56-60, Cr#~78-80). The cpx has melt inclusions of basaltic to trachyandesitic composition.
Our results indicate that the trachytic melt was in equilibrium with Group-A minerals. The absence of chemical zoning in Group-A minerals as well as the chemical homogeneity of the co-existing melt suggest that thermal-chemical fluctuation was negligible right before the eruption and/or that the duration between the thermal-chemical instability to the eruption was short; the diffusion time of Ti in magnetite to form detectable chemical zoning of 5 micron thickness is estimated to be ~7hr. Using the plagioclase-melt and the cpx-melt geothermo-hygrometers calibrated for trachytic melt, the equilibrium temperature-melt H2O content conditions are estimated to be ~935 ºC and ~3.7 wt%, respectively. Assuming the crustal density of 2390 kg/m3, the pressure of H2O-saturation is calculated to be ~69 MPa, which corresponds to a depth of ~3 km. The estimated depth is consistent with the previous geophysical observations that the low-seismic velocity, high-attenuation, and non-magnetic zone exist at a depth > 2km beneath the volcano.
The CaO-MnO-Fo content relation of Group-B olivine and Cr# of the spinel inclusions suggest that they crystallized from a primitive melt derived from a highly depleted mantle. The low Ni content of the olivine may be attributed to the coexistence of the sulfide phase, implying the contribution of subduction-related sediment component. Cr# of Group-B cpx positively correlates with Mg#, and the highest Cr# cpx may have crystallized with Group-B olivine and spinel inclusions. The continuous variation of core composition and chemically zoned rim of the cpx suggest the existence of another magma chamber where Group-B minerals crystallized, located at a larger depth than the chamber of the trachytic melt. The less-evolved, mafic magma containing Group-B minerals intruded into the shallower trachytic magma chamber right before the eruption, which may have increased the overpressure to trigger the eruption, but mixing between the trachytic and the mafic magmas was minor.