[SVC39-P08] Plagioclase-hosted melt inclusions of the 1986 eruption at Izu-Oshima volcano, Japan: Implication for pre-eruptive process
Keywords:Izu-Oshima volcano, melt inclusions, pre-eruptive process, plagioclase, magma mixing
Scoriae from the A and B vents of the 1986 eruption, sampled in the summit caldera, are investigated. Bulk rock compositions of A and B scoria were measured using a XRF (RIGAKU ZCX primus ２) at the Earthquake Research Institute, University of Tokyo (ERI). Compositions of plagioclase-hosted melt inclusions and their host plagioclase crystals, and groundmass glasses were analyzed using EPMA (JEOL8800R) at the ERI.
Bulk rock major element compositions of A and B scoria are consistent with those reported in previous studies. Bulk rock trace element compositions were almost the same between A and B scoria. Plagioclases in A and B scoria show similar ranges of An# [=100Ca/(Ca+Na)] and trace element compositions. SiO2 and SO3 contents of melt inclusions in A plagioclases are about 55-57 wt.% and < 0.1wt.%, respectively; basaltic melt inclusion is not found. On the other hand, compositions of melt inclusions in S-rich, SiO2-poor (SO3~0.12-0.3wt.%, SiO2∼49-53wt.%) and S-poor, SiO2-rich melt inclusions (SO3 < 0.12wt.%, SiO2~53-60wt.%) are found in B plagioclase crystals. The wide compositional variation of melt inclusions in B plagioclase is explained by mixing of basaltic and andesitic melts and post-entrapment overgrowth of their host plagioclase. In addition, compositions of groundmass glasses of the A and B scoria and A plagioclase-hosted melt inclusions are also explained by the two melt mixing.
The origin of A magma is explained by incorporating plagioclase and interstitial melt as a parental magma passes through the B reservoir. Bulk rock trace element compositions indicate that A and B magmas were derived from the common parental magma. The A plagioclase is thought to be crystallized in B reservoir and incorporated into the parental magma. This is because (1) it shows the same ranges of An# and trace element compositions to B plagioclase, (2) compositions of melt inclusions in A plagioclase are in the compositional range of B plagioclase-hosted melt inclusions, and (3) melt inclusions in A plagioclase have different composition from the groundmass melt in A scoria. The passing of the parental magma induces pressurization of B reservoir and might trigger the fissure eruptions from B and C vents.
The B magma is thought to be formed from combination of supply of basaltic melt from deeper source, magma mixing, and crystallization. S-rich, SiO2-poor melt inclusions found in B plagioclases were entrapped at the depth deeper than B magma reservoir because of S enrichment. This suggests that basaltic melts have been supplied into the B magma reservoir from the deeper source. Also all melts found in the 1986 A and B scoriae are explained by mixing of the basaltic and andesitic endmember melts. Moreover, mixed melts of the two endmembers are found as inclusions in plagioclase crystals. This suggests that crystallization occurred after melt mixing.