Thermal History of Japan Sea Sediments from Isotopic Studies of Diagenetic Silica and Associated Pore Waters

Abstract

Over a broad region of the eastern Japan Sea, Neogene opaline diatomaceous sediments alter with depth to hard porcellanites and cherts composed of opal-CT and quartz. We examined the oxygen isotopic compositions of these diagenetic silica minerals at four widely spaced sites occupied during ODP Leg 127 in order to investigate the thermal history of the region. Formation temperatures computed from these isotopic data range from 22° to 68° C for opal-CT and from 44° to 92° C for diagenetic quartz, quite similar to temperature ranges estimated from the extrapolated modern gradients, 36°-43°C and 49°-64°C, respectively. At each site the isotopic temperature values cluster near the extrapolated ambient sediment temperatures. As a first approximation, the similarities suggest that the positions of the silica transformations in the basin are controlled by the present thermal regime.In detail, isotopic and ambient temperatures differ. If these differences are real, then they reflect variations in the thermal histories at these sites. At Sites 794 and 797 in the Yamato Basin, isotopic temperatures and gradients computed from these data are lower than or comparable to ambient temperatures and gradients. We suggest that the silica zones have roughly equilibrated with the modern gradients at these localities. At Site 795 in the Japan Basin, isotopic temperatures are also lower than ambient sediment temperatures at comparable depths, but the gradient computed from the isotopic temperatures is higher than the present measured gradient. For both scenarios to hold, the silica zones must have formed under initially high gradients during the early post-rift period at this locality. These zones were then rapidly buried and have yet to equilibrate with the modern lower gradient. At Site 796 on Okushiri Ridge, isotopic temperatures exceed present temperatures as expected for an area of recent uplift. The gradient computed from our isotopic data and the thickness of the opal-CT zone indicate a higher gradient than at present at this site, apparently reflecting higher heat fluxes during the early post-rift period or recent frictional heating from nearby reverse fault activity.

Pore fluid chemistry from the Second Gas Hydrate Drilling Expedition in the Ulleung Basin (UBGH2): Source, mechanisms and consequences of fluid freshening in the central part of the Ulleung Basin, East Sea

“…Clay mineral analyses of deep samples from ODP Site 808, has been used to document an in situ conversion of smectite to illite, which explain the bulk of the Cl À anomaly observed at this site (Brown et al, 2001). In contrast, data from ODP Site 797 (Yamato Basin), where the pore fluid freshening is driven by silica diagenesis below 300 mbsf (Matsumoto, 1992;Pisciotto et al, 1992), show trends that deviate from those of ODP Site 808 and our Group III samples (Fig. 9D, E and F).…”

“…Two dehydration reactions commonly associated with deep fluid freshening in marine sediments are opal A to opal CT (silica diagenesis) and smectite to illite transformations (illitization) (Perry and Hower, 1972;Moore and Gieskes, 1980;Kastner et al, 1991;Matsumoto, 1992;Pisciotto et al, 1992;Martin et al, 1996;Aquilina et al, 1997;Fitts and Brown, 1999;Brown et al, 2001;Dählmann and de Lange, 2003;Torres et al, 2004a;Teichert et al, 2005;Vanneste et al, 2011). The opal A to opal CT transformation occurs at temperatures ranging from 22 to 68 C (e.g., Murray et al, 1992;Pisciotto et al, 1992), and the onset of smectite-to-illite transformation occurs at 60e90 C (Freed and Peacor, 1989).…”

“…The opal A to opal CT transformation occurs at temperatures ranging from 22 to 68 C (e.g., Murray et al, 1992;Pisciotto et al, 1992), and the onset of smectite-to-illite transformation occurs at 60e90 C (Freed and Peacor, 1989). These reactions have been documented by analyses of sediment and pore fluid samples collected by deep drilling in the Yamato Basin and at the toe of the Nankai subduction zone (Brown et al, 2001;Saffer et al, 2008;Saffer and McKiernan, 2009).…”

Fossilisation of a silica diagenesis reaction front on the mid-Norwegian margin

“…This work has been mainly based on core, field or experimental data (e.g. Murata and Larson, 1975;Kastner et al, 1977;Mizutani, 1977;Isaacs, 1981;Isaacs et al, 1983;Hinman, 1990;Compton, 1991;Matheney and Knauth, 1993;Murray et al, 1992a,b;Pisciotto et al, 1992;Eichhubl and Boles, 2000). Less attention has been paid to the seismic expression of silica diagenesis even though the remarkable change in physical properties of many biosiliceous sediments during the diagenetic pathways commonly results in a prominent seismic reflection marking the silica diagenetic boundary (e.g.…”