Seasonal variation in chemical and isotopic compositions of Kashio brines and its interpretation

Abstract

Kashio brines in the deep mountain area in the central Honshu have been noted for the high salinity, the non-meteoric δ¹⁸O and δ²H values of water, and its close relation to the Median Tectonic Line, the largest fault in Japan. Similar brines discharge in other localities along the fault. The Cl^- content of the brine seasonally varied from 670 to 60 meq/l in harmony with the variations of δ¹⁸O from -2.4 to -8.7‰ and δ²H from -52 to -64‰. The stable isotopic and chemical data indicate that these variations are caused by dilution of ascending deep endmembers with two types of meteoric circulation. The first is a local meteoric circulation directly supplied from precipitation to the drainage basin of Kashio, having δ¹⁸O of -10.5 to -13‰ and δ²H of -72 to -86‰. The second is a circulation whose isotopic values are greater (δ¹⁸O: -9 to -10‰ and δ²H: -60 to -70‰) than the local meteoric water. The enrichment of heavy isotopes in the latter circulation may have resulted from ultra or ion-filtration of deep-circulating water during its passage through sedimentary rocks. A heavy precipitation in the drainage area increased after a few months the proportion of the first meteoric component and decreased that of the second.