09:00 〜 09:15
[SCG52-01] 斑晶メルト包有物の水素同位体比による沈み込む太平洋プレートの追跡
★招待講演
Global water cycles involving the Earth’s deep interior have played a critical role in Earth’s evolution. Water in the Earth’s mantle affects its rheological and melting behaviours, thereby controlling the thermal evolution and chemical differentiation of the solid Earth. The ocean’s mass is primarily controlled by the balance between the water influx to the interior through subducting oceanic plates and the water outflux to the surface through magmatism. Understanding the origin of terrestrial water and the cycling and distribution of water in the Earth’s interior has been significantly improved by the application of stable hydrogen isotopes, which are powerful tracers. As materials transported into the Earth’s interior by subduction decrease the hydrogen isotope (D/H) ratios by releasing Deuterium (D)-enriched fluids, D/H ratios of deep-source magmas allow us to identify the involvement of the recycled water as well as distinguish the recycled water from the Earth’s primordial water.
Little is understood about the changes in the D/H ratio of subducting slabs with increasing depth, which is a key issue in estimating the characteristic D/H ratios of subducted recycled materials in the deep mantle reservoir. Using the results of hydrogen isotopic analyses on olivine-hosted melt inclusions for some Mariana arc volcanoes, Shaw et al. (2008) estimated the dD‰ values (dD‰ = [(D/H)Sample -(D/H)SMOW]/(D/H)SMOW × 1000, where SMOW refers to the standard mean ocean water, and dD = 0‰) of the subducted materials to be as low as -234‰. Walowski et al. (2015) estimated the dD‰ values of dehydrated residual slab materials to be -100‰ to -120‰ by combining the results of hydrogen isotopic analyses on melt inclusions from Cascadia arc basalts with a model that considered the thermal and petrological structures of a hot subducting slab. So far, however, the evolution of the D/H ratios of the subducting materials beyond the frontal arc with slab depths (Z) of >120 km has not been well defined by natural observations.
In this study, to trace the evolution of the D/H ratios of the subducting Pacific slab from the volcanic front to the mantle transition zone, the hydrogen isotopic compositions of olivine-hosted melt inclusions were determined for basaltic scoria samples from six active volcanoes, including Iwate (Z = ~90 km), Akita-Komagatake (Z = ~100 km), Me-Akan (Z = ~110 km), Oshima-Oshima (Z = ~180 km), Rishiri (Z = ~300 km), and Fukue (Z = ~550 km).
Hydrogen isotopic compositions, as well as volatile (H2O, CO2, F, S, and Cl) contents, were analysed on melt inclusions using the ion microprobe (Cameca IMS-1280HR, Ametek Cameca) at the Kochi Institute for Core Sample Research, JAMSTEC. It is well established that the D/H ratios of olivine-hosted melt inclusions do not necessarily represent those of mantle-derived magmas (e.g., Bucholz et al., 2013). To carefully examine the effect of the post-entrapment modification of melt inclusion compositions, volatile content analyses were conducted on at least 25 melt inclusions for each volcano, and the data of the melt inclusions with the three highest CO2 contents were selected as the representative data sets for each volcano.
The results show that the D/H ratio of the slab fluid at the volcanic front (Iwate) is lower than that of the slab fluid just behind the volcanic front (Akita-Komagatake). This demonstrates that fluids with different D/H ratios were released from the crust and the underlying peridotite portions of the slab around the volcanic front. The results also show that the D/H ratios of slab fluids do not change significantly with slab depths from 300 to 550 km, which demonstrates that slab dehydration did not occur significantly beyond the arc. Our estimated dD‰ value for the slab materials that accumulated in the mantle transition zone is >-90‰, a value which is significantly higher than previous estimates.
Little is understood about the changes in the D/H ratio of subducting slabs with increasing depth, which is a key issue in estimating the characteristic D/H ratios of subducted recycled materials in the deep mantle reservoir. Using the results of hydrogen isotopic analyses on olivine-hosted melt inclusions for some Mariana arc volcanoes, Shaw et al. (2008) estimated the dD‰ values (dD‰ = [(D/H)Sample -(D/H)SMOW]/(D/H)SMOW × 1000, where SMOW refers to the standard mean ocean water, and dD = 0‰) of the subducted materials to be as low as -234‰. Walowski et al. (2015) estimated the dD‰ values of dehydrated residual slab materials to be -100‰ to -120‰ by combining the results of hydrogen isotopic analyses on melt inclusions from Cascadia arc basalts with a model that considered the thermal and petrological structures of a hot subducting slab. So far, however, the evolution of the D/H ratios of the subducting materials beyond the frontal arc with slab depths (Z) of >120 km has not been well defined by natural observations.
In this study, to trace the evolution of the D/H ratios of the subducting Pacific slab from the volcanic front to the mantle transition zone, the hydrogen isotopic compositions of olivine-hosted melt inclusions were determined for basaltic scoria samples from six active volcanoes, including Iwate (Z = ~90 km), Akita-Komagatake (Z = ~100 km), Me-Akan (Z = ~110 km), Oshima-Oshima (Z = ~180 km), Rishiri (Z = ~300 km), and Fukue (Z = ~550 km).
Hydrogen isotopic compositions, as well as volatile (H2O, CO2, F, S, and Cl) contents, were analysed on melt inclusions using the ion microprobe (Cameca IMS-1280HR, Ametek Cameca) at the Kochi Institute for Core Sample Research, JAMSTEC. It is well established that the D/H ratios of olivine-hosted melt inclusions do not necessarily represent those of mantle-derived magmas (e.g., Bucholz et al., 2013). To carefully examine the effect of the post-entrapment modification of melt inclusion compositions, volatile content analyses were conducted on at least 25 melt inclusions for each volcano, and the data of the melt inclusions with the three highest CO2 contents were selected as the representative data sets for each volcano.
The results show that the D/H ratio of the slab fluid at the volcanic front (Iwate) is lower than that of the slab fluid just behind the volcanic front (Akita-Komagatake). This demonstrates that fluids with different D/H ratios were released from the crust and the underlying peridotite portions of the slab around the volcanic front. The results also show that the D/H ratios of slab fluids do not change significantly with slab depths from 300 to 550 km, which demonstrates that slab dehydration did not occur significantly beyond the arc. Our estimated dD‰ value for the slab materials that accumulated in the mantle transition zone is >-90‰, a value which is significantly higher than previous estimates.