GEOCHEMISTRY OF A SEALED DEEP-SEA BOREHOLE ON THE CASCADIA MARGIN

The deep-sea borehole seal CORK was deployed for the first time on a m odern accretionary prism during ODP Leg 146 to the Cascadia Margin. Te n months after the deployment the fluid flow and geochemistry of the b orehole fluids was investigated during several dives by DSRV Alvin. Th e chemical analysis of the borehole fluids revealed methane concentrat ions of more than 3.5 mM, whereas oxygen and dissolved ions as Cl, NO3 , or PO4 are still close to the ambient seawater composition. The exce edingly high methane content measured at the top of the sealed borehol e and the observed degassing during the ascent of the submersible indi cates that the sampled fluid was initially saturated or close to satur ation with respect to CH4 The hydrocarbons are characterized by C-1/C2 + ratios of 170-200 and delta(13)C values of -59.5 to -62.4% which ind icates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that C RI enters the sealed borehole in the lower, perforated section (94-178 mbsf) and accumulates at the top of the borehole. This suggests the o ccurrence of free gas within the encapsulated borehole. Considering th e stability field of CH4-hydrates, the formation of these ice-like str uctures may take place and potentially results in a clogging of the to p of the borehole. Such precipitates could result in a decoupling of t he top of the borehole from the hydraulic and geochemical regime of th e accretionary complex, an important aspect for future plans of CORK d eployments. (C) 1998 Elsevier Science B.V. All rights reserved.