Sediments and rocks recovered during Ocean Drilling Program Leg 126 at Sites 792 and 793 in the Izu-Bonin forearc basin are described with a primary focus on clay mineralogy. Evidence for diagenetic hydrothermal alteration processes is present in the upper Oligocene to lower Miocene sediments at these sites. The vitric and pumiceous sand/sandstone and pumiceous gravel contain high concentrations of smectites, zeolites, and gypsum. Microscopic observations show that the volcanic glass and feldspars have been altered to smectites and zeolites. The authigenic mineral assemblages indicate that these minerals resulted from precipitation from circulating fluids, as well as from the alteration of glass and feldspar under temperature conditions that may have reached 200°-300°C. Mineral assemblages in microfractures display thermal gradients that possibly reflect cooling effects.

Sediment depth is given in mbsf. The parameter comments describe the strongest peak intensity that was used to determine the cps value for each mineral. <1 = traces.

Supplement to: Tazaki, Kazue; Fyfe, William S (1992): Diagenetic and hydrothermal mineral alteration observed in Izu-Bonin deep-sea sediments, Leg 126. In: Taylor, B; Fujioka, K; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 126, 101-112