Strontium Isotope Characteristics (δ88/86Sr, 87Sr/86Sr) of Arima-Type Brines Originated From Slab-Fluids

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 (delta Sr-88/86 = 0.122-0.157 parts per thousand) 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.