1:45 PM - 3:15 PM
[SVC36-P08] Differentiation process of rhyolite magma of Kawago-daira volcano inferred from trace element composition of amphibole phenocryst
Keywords:Rhyolite magma, Crystal fractionation, Monogenetic volcano, Izu arc
Introduction
Kawagodaira volcano, located in the northwestern part of the Izu Peninsula, is a monogenetic volcano that erupted about 3100 years ago (Tani et al., 2013). Kawagodaira is the first volcano to erupt rhyolitic magma in the Higashi-Izu monogenetic volcano group, which is dominated by basaltic to andesitic magma. In this study, we focus on the trace element composition of amphibole phenocrysts to discuss the geochemical processes that derived the rhyolite magma of the Kawagodaira volcano.
Analysis and Result
We analyzed the major and trace element composition of amphibole phenocrysts in the Kawagohira rhyolite using the Laser-ablation ICP-MS (Japan Agency for Marine-Earth Science and Technology). The amphibole phenocrysts exhibit a wide range of compositions. For example, the concentration of lutetium, the heavy rare earth element, ranges from 0.47 to 2.64 ppm. Both major and trace elements exhibit continuous compositional variations. Rare earth elements (REE) exhibit parallel patterns regardless of concentration on the spidergram, while strontium (Sr) and europium (Eu) anomalies are noticeable. Based on multivariable statistical analysis (principal component analysis; Ueki and Iwamori, 2017), it was found that the trace element variations can be explained by two principal components. The first principal component involves only REE and Sr, which are inversely correlated, i.e., REE enrichment and Sr depletion. The second principal component involves LILE (large-ion lithophile elements) and some HFSE (high-field strength elements), indicating a process that enriches LILE and HFSE independently of REE and Sr.
Discussion
SiO2 concentrations of host melts of amphibole phenocrysts estimated by Suwa et al. (2018) exhibit a series of compositions ranging from andesite to rhyolite, although all amphiboles were found in rhyolite. The SiO2 concentrations of the estimated host melts are correlated with the REE enrichment, Sr depletion, and Eu anomaly in the amphibole. In other words, the SiO2 concentration of the melt that crystallized amphibole increased with increasing REE concentrations and negative Sr and Eu anomalies. The parallel REE patterns on the spidergrams and the result from the principal component analysis rule out the possibility of magma mixing or partial melting trend for the compositional variations of amphiboles. On the other hand, the negative the Sr and Eu anomalies with increasing SiO2 and REE strongly suggest the fractionation of plagioclase. Based on the mass-balance calculation on the trace element composition of the host melt calculated using the melt-amphibole partition coefficient, the Sr and Eu anomalies and the increase in REE of the estimated host melts are consistently reproduced by the plagioclase fractionation. Also, mass-balance of major elements and the petrological analysis by Suwa et al. (2018) suggest the fractionations of small amounts of other phenocryst minerals of the Kawagodaira rhyolite (orthopyroxene, Ti-Fe oxide, and amphibole) in addition to plagioclase. The REE ratios of the andesites of the estimated host melts exhibit a compositional range that overlaps with that of the andesites that erupted as the Higashi-Izu monogenetic volcanoes (Arakawa et al., 2022).
Summary
The results of this study suggest that the compositional variation from andesite to rhyolite recorded in amphibole phenocrysts in the Kawagodaira rhyolite represents plagioclase-dominated crystal fractionation. In addition, LILE and HFSE behave independently of REE and Sr, suggesting that magma mixing or assimilation could have contributed in addition to crystal fractionation. Indeed, previous studies (Suzuki, 2000; Arakawa et al., 2022) have suggested that magma mixing was also involved in the generation of felsic magma in the Higashi-Izu group. In this presentation, we will further discuss the magma-feeding system of the monogenetic volcanoes that derived the Kawagodaira rhyolite.
Kawagodaira volcano, located in the northwestern part of the Izu Peninsula, is a monogenetic volcano that erupted about 3100 years ago (Tani et al., 2013). Kawagodaira is the first volcano to erupt rhyolitic magma in the Higashi-Izu monogenetic volcano group, which is dominated by basaltic to andesitic magma. In this study, we focus on the trace element composition of amphibole phenocrysts to discuss the geochemical processes that derived the rhyolite magma of the Kawagodaira volcano.
Analysis and Result
We analyzed the major and trace element composition of amphibole phenocrysts in the Kawagohira rhyolite using the Laser-ablation ICP-MS (Japan Agency for Marine-Earth Science and Technology). The amphibole phenocrysts exhibit a wide range of compositions. For example, the concentration of lutetium, the heavy rare earth element, ranges from 0.47 to 2.64 ppm. Both major and trace elements exhibit continuous compositional variations. Rare earth elements (REE) exhibit parallel patterns regardless of concentration on the spidergram, while strontium (Sr) and europium (Eu) anomalies are noticeable. Based on multivariable statistical analysis (principal component analysis; Ueki and Iwamori, 2017), it was found that the trace element variations can be explained by two principal components. The first principal component involves only REE and Sr, which are inversely correlated, i.e., REE enrichment and Sr depletion. The second principal component involves LILE (large-ion lithophile elements) and some HFSE (high-field strength elements), indicating a process that enriches LILE and HFSE independently of REE and Sr.
Discussion
SiO2 concentrations of host melts of amphibole phenocrysts estimated by Suwa et al. (2018) exhibit a series of compositions ranging from andesite to rhyolite, although all amphiboles were found in rhyolite. The SiO2 concentrations of the estimated host melts are correlated with the REE enrichment, Sr depletion, and Eu anomaly in the amphibole. In other words, the SiO2 concentration of the melt that crystallized amphibole increased with increasing REE concentrations and negative Sr and Eu anomalies. The parallel REE patterns on the spidergrams and the result from the principal component analysis rule out the possibility of magma mixing or partial melting trend for the compositional variations of amphiboles. On the other hand, the negative the Sr and Eu anomalies with increasing SiO2 and REE strongly suggest the fractionation of plagioclase. Based on the mass-balance calculation on the trace element composition of the host melt calculated using the melt-amphibole partition coefficient, the Sr and Eu anomalies and the increase in REE of the estimated host melts are consistently reproduced by the plagioclase fractionation. Also, mass-balance of major elements and the petrological analysis by Suwa et al. (2018) suggest the fractionations of small amounts of other phenocryst minerals of the Kawagodaira rhyolite (orthopyroxene, Ti-Fe oxide, and amphibole) in addition to plagioclase. The REE ratios of the andesites of the estimated host melts exhibit a compositional range that overlaps with that of the andesites that erupted as the Higashi-Izu monogenetic volcanoes (Arakawa et al., 2022).
Summary
The results of this study suggest that the compositional variation from andesite to rhyolite recorded in amphibole phenocrysts in the Kawagodaira rhyolite represents plagioclase-dominated crystal fractionation. In addition, LILE and HFSE behave independently of REE and Sr, suggesting that magma mixing or assimilation could have contributed in addition to crystal fractionation. Indeed, previous studies (Suzuki, 2000; Arakawa et al., 2022) have suggested that magma mixing was also involved in the generation of felsic magma in the Higashi-Izu group. In this presentation, we will further discuss the magma-feeding system of the monogenetic volcanoes that derived the Kawagodaira rhyolite.