Journal de géologie et géophysique
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Rare Earth Elements of the Arima Spring Waters, Southwest Japan: Implications for Fluid Crust Interaction during Ascent of Deep Brine

Hitomi Nakamura, Kotona Chiba, Qing Chang, Shunichi Nakai, Kohei Kazahaya, and Hikaru Iwamori

Rare earth elements (REEs) of the eight Arima spring waters in southwest Japan, including Arima-type brine that represents a specific type of deep-seated brine of up to 6 wt.% NaCl in the non-volcanic fore-arc region, have been investigated in order to discuss their upwelling processes and origins. We found four distinct patterns of REE composition of the spring waters within the Arima area of ~1 km² , based on which two sources for REEs and two aquifers are inferred in the modification of the original deep-seated brine composition. On the basis of the REEs and isotopic compositions of the original deep brine, one of the two sources is thought to be slab-derived fluid dehydrated from the subducted Philippine Sea slab beneath the Arima area, represented by the ‘Kinsen’ hot spring water [1]. The convex-down REE pattern of most Arima spring waters, except for ‘Kinsen’ and ‘Tansansen’, suggests the presence of an oxidizing aquifer deeper than 160 m that causes co-precipitation of REEs with oxyhydroxides. CO₂ and He degassed from this aquifer flux the overlying shallow aquifer less than ~50 m in depth, producing highly carbonated water such as ‘Tansansen’ water that was originally derived from meteoric water. The carbonated water may dissolve a significant amount of REEs to the ‘Tansansen’ spring water from the host rocks, which are possibly silicic igneous rocks with Eu-negative anomalies. The four types of REE patterns with a wide concentration range, therefore, provide invaluable information concerning fluid–crust interaction during ascent of the deep brine.