LAUSR

In the southwest Japan forearc, slab‐fluids produced from subducted materials migrate to crustal levels and appear as deep‐seated brine. We have analyzed for the first‐time stable strontium isotopes in non‐volcanic spring water with high salinity, referred to as Arima‐type saline water that likely originate from slab‐fluid that upwelled along major faults. The stable strontium isotope compositions of the saline water are isotopically light (δ88/86Sr = 0.122–0.157‰) and different from those of local bedrock and near‐surface water. The light strontium‐enriched and radiogenic signature of the saline water reflects the primary characteristic of slab‐fluids without an isotopic overprint in the crust. The Arima‐type brines show signatures of slab‐fluids at forearc depth, which is different from the slab‐fluids at subarc depth estimated from arc lavas. The characteristic features of the Arima‐type brines are explained by a larger contribution of subducted sediments and strontium isotope fractionation during fluids generation at shallower depth.