17:15 〜 19:15
[SGC37-P09] Contribution of crustal material inferred from δ26Mg ratios from intraplate volcanics of southwest Japan
キーワード：Mg isotope, Intraplate volcanism, Southwest Japan, Subduction recycling, Mantle enrichment
Deep subduction of crustal material contributes to enrichment of the asthenospheric mantle. Evidence of such subduction and enrichment can be interpreted from chemical compositions of intraplate basalts in both oceanic and continental settings. However, interpreting the source and enrichment processes for the sub-continental asthenospheric mantle has been challenging due to chemical interaction of upwelling magma with the thick lithospheric mantle and continental crust. Previous research from southwest Japan intraplate basalts demonstrated that ancient sediments subducted in the Paleoproterozoic contribute to the asthenospheric mantle, through Pb isotopic modelling (Dey et al., 2024). Trace element models for these intraplate basalts, generated below the lithosphere-asthenosphere boundary, suggests that both carbonate and siliciclastic sediments are present as part of the subducted component. Furthermore, radiogenic isotopic ratios also suggest evidence of interaction with the lithospheric mantle (Dey et al. 2024).
Magnesium isotope geochemistry is a useful tool to probe the crustal components recycled into the mantle, as low temperature surface processes (i.e., precipitation of carbonate sediments, terrestrial weathering) fractionate these isotopes to a much higher degree than the high temperature processes (i.e., melting, fractionation) in the mantle. In this research we aim to identify the variability in Mg isotope geochemistry, which may correspond to the recycled crustal components in the mantle below southwest Japan. However, Mg isotopic variation within volcanic rocks is quite small and comparable to the reproducibility of measurement for natural samples (~0.1‰). Therefore, we improvised the analytical methods (based on An et al., 2014) to measure low blank (~1 ng), high precision data for natural samples. Our measurements yield an external reproducibility of 0.05‰ (n=10) for basaltic rock standards which is sufficient to determine the variation in volcanic rocks from southwest Japan.
In this research, we present high-precision Mg isotope data from Kita-Matsuura, Higashi-Matsuura and Pre-Unzen, which demonstrates a variability correlated with other geochemical parameters. The data shows a variation from -0.04‰ for Pre-Unzen which is significantly higher than the mantle average (-0.236‰, Liu et al., 2023) to -0.32‰ for Higashi-Matsuura which is below the mantle average. The results show lower than mantle values correlated to low Pb isotope ratios and Ce/Pb ratios for Higashi-Matsuura alkali basalts (δ26Mg -0.21‰ to -0.34‰), which indicates a contribution from ancient carbonate sediments. Similar trends are seen for geochemical data reported from intraplate basalts from mainland East Asia (e.g., Changbaishan, Wudalianchi, Erkeshan etc, Wang et al., 2017) and Petit Spot (Liu et al., 2020) on the Pacific Plate, which had been explained in terms of mixing with recycled carbonate sediments and oceanic crust.
Basalts and basaltic andesites from Kita-Matsuura show almost no variation in δ26Mg (~-0.23‰) between relatively primary (SiO2 48%) to evolved (SiO2 55%) magma indicating a lack of crustal component derived material in the studied samples.
Higher than mantle values (-0.23‰ to -0.04‰) are seen for Pre-Unzen samples which correlate to SiO2 content and inversely correlate to MgO content and Ce/Pb, indicating an influence of continental crust derived material. Whether this is an effect of recycled terrestrial sediments (with high δ26Mg) in the mantle source, or interaction of the magma with the overlying crust is unclear at the moment.
References: An et al., 2014. Chemical Geology 390, 9–21; Dey et al., 2024. Journal of Petrology 65, 104; Liu et al., 2023. Geochimica et Cosmochimica Acta 358, 12–26; Liu et al. (2020). Nature Communications 11, 1–9; Wang et al., 2017. Earth and Planetary Science Letters 465, 16–28.
Magnesium isotope geochemistry is a useful tool to probe the crustal components recycled into the mantle, as low temperature surface processes (i.e., precipitation of carbonate sediments, terrestrial weathering) fractionate these isotopes to a much higher degree than the high temperature processes (i.e., melting, fractionation) in the mantle. In this research we aim to identify the variability in Mg isotope geochemistry, which may correspond to the recycled crustal components in the mantle below southwest Japan. However, Mg isotopic variation within volcanic rocks is quite small and comparable to the reproducibility of measurement for natural samples (~0.1‰). Therefore, we improvised the analytical methods (based on An et al., 2014) to measure low blank (~1 ng), high precision data for natural samples. Our measurements yield an external reproducibility of 0.05‰ (n=10) for basaltic rock standards which is sufficient to determine the variation in volcanic rocks from southwest Japan.
In this research, we present high-precision Mg isotope data from Kita-Matsuura, Higashi-Matsuura and Pre-Unzen, which demonstrates a variability correlated with other geochemical parameters. The data shows a variation from -0.04‰ for Pre-Unzen which is significantly higher than the mantle average (-0.236‰, Liu et al., 2023) to -0.32‰ for Higashi-Matsuura which is below the mantle average. The results show lower than mantle values correlated to low Pb isotope ratios and Ce/Pb ratios for Higashi-Matsuura alkali basalts (δ26Mg -0.21‰ to -0.34‰), which indicates a contribution from ancient carbonate sediments. Similar trends are seen for geochemical data reported from intraplate basalts from mainland East Asia (e.g., Changbaishan, Wudalianchi, Erkeshan etc, Wang et al., 2017) and Petit Spot (Liu et al., 2020) on the Pacific Plate, which had been explained in terms of mixing with recycled carbonate sediments and oceanic crust.
Basalts and basaltic andesites from Kita-Matsuura show almost no variation in δ26Mg (~-0.23‰) between relatively primary (SiO2 48%) to evolved (SiO2 55%) magma indicating a lack of crustal component derived material in the studied samples.
Higher than mantle values (-0.23‰ to -0.04‰) are seen for Pre-Unzen samples which correlate to SiO2 content and inversely correlate to MgO content and Ce/Pb, indicating an influence of continental crust derived material. Whether this is an effect of recycled terrestrial sediments (with high δ26Mg) in the mantle source, or interaction of the magma with the overlying crust is unclear at the moment.
References: An et al., 2014. Chemical Geology 390, 9–21; Dey et al., 2024. Journal of Petrology 65, 104; Liu et al., 2023. Geochimica et Cosmochimica Acta 358, 12–26; Liu et al. (2020). Nature Communications 11, 1–9; Wang et al., 2017. Earth and Planetary Science Letters 465, 16–28.