Chemical, and Sr and Li isotopic composition of pore fluids and sediments from ODP Leg 170 and Leg 205 sites

Pore fluid and sediment chemical and isotopic data were obtained for samples from Ocean Drilling Program (ODP) Leg 205 Sites 1253, 1254, and 1255 in the Costa Rica subduction zone. The chemical and isotopic data reported here were generated in our shore-based laboratories to complement shipboard inorganic geochemical data. Li isotopic analyses were carried out by L.-H. Chan at Louisiana State University (USA). The data reported herein include fluoride, bromide, rubidium, cesium, and barium concentrations; Li and Sr isotopic compositions in pore fluids; and Rb, Cs, and Ba concentrations in representative bulk sediments. The data also include new pore fluid fluoride and bromide concentrations from corresponding ODP Leg 170 Sites 1039, 1040, and 1043. O.M. Saether's Site 1039 and 1040 fluoride concentration data are shown for comparison. Basal sediment fluoride concentrations and Li and Sr isotope ratios at both Sites 1253 and 1039 show reversals that approach modern seawater values. Br/Cl ratios are, however, conservative throughout the sediment section at Sites 1039 and 1253. The observed sharp F and Br concentration maxima, Rb and K concentration minima, the most radiogenic 87Sr/86Sr ratios, and highest 7Li values along the décollement and fracture zone (Sites 1040, 1043, 1254, and 1255) strengthen the evidence obtained during Leg 170 that a deeply sourced fluid, originating from fluid-rock reactions at ~150°C and corresponding to between 10 and 15 km depth, is transporting solutes to the ocean.

Pore fluid pressure at DSDP Leg 84 Holes

Evidence of considerable overpressuring of pore fluids in the sediment drilled during Leg 84 was obtained from direct measurement of pressure by two methods. The first involved measurement of back pressure when the annulus of the drill hole became constricted with unremoved drill cuttings or constriction was caused by plastic inflow of the drill hole walls. The second involved measurement of pressure ahead of the bit in conjunction with in situ water samples and heat flow. All measurements indicated abnormally high pore pressure even in slope deposits of the Middle America Trench off Guatemala.

Cloride and bromide concentrations and Br-/Cl- molar ratios in pore fluids at ODP Site 134-833

Other than halite diagenesis and organic matter degradation, Cl- and Br- are considered to be conservative in marine pore fluids. Consequently, Br-/Cl- ratios should remain constant during most diagenetic reactions. Nonetheless, Br-/Cl- molar ratios decrease to 0.00127 (~18% less than seawater value) in pore fluids from Site 833 in the Aoba Basin of the New Hebrides convergent margin despite the lack of halite diagenesis and little organic matter. Sediment at this site is largely volcanic ash, which becomes hydrated with depth as it converts to clay and zeolite minerals. These hydration reactions remove sufficient water to increase the concentrations of most solutes including Cl- and Br-. The resulting concentration gradients drive diffusion, but calculations indicate that diffusion does not decrease the Br-/Cl- ratio. Some Cl- may be leached from the ash, but insufficient amounts are available to cause the observed decrease in Br-/Cl- ratio. The limited source of Cl- suggests that proportionately more Br- than Cl- is lost from the fluids to the diagenetic solids. Similar nonconservative behavior of Cl- and Br- may occur during fluid circulation at ridge crests and flanks, thereby influencing the halide distribution in the crust.

(Table 2) Uranium concentrations and isotopic ratios in surface waters of the Atlantic Ocean in April 1977

Uranium concentrations and isotope ratios were measured in thirteen surface-water samples collected across the entire Atlantic Ocean. The mean isotope ratio was 1.15+/-0.01, and the mean concentration 3.1+/-0.2 µg/l.