ORIGIN OF LOW-CHLORIDE PORE FLUIDS ON A PASSIVE CONTINENTAL MARGIN, OFFSHORE TASMANIA (ODP LEG 189)

We use oxygen and chlorine isotopic measurements of interstitial waters (IWs) recovered from ODP Leg 189 to assess the origin of low Cl anomalies. Four sites on the west Tasman margin, south Tasman Rise, and the East Tasman Plateau were cored to depths ranging from 766-958 mbsf in 2145-2704 m of water. Sediments recovered are Neogene-Oligocene calcareous biogenic ooze/chalk underlain by Paleogene-Cretaceous siliciclastic mudstones. In situ temperature measurements indicate an average geothermal gradient of 55°C/km. Shipboard results show the presence of regionally extensive low Cl IW in the older sediments, which are manifested by multiple distinct Cl extrema rather than smoothly decreasing values. Minimum Cl values at the four sites range from 486 to 440 mM, a 13-21% decrease relative to the mean seawater (559 mM). Possible origin of the low Cl fluids include (1) gas hydrate dissociation, (2) dehydration and/or transformation of hydrous minerals, (3) clay membrane ion filtration, and (4) advection of meteoric waters. We observe no evidence in IW profiles to indicate lateral advection of fluids, and the geographic and geologic setting make it improbable that there are links to continental recharge areas. Gas hydrate dissociation is a common cause of such profiles in continental margin settings At three of the sites, IW freshening coincides with the onset of methanogenesis; however, low Cl fluids also are encountered at Site 1172 where only traces quantities of methane are present. Hydrate stability calculations indicate the low Cl fluids occur well below the base of the hydrate stability zone. Furthermore, IW oxygen isotopic values decrease with depth (minimum=-5.24 permil) and do not show the expected increase associated with hydrate decomposition. Cl isotopic values range from seawater-like values near the seawater/sediment interface to -7.3 permil at depth. Values significantly lower than seawater indicate fluid/mineral exchange of Cl. Ion membrane filtration should produce fluids enriched in ³⁷Cl, which we do not observe. Dehydration (expulsion of water) may play a role in Cl dilution, but does not fractionate Cl isotopes. We believe the low Cl isotopic values and Cl concentrations are likely the result of hydrous mineral transformations or the combination of dehydration and the uptake of Cl.