Chemical and Isotopic Evidence for Sources of Fluids in a Mud Volcano Field Seaward of the Barbados Accretionary Wedge


Martin, J.B., Kastner, M., Henry, P., Le Pichon, X., Lallemant, S. (1996), Journal of Geophysical Research 101: 20,325-20,345.

Abstract


Chemical, isotopic and heat budgets are controlled by fluid flow and venting at convergent margins. Fluids vent from accretionary complex sediments via numerous pathways including mud volcanoes, which are common at the Barbados convergent margin. Chemical and isotopic compositions of fluids collected from four mud volcanoes, located east of the Barbados deformation front at 13 deg. 50'N, indicate venting is sufficiently rapid to prevent diffusion from seawater. In sediments surrounding the mud volcanoes, the rate of non-focused fluid flow is ~0.26 cm/yr. Although this rate is ~3 orders of magnitude slower than the flow from the mud volcanoes, significantly more fluid vents from the sediments surrounding the mud volcanoes than from the restricted areas of rapid flow from the volcanoes. Chloride concentrations are depleted to a minimum of 211 mM (~45% of seawater value) primarily by mixing diagenetically altered pore fluids with an 18O enriched fresh water released from the dissociation of methane hydrate. This reaction is accompanied by the release of sufficient methane to form a free gas phase and initiate eruption of the mud volcanoes. The geochemical compositions of the diagenetically altered pore fluids reflect the interplay between five major reactions: (1) clay mineral dehydration, which releases H2O and influences a range of chemical and isotopic compositions; (2) organic matter regeneration, which increases alkalinity, NH4+, and Br-, and decreases SO42- concentrations; (3) cation exchange with clay minerals which increases Na+ and decreases NH4+, concentrations; (4) carbonate recrystallization and precipitation, which decreases Ca2+ but increases Sr2+ concentrations; and (5) conversion of volcanic ash to clay minerals, which utilizes H2O and decreases Mg2+ concentrations. The geochemistry of the pore fluids suggests that these reactions occurred at temperatures ranging from ~75 deg. to 115 deg. C. Thus, at the regional geothermal gradients of ~27 to 40 deg. C/km, these temperatures indicate that the source regions are at ~2 to 4.5 km depth.


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