11:00 AM - 1:00 PM
[SVC32-P02] Quartz bearing plutonic xenoliths of Izu-Oshima 1986B eruption: Implication for mushy magma reservoir process
Keywords:Izu-Oshima volcano, Crystal mush, magma reservoir, Plutonic xenoliths, Quartz
Interstitial glass-bearing plutonic xenoliths are thought to be fragments of crystal mush and have information on the magmatic process in the mush reservoir. In this study, petrological, textural, and chemical analyses were performed on the interstitial glass-bearing plutonic xenoliths included in the deposits of the 1986B eruption of Izu-Oshima volcano, to investigate the pre-eruptive process in the shallow magma chamber related to the B eruption. Plagioclase, clinopyroxene, orthopyroxene, and interstitial glass are found in all of the investigated xenolith samples, and some samples contain Fe-Ti oxide and olivine. About 70% and 30% of these samples are classified into gabbro-gabbronorite and anorthosite, respectively. One sample (OSNXe-3B) has ~16 vol.% of coarse-grained quartz. Quartz is rare in the deposits of the volcano, but it is described in the dacitic deposits which were erupted only small amount from the B vents, suggesting the petrogenetic relevance between the dacitic magma and the xenolith sample.
The xenolith sample (OSNXe-3B) is composed of two regions; one is the gabbroic region chiefly composed of pyroxene, Fe-Ti oxides and plagioclase (referred to as the M-region) and another is the tonalitic region composed of quartz, plagioclase and interstitial silicic glass (referred to as the S-region). The S-region is distributed on the patch in the M-region, and interstitial space in the M-region is filled by quartz-bearing silicic glass. There is no significant compositional difference between the interstitial glass in the M- and S-regions and they are dacitic to rhyolitic. The 1986B dacite is within the compositional ranges of the interstitial glass, suggesting that the xenoliths are derived from the reservoir of the magma.
An# [=100Ca/(Ca+Na)] of plagioclase ranges from 60 to 82 in the M-region and from 27 to 47 in the S-region. An# changes within the narrow boundary region between the two regions. An# of plagioclase in the M-region is close to that of plagioclase phenocrysts in the 1986 B basaltic andesite. While plagioclase in the S-region is anorthite-poor compared to those of plagioclase in equilibrium with rhyolitic melts of Izu peninsula and Hakone volcano of the Izu arc. Exsolution structures are developed in both pyroxene and Fe-Ti oxides, suggesting the crystals of the M-region had slowly cooled after they were crystallized. Symplectite composed of clinopyroxene, plagioclase, and glass are found at the boundary between pyroxene and the interstitial glass, suggesting the M- and S-regions were in disequilibrium. Magnetite is chemically heterogeneous and Ti-rich around cracks, suggesting that the xenolith was rapidly heated right before the eruption.
These results indicate that the xenolith experienced the following three stages. First, the M-region was formed as a cumulate mush from mafic magma (stage-1). Then, a tonalitic melt flowed into the interstitial space of the M-region and replaced the original interstitial melt, followed by slow cooling to crystallize coarse quartz (stage-2). Finally, the composite mush was captured as xenolith, heated in the 1986B magma, and transported to the surface (stage-3). Heating in stage-3 induced minor melting of constituent minerals, which may partly have contributed to the chemical heterogeneity of the interstitial glass. The inflow of tonalitic melt in stage 2 is thought to have induced as follows; the tonalitic melt was formed by partial melting of perfectly-solidified cumulate rock at the base of the crystal mush layer and/or the wall crustal rock surrounding the magma reservoir. Because the melt is less-dense compared to mafic melt filling the interstitial space of the mush layer, buoyancy-driven permeable flow of the melt to the upper part of the mush was induced. The migration of the differentiated melt from the bottom of the crystal mush may have contributed to evolving the mafic melt pooled in the chamber above the mush layer.
The xenolith sample (OSNXe-3B) is composed of two regions; one is the gabbroic region chiefly composed of pyroxene, Fe-Ti oxides and plagioclase (referred to as the M-region) and another is the tonalitic region composed of quartz, plagioclase and interstitial silicic glass (referred to as the S-region). The S-region is distributed on the patch in the M-region, and interstitial space in the M-region is filled by quartz-bearing silicic glass. There is no significant compositional difference between the interstitial glass in the M- and S-regions and they are dacitic to rhyolitic. The 1986B dacite is within the compositional ranges of the interstitial glass, suggesting that the xenoliths are derived from the reservoir of the magma.
An# [=100Ca/(Ca+Na)] of plagioclase ranges from 60 to 82 in the M-region and from 27 to 47 in the S-region. An# changes within the narrow boundary region between the two regions. An# of plagioclase in the M-region is close to that of plagioclase phenocrysts in the 1986 B basaltic andesite. While plagioclase in the S-region is anorthite-poor compared to those of plagioclase in equilibrium with rhyolitic melts of Izu peninsula and Hakone volcano of the Izu arc. Exsolution structures are developed in both pyroxene and Fe-Ti oxides, suggesting the crystals of the M-region had slowly cooled after they were crystallized. Symplectite composed of clinopyroxene, plagioclase, and glass are found at the boundary between pyroxene and the interstitial glass, suggesting the M- and S-regions were in disequilibrium. Magnetite is chemically heterogeneous and Ti-rich around cracks, suggesting that the xenolith was rapidly heated right before the eruption.
These results indicate that the xenolith experienced the following three stages. First, the M-region was formed as a cumulate mush from mafic magma (stage-1). Then, a tonalitic melt flowed into the interstitial space of the M-region and replaced the original interstitial melt, followed by slow cooling to crystallize coarse quartz (stage-2). Finally, the composite mush was captured as xenolith, heated in the 1986B magma, and transported to the surface (stage-3). Heating in stage-3 induced minor melting of constituent minerals, which may partly have contributed to the chemical heterogeneity of the interstitial glass. The inflow of tonalitic melt in stage 2 is thought to have induced as follows; the tonalitic melt was formed by partial melting of perfectly-solidified cumulate rock at the base of the crystal mush layer and/or the wall crustal rock surrounding the magma reservoir. Because the melt is less-dense compared to mafic melt filling the interstitial space of the mush layer, buoyancy-driven permeable flow of the melt to the upper part of the mush was induced. The migration of the differentiated melt from the bottom of the crystal mush may have contributed to evolving the mafic melt pooled in the chamber above the mush layer.