29. DATA REPORT: CHEMICAL AND ISOTOPIC COMPOSITIONS OF PORE FLUIDS IN ACCRETED AND UNDERTHRUST SEDIMENTS OF THE NORTHERN BARBADOS RIDGE 1

Miriam Kastner 2 and Yan Zheng 3

ABSTRACT

The chemical and isotopic data reported here were carried out in our shore-based laboratories to complement the shipboard pore-fluid chemical data from the northern Barbados Ridge. The data include (1) pore-fluid bromide, iodide, and strontium concentrations and strontium and oxygen isotope ratios at both sites drilled, and (2) strontium, oxygen, and carbon isotope ratios, and Sr concentrations of vein-filling carbonates from Hole 948C. The data suggest that the fluid-rock reactions responsible for the source of the low Cl- fluid at the décollement occur arcward from the sites drilled, at greater burial depth than penetrated by drilling, and that organic matter-rich sediments are involved in the fluid-sediments reactions at the origin. This fluid has more radiogenic Sr isotopes than the in situ pore fluids and is enriched in Sr and I, and the carbon isotopes of the dissolved carbonate are strongly negative. The chemistry and isotopic compositions of the vein-filling carbonates indicate that they did not precipitate from the in situ pore fluid. A past pulse of fluid injection caused high pore-fluid pressure, hydrofracturing, and vein-filling carbonate mineralization. The present-day pore fluid is strongly overprinted by in situ diagenesis of volcanic glass, suggesting that no major episode of fluid flow has occurred in the recent past.

1 Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results,156: College Station, TX (Ocean Drilling Program).
2 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212, U.S.A. mkastner@ucsd.edu
3 Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, U.S.A.