Barium-rich authigenic clinoptilolite in sediments from the Japan Sea - a sink for dissolved barium?

Exceptional barium-rich clinoptilolite, a heulandite-group zeolite, with Ba contents of up to 1.83 wt.% BaO occurs in Miocene sediments from the Yamat o Basin of the Japan Sea (ODP Site 797). The clinoptilolites with highest B a-concentrations occur directly below the base of the sulfate reduction zon e, where pore water Ba concentrations are high due to the dissolution of bi ogenic barite. Downcore variations of the Ba content in clinoptilolites and pore water show that the formation of Be-rich crystals at 299 m depth coin cides with a decrease of the pore water Ba concentration. Thus, formation o f Ba-rich zeolites seems to provide a sink for dissolved Ba, although the o ccurrence of authigenic barite near the base of sulfate reduction might als o contribute to the depletion of pore water Ba. These findings show that th e intrachannel cation content of authigenic clinoptilolite is not directly controlled by a mineralogical precursor, but seems to reflect the geochemic al conditions of the surrounding pore fluids. The formation of Ba-rich clin optilolites in a sulfate-depleted environment with elevated pore water Ba c oncentrations and the concurrent decrease of dissolved Ba in the pore fluid s documents the mutual influence of pore water composition and mineral chem istry during diagenetic processes in marine sediments. (C) 1999 Elsevier Sc ience B.V. All rights reserved.