Rare Earth Elements and Sr-Nd-Pb Isotopic Analyses of the Arima Hot Spring Waters, Southwest Japan: Implications for Origin of the Arima-type Brine

Abstract

Rare earth elements (REEs) and Sr-Nd-Pb isotopic compsotions of the Arima hot spring waters, a specific type of deep-seated brine (up to 6 wt.% NaCl) in the non-volcanic fore-arc region of southwest Japan, have been analyzed in order to discuss their source materials and origins. We have first examined the matrix effect associated with variable salinity (0 to 5 wt.% NaCl), and found that above 1 wt.% NaCl, the intensity of REE in ICP-MS measurement is drastically reduced due to the matrix effect. Accordingly, we have diluted the sample waters to contain ~0.6 wt.% NaCl, and then analyzed them by the standard addition method. The result shows that the abundance is appreciably high compared to near-surface waters, and exhibits almost a flat DMM-normalized pattern, which can be explained by mixing of a slab-derived fluid at relatively low temparature (400 to 500oC) and a near-surface water. The Sr-Nd- Pb isotopic compositions of the brineare consistent with the above interpretation on REEs, in that the deep brine is isotopically similar to a slab-derived fluid of the subducted Philippine Sea slab. These evidences suggest that the slabderived fluid is upwelling even in the non-volcanic region possibly through a fault zone along the tectonic line.