[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-07] Geochemical Study on melt inclusion, Kannabe volcano, Japan

*Haruki Maeda^1,3, Tetsuya Sakuyama¹, Kenji Shimizu², Takayuki Ushikubo², Takeshi Hanyu⁴, Qing Chang⁴, MINJAE KANG¹ (1.Faculty of Science, Osaka Metropolitan University, 2.Kochi Institute of Core Research, Japan Agency for Marine-Earth Science and Technology, 3.DAIDO LIFE INSURANCE COMPANY, 4.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology)

Keywords:Kannabe, SW Japan, melt inclusion, volatile, H isotope

Introduction
It is said that most of the Quaternary volcanism around the Chugoku area is igneous activity caused by the formation of mantle wedges and dehydrated fluids derived from the PHS slab. Meanwhile, some volcanoes such as Kannabe volcano have produced basalts with trace element characteristics of oceanic island-type volcanoes (Kimura et al., 2014), suggesting that an ascending mantle contribution is considered (Nguyen et al., 2020). The existence of a seismic low-velocity zone (LVZ) extending from a depth of about 300km to 50km beneath the PHS slab has been reported in the upper mantle from Hyogo to Wakayama (Nakajima & Hasegawa, 2007). While the relationship of this region to hot springs and volcanic activity has been discussed, geochemical studies on the relationship between this region and volcanic activity have not yet been carried out. In this study, we attempted to elucidate the source of materials involved in igneous activity based on the results of EPMA analysis of major elements, SIMS analysis of volatile elements (Cl,F,S,H₂O,CO₂), hydrogen isotope ratio (δD), LA-ICP-MS analysis of trace elements and Pb isotopic ratio (²⁰⁷Pb/²⁰⁶Pb, ²⁰⁸Pb/²⁰⁶Pb) in the olivine hosted melt inclusions (MI) from the scoria of Kannabe Volcano.
geochemical features
The H₂O and CO₂ concentration range is mostly 1.4~1.7wt%, 2000~3000 ppm, both of which are high, indicating that the primary information of volatile is retained in MI. MI shows a range of SiO₂ =48~52wt%, where Al₂O₃ and CaO, Na₂O increase while SiO₂ increases, and MgO, FeO, K₂O, P₂O₅ decreases. While there are no significant differences among the major elements, the trace element compositions are divided into two groups: a group with constant Nb/Y and a large increase in Ba/Zr (High-LILE) and a group with a slight increase in Ba/Zr with an increase in Nb/Y (Low-LILE). The H₂O/F, Cl/F, and δD of the High-LILE ranged from 20~35, 0.3~1.0, and -50~-75(⁰/₀₀), respectively. Whereas H₂O/F, Cl/F and δD of Low-LILE were 25~35, 0.1~0.4 and -60~-90(⁰/₀₀), respectively. ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁸Pb/²⁰⁶Pb were 0.85~0.86 and 2.11~2.13 in the High-LILE, and 0.86~0.88 and 2.12~2.15 in the Low-LILE, respectively. Pb isotope ratios tended to increase with decreasing Cl/F in the Low-LILE, but did not change much in the High-LILE.
Examination
Kuritani et al. (2021) measured the volatile content and δD of MI in olivine found that Cl/F and H₂O/F systematically decrease with depth in PAC slabs, that the δD at slab depths of 300 km The effect of dehydrated fluid from PAC slabs on igneous activity is discussed based on the fact that the δD is constant at -60~-70 (⁰/₀₀) and that both values are higher than the Depleted MORB Mantle (DMM) composition. High-LILE is relatively close to Fukue Island, which is no contradiction in igneous activity caused by dehydrated fluids from Pacific slabs.
Whereas the Cl/F of Low-LILE is similar to the DMM; H₂O/F is higher than that of DMM (Shimizu et al., 2019) and there was a supply of dehydrated fluid containing less Cl than in High-LILE. The Cl/F of the High-LILE is considered to be the high Pb isotope ratios exhibited by Low-LILE samples with particularly low Cl/F deviate from the compositional range of slab materials and DMM, and require a different source material. The possibility of a PAC slab fracture directly beneath Kannabe has been reported (Obayashi et al., 2009). A LVZ extending from near the outer ridge to a depth of about 400km has been confirmed to exist in the PAC sub-slab asthenosphere, and petit-spot volcanoes are thought to originate from the asthenosphere mantle of the area. The ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁸Pb/²⁰⁶Pb values reported for petit-spot volcanoes are high, around 0.87~0.91 and 2.13~2.20(Machida et al., 2009), respectively, and the trend created by the Low-LILE can be explained by mixing of slab materials and petit-spot volcanic samples. The contribution of fluids originating from the sub-PAC slab asthenosphere mantle is therefore suggested.