17:15 〜 18:45
[SGC32-P06] Tracing the subducting Pacific slab with hydrogen and sulfur isotopic ratios in olivine-hosted melt inclusions
キーワード：硫黄同位体、メルト包有物、沈み込み帯
Global volatile cycles involving the Earth’s deep interior have played a critical role in Earth’s evolution, because volatiles in the Earth’s mantle affect its rheological and melting behaviors, thereby controlling the thermal evolution and chemical differentiation of the solid Earth. Stable isotopes, including hydrogen and sulfur, are expected to provide useful information on understanding the cycling and distribution of volatiles in the Earth’s interior, because of possible fractionation of the stable isotopes through dehydration during the transport to the Earth’s interior by subduction. However, little is known about the changes in the stable isotopic ratios of subducting slabs with increasing depth, partly because the evolution of the hydrogen and sulfur isotopic ratios of the subducting materials beyond the frontal arc with slab depths (Z) of >120 km has not been well defined by natural observations.
Kuritani et al. (2021) measured volatile (H2O, CO2, F, S, and Cl) contents and hydrogen isotopic compositions of olivine-hosted melt inclusions 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), to trace the evolution of the D/H ratios of the subducting Pacific slab from the volcanic front to the mantle transition zone. In this study, we obtained sulfur isotopic ratios of the representative olivine-hosted melt inclusions from the six volcanoes that were studied by Kuritani et al. (2021).
Sulfur isotopic compositions were analyzed on melt inclusions using the ion microprobe (Cameca IMS-1280HR, Ametek Cameca) at the Kochi Institute for Core Sample Research, JAMSTEC (Shimizu et al., 2019). It is well established that stable isotopic ratios of olivine-hosted melt inclusions do not necessarily represent those of mantle-derived magmas because of the effects of degassing and post-entrapment modification of melt inclusion compositions at shallow levels (e.g., Bucholz et al., 2013). To minimize these effects, volatile content analyses were conducted on at least 25 melt inclusions for each volcano, and sulfur isotopic analyses were conducted on melt inclusions with relatively high CO2 and S contents for each volcano.
We obtained the d34S values of 3.0±0.2 (n=10, 1s) for Iwate, 1.8±0.4 (n=10, 1s) for Akita-Komagatake, 2.1±0.4 (n=10, 1s) for Me-Akan, 4.5±0.4 (n=10, 1s) for Oshima-Oshima, 4.1±0.4 (n=10, 1s) for Rishiri, and 2.2±0.3 (n=10, 1s) for Fukue. These results show that, with increasing the slab depth, the d34S values decreases from ~3.0 at Z = ~90 km to ~2.1 at Z = ~110 km, and jump up to ~4.5 at Z = ~180 km. Then, the d34S values decreases from ~4.5 at Z = ~180 km through ~4.1 at Z = ~300 km to ~2.2 at Z = ~550 km. These observations cannot be explained by a single reservoir model (e.g., Shaw et al. 2008 for hydrogen) in which dehydration of the subducting slab occurred progressively as a single sulfur reservoir. Rather, the results demonstrate the involvement of at least two sulfur reservoirs with different d34S values in the generation of slab fluids that were responsible for the magmatism of the studied volcanoes with variable slab depths.
Kuritani et al. (2021) measured volatile (H2O, CO2, F, S, and Cl) contents and hydrogen isotopic compositions of olivine-hosted melt inclusions 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), to trace the evolution of the D/H ratios of the subducting Pacific slab from the volcanic front to the mantle transition zone. In this study, we obtained sulfur isotopic ratios of the representative olivine-hosted melt inclusions from the six volcanoes that were studied by Kuritani et al. (2021).
Sulfur isotopic compositions were analyzed on melt inclusions using the ion microprobe (Cameca IMS-1280HR, Ametek Cameca) at the Kochi Institute for Core Sample Research, JAMSTEC (Shimizu et al., 2019). It is well established that stable isotopic ratios of olivine-hosted melt inclusions do not necessarily represent those of mantle-derived magmas because of the effects of degassing and post-entrapment modification of melt inclusion compositions at shallow levels (e.g., Bucholz et al., 2013). To minimize these effects, volatile content analyses were conducted on at least 25 melt inclusions for each volcano, and sulfur isotopic analyses were conducted on melt inclusions with relatively high CO2 and S contents for each volcano.
We obtained the d34S values of 3.0±0.2 (n=10, 1s) for Iwate, 1.8±0.4 (n=10, 1s) for Akita-Komagatake, 2.1±0.4 (n=10, 1s) for Me-Akan, 4.5±0.4 (n=10, 1s) for Oshima-Oshima, 4.1±0.4 (n=10, 1s) for Rishiri, and 2.2±0.3 (n=10, 1s) for Fukue. These results show that, with increasing the slab depth, the d34S values decreases from ~3.0 at Z = ~90 km to ~2.1 at Z = ~110 km, and jump up to ~4.5 at Z = ~180 km. Then, the d34S values decreases from ~4.5 at Z = ~180 km through ~4.1 at Z = ~300 km to ~2.2 at Z = ~550 km. These observations cannot be explained by a single reservoir model (e.g., Shaw et al. 2008 for hydrogen) in which dehydration of the subducting slab occurred progressively as a single sulfur reservoir. Rather, the results demonstrate the involvement of at least two sulfur reservoirs with different d34S values in the generation of slab fluids that were responsible for the magmatism of the studied volcanoes with variable slab depths.