EPISODIC FLUID FLOW AND DOLOMITIZATION BY METHANE-BEARING PORE WATER OF MARINE PARENTAGE IN AN ACCRETIONARY PRISM SETTING, BARBADOS, WEST INDIES

摘要

Barbados is a small island located on the crest of an accretionary prism at the eastern edge of the Caribbean Sea. Dolomite occurs in relatively small amounts in Pleistocene carbonates and in the underlying Oligocene–Miocene chalk, which also contains a few small caves, distinctive authigenic calcites, and oil that ascended through a dense networks of fractures. The stable isotopes of calcites form a positive trend, while those of the dolomites form a negative trend. Dolomite d18O values range from about +1 to +8 PDB, and d~(13)C values range from about +2 to 225 PDB. Sr-isotope ratios range from 0.7079 to 0.7094, which corresponds to a secular range from the Oligocene to the Holocene. The data presented in this study support the notion that the Barbados accretionary wedge has undergone a prolonged history of compression with concomitant expulsion of fluids that varied through time and space. At times these fluids were diluted relative to seawater and also undersaturated with respect to calcite, which resulted in limited dissolution along fractures and in their immediate vicinity. At other times the expelled pore fluids were prone to limited dolomitization, forming pods of dolomite. At yet other times the expelled fluids formed calcite as interstitial cements and around submarine points of exit in what appears to be ‘‘cold seeps’’ with calculated temperatures as low as +4 oC. The expelled fluids were chemically slightly modified seawater that contained thermogenic methane. Calculated temperatures of dolomitization range between +4 and +25 uC, which suggest fluid expulsion from sub-sea depths of zero to about –650 m. A temperature of +4 uC is the lowest ever determined for dolomite formation. The Sr-isotope data suggest that the hypogene fluids were expelled episodically between about 8 and 2 My ago. This study provides support for the hypothesis that dolomitization can result from fluid expulsion in an accretionary prism setting. However, the volumes of dolomite formed in Barbados are small and probably are similarly small in most other accretionary prism settings. Thus, regionally extensive, pervasive dolomitization is not to be expected from the dolomitizing process identified here. If elevated to the status of a model, dolomitization via fluid expulsion from an accretionary prism would be comparable in importance to the mixing zone model of dolomitization.