Arima-type hot spring water components discovered in Okayama Prefecture, where active faults are rare
―The possibility that slab-derived deep fluids are rising universally in the foreland region of southwestern Japan―
Aito Akishiba, a second-year graduate student in the Department of Agricultural, Forestry, and Marine Sciences, and Associate Professor Yoshiaki Nishio of the Division of Interdisciplinary Sciences in the School of Integrated Sciences have discovered lithium isotope compositions in hot spring water from Okayama Prefecture that are similar to those found in “Arima-type hot spring water” (*1).The forarc region of southwestern Japan (*2) lacks active volcanoes, and fluids derived from the dehydration of the subducting Philippine Sea Plate exist deep underground. In particular, it has been suggested that in regions where the young and hot Philippine Sea Plate is subducting, slab-derived fluids (*3) may rise to the surface.
Until now, the ascent of slab-derived fluids had only been confirmed in hot spring water from certain areas near major faults, but the discovery in Okayama Prefecture suggests that slab-derived fluids may be present in hot spring water throughout the forarc region of southwestern Japan.
These research findings were published on February 5, 2025, in the online scientific journal *Progress in Earth and Planetary Science*, operated by the Japan Geoscience Union.
*1 Arima-type hot spring water: A classification of hot spring water typified by Arima Onsen. Based on the results of isotopic ratios of hydrogen and oxygen in the water and helium isotopic ratios in the hot spring gases, it has been suggested that Arima-type hot spring water may originate from the subducted Philippine Sea Plate. It is also referred to as Arima-type fluid or Arima-type deep fluid.
*2 Foreland region: A non-volcanic zone located between a trench and a volcanic arc.
*3 Slab-derived fluid: Fluid originating from a subducted oceanic plate. When an oceanic plate subducts into the Earth’s interior and is subjected to high temperatures and pressures, large amounts of water are squeezed out from the pores of sediments and minerals. The fluid generated during this process is called slab-derived fluid.
[Press Release] Arima-type Hot Spring Water Components Discovered in Okayama Prefecture, an Area with Few Active Faults [PDF: 536 KB]
[Publication Information]
<strong>Lithium and strontium isotope hydrogeochemistry in Okayama and Tottori Prefectures, southwest Japan: Implications for tracing slab-derived fluid</strong></p>
<p> URL: <a href="https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-024-00679-y"><span class="has-icon is-icon-external">http://dx.doi.org/10.1186/s40645-024-00635-w</span></a></p>
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