[S-CG45] Science of slow-to-fast earthquakes

[SCG45-30] Arima-type fluids derived from subducted slab: Li isotope constraint and implications for earthquake-related fluids

*石川 剛志¹、谷水 雅治² (1.海洋研究開発機構高知コア研究所、2.関西学院大学)

キーワード：流体、沈み込み帯、地震、地球化学

Recent investigations suggest that saline Arima-type fluids found in SW Japan are derived from dehydration of subducted Philippine Sea (PHS) Plate slab (e.g., Kusuda et al., 2014; Nakamura and Iwamori, 2022). The occurrence of this type of fluids is widely reported in non-volcanic regions including around the Median Tectonic Line (MTL), and some of them spatially overlap with the areas under which deep low-frequency (DLF) tremors or DLF earthquakes are observed (Kazahaya et al., 2014). Thus, it is important to reveal whether such slab-derived saline fluids influence slow and fast earthquake events occurring around the subducted slab surface. The Arima-type and related fluids are known to be highly enriched in Li (Takamatsu et al., 1986), which accords with experimental observations showing very high mobility of Li during fluid-sediment interactions at high temperatures (You et al., 1996). However, no Li isotope study had been carried out for Arima-type fluids. We analyzed Li isotope ratios (δ⁷Li values) of hot spring waters from Arima and ground waters from east Kii Peninsula (Umam et al., 2022). Arima hot spring waters showed Cl/Li ratios of 560 to 800 and δ⁷Li values of +1.4 to +3.1 ‰, which are very low compared with the seawater values of 110,000 and +31 ‰, respectively. These low δ⁷Li values are consistent with equilibration with PHS sediments at 400 to 500 °C, the temperatures inferred for PHS slab dehydration associated with formation of the Arima fluids (Nakamura and Iwamori, 2022). Thus, the Li isotope data support that the Arima fluids are of PHS slab origin. Some of the east Kii groundwaters also showed low Cl/Li-δ⁷Li characteristics, and the observed compositions overlap with the data of the PHS slab fluids inferred for the Kumano mud volcano, off Kii Peninsula (Nishio et al., 2015) and the data of the Arima fluids described above. This suggests that the low Cl/Li-δ⁷Li components in these east Kii fluids are also of PHS slab origin. Interestingly, the occurrence of such Arima-like low Cl/Li-δ⁷Li fluids is limited only along or south of MTL, and seems to lack in large areas under which extensive DLF tremors are observed. This may be because relatively high permeability of the slab-crust interface in southern area combined with extensive crustal faulting including at MTL allows slab fluids to migrate to the surface, while in northern area, low permeability of the tremor-occurring slab-crust interface suppresses ascent of the slab fluids. However, it is possible that the fluids associated with the DLF tremors are transported upward along the slab-crust interface and sampled as the low Cl/Li-δ⁷Li fluids at the southern east Kii Peninsula. Further studies are required for understanding the role of such saline, high-Li slab fluids in slow and fast earthquakes.

References: Kazahaya, J. Jap. Assoc. Hydrol. Sci., 44, 3-16, 2014; Kusuda et al., EPS, 66, 119, 2014; Nakamura and Iwamori, Chikyukagaku, 56, 76-86, 2022; Nisho et al., EPSL, 414, 144-155, 2015; Takamautsu, Geochem. J., 20, 143-151, 1986; Umam et al., Geochem. J., 56, e8-e17, 2022; You et al., EPSL, 140, 41-52, 1996.