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[SGL17-P05] Excess argon in nosean from phonolite of Olbrück volcano, East Eifel, Germany by Ar/Ar dating and SEM-EDS analysis
Keywords:Ar/Ar dating, excess argon, Quaternary, Eifel volcano group, magma, carbonatite
Excess argon has been reported by Lippolt et al. (1990) in several phonolites from the East Eifel Monogenetic Volcano Group, but its petrological significance was not clear (Lippolt et al., 1990). The in-situ Ar isotope analyses of historic lava of Unzen volcano showed that the carrier in plagioclase with excess argon was melt inclusions making zones by Sumino et al. (2008). With the similar method, we are investigating the carrier of excess argon in nosean in the phonolite of Olbrück volcano, East Eifel, by petrography, mineral chemistry and in-situ Ar/Ar dating. The petrological significance of the results so far is discussed in this study.
East Eifel volcano group includes volcanoes showing explosive eruptions as Laacher See volcano with ages ranging from 0.4 Ma to Laacher See eruption at 12,900 years ago (Schmincke, 2004). Olbrück volcano consists of a scoria cone and a phonolitic lava flow with the Ar/Ar age of 0.41 ± 0.01 Ma (Lippolt et al. 1990). In this time, a fresh phonolitic lava flow sample was collected from the outcrop of the lava.
Thick sections of 12 mm long and 0.6 mm thickness were prepared from the phonolite as the samples for SEM-EDS and in situ Ar/Ar dating. Fast neutron irradiation was carried out for 4 hours in the facility with Cd tube of TRIGA reactor at Oregon State University.
The rock shows that the phenocrysts are nosean of up to 4 mm, sanidine of up to 1.5 mm and leucite of up to 500 micron, and rarely quartz. For nosean, abundant melt inclusions linearly distributed in two orthogonal directions occur. Nosean was also often observed to coexist with carbonatite melts, and their euhedral shapes were changed to corrosion rims by the carbonatite melts. The groundmass shows an intersertal structure with glass. The composition of groundmass was phonolitic, but Ca-rich, preliminary mafic basaltic composition was also partly observed, indicating that the preliminary mafic basaltic magma was mixed with phonolitic magma. EDS analysis of the areas just above the melt inclusions showed that the Cl concentration was 30-40% higher than the inclusion-free area, indicating that Cl was concentrated in melt inclusions.
The Ar/Ar dating was conducted with three separate neutron irradiations. To obtain sufficient gas, five 200 micron spots were heated with 5-20 Hz for one minute for an analysis. The weighted average of ages were 0.43 ± 0.21 Ma for sanidine, 0.71 ± 0.17 Ma for groundmass and 0.08 ± 0.16 Ma for leucite, which all agree with the age of Olbrück volcano within 2 sigma error. However, the ages of 3.14±0.45 Ma and 31.05±4.11 Ma from two thick sections for Nosean do not agree within error, and also differ from results of bulk analyses of Lippolt (11.2 and 12.2 Ma), thus ages vary unevenly from mineral grain to grain. In nosean, it was not possible to compare the results with and without melt inclusion due to lacking spatial resolution of laser. Therefore, a correlation between apparent radiogenic 40Ar and Cl-derived 38Ar was examined because Cl is concentrated in melt inclusions, and a positive correlation was observed, indirectly suggesting that excess argon is concentrated in the melt inclusions.
Recently, immiscible separations of carbonatite melts were discovered in the melt inclusions of phonolitic composition in hauyne from Laacher See volcano (Berndt & Klemme, 2022). Excess argon was also reported for this hauyne by Lippolt. At Olbrück volcano, it is highly likely that excess argon was trapped in parallel with a similar mechanism of trapping inclusions of phonolitic magma in nosean and subsequent segregation of carbonatites, suggesting a close relationship between the formation of carbonatite melts and excess argon.
Literature:
Lippolt, H.J., Troesch, M., Hess, J.C. (1990): Earth Planet Sci Lett, 101. 19-33
Sumino, H., Ikehata, K., Shimizu, A., Nagao, K., Nakada, S. (2008): J Volcanol Geotherm Res, 175, 189-207
Schminke, H.U. (2004): Volcanism, Springer, 324pp
Berndt, J. and Klemme, S. (2022): Nature Communications, 13: 2892
East Eifel volcano group includes volcanoes showing explosive eruptions as Laacher See volcano with ages ranging from 0.4 Ma to Laacher See eruption at 12,900 years ago (Schmincke, 2004). Olbrück volcano consists of a scoria cone and a phonolitic lava flow with the Ar/Ar age of 0.41 ± 0.01 Ma (Lippolt et al. 1990). In this time, a fresh phonolitic lava flow sample was collected from the outcrop of the lava.
Thick sections of 12 mm long and 0.6 mm thickness were prepared from the phonolite as the samples for SEM-EDS and in situ Ar/Ar dating. Fast neutron irradiation was carried out for 4 hours in the facility with Cd tube of TRIGA reactor at Oregon State University.
The rock shows that the phenocrysts are nosean of up to 4 mm, sanidine of up to 1.5 mm and leucite of up to 500 micron, and rarely quartz. For nosean, abundant melt inclusions linearly distributed in two orthogonal directions occur. Nosean was also often observed to coexist with carbonatite melts, and their euhedral shapes were changed to corrosion rims by the carbonatite melts. The groundmass shows an intersertal structure with glass. The composition of groundmass was phonolitic, but Ca-rich, preliminary mafic basaltic composition was also partly observed, indicating that the preliminary mafic basaltic magma was mixed with phonolitic magma. EDS analysis of the areas just above the melt inclusions showed that the Cl concentration was 30-40% higher than the inclusion-free area, indicating that Cl was concentrated in melt inclusions.
The Ar/Ar dating was conducted with three separate neutron irradiations. To obtain sufficient gas, five 200 micron spots were heated with 5-20 Hz for one minute for an analysis. The weighted average of ages were 0.43 ± 0.21 Ma for sanidine, 0.71 ± 0.17 Ma for groundmass and 0.08 ± 0.16 Ma for leucite, which all agree with the age of Olbrück volcano within 2 sigma error. However, the ages of 3.14±0.45 Ma and 31.05±4.11 Ma from two thick sections for Nosean do not agree within error, and also differ from results of bulk analyses of Lippolt (11.2 and 12.2 Ma), thus ages vary unevenly from mineral grain to grain. In nosean, it was not possible to compare the results with and without melt inclusion due to lacking spatial resolution of laser. Therefore, a correlation between apparent radiogenic 40Ar and Cl-derived 38Ar was examined because Cl is concentrated in melt inclusions, and a positive correlation was observed, indirectly suggesting that excess argon is concentrated in the melt inclusions.
Recently, immiscible separations of carbonatite melts were discovered in the melt inclusions of phonolitic composition in hauyne from Laacher See volcano (Berndt & Klemme, 2022). Excess argon was also reported for this hauyne by Lippolt. At Olbrück volcano, it is highly likely that excess argon was trapped in parallel with a similar mechanism of trapping inclusions of phonolitic magma in nosean and subsequent segregation of carbonatites, suggesting a close relationship between the formation of carbonatite melts and excess argon.
Literature:
Lippolt, H.J., Troesch, M., Hess, J.C. (1990): Earth Planet Sci Lett, 101. 19-33
Sumino, H., Ikehata, K., Shimizu, A., Nagao, K., Nakada, S. (2008): J Volcanol Geotherm Res, 175, 189-207
Schminke, H.U. (2004): Volcanism, Springer, 324pp
Berndt, J. and Klemme, S. (2022): Nature Communications, 13: 2892