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Chemical and isotopic evidence for sources of fluids in a mud volcano field seaward of the Barbados accretionary wedge

1996; American Geophysical Union; Volume: 101; Issue: B9 Linguagem: Inglês

10.1029/96jb00140

2156-2202

Jonathan B. Martin, Miriam Kastner, Pierre Henry, Xavier Le Pichon, Seigfried Lallement,

Geology and Paleoclimatology Research

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°50′N, indicate venting is sufficiently rapid to prevent diffusion from seawater. In sediments surrounding the mud volcanoes, the rate of nonfocused 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 m M (∼45% of seawater value) primarily by mixing diagenetically altered pore fluids with an 18 O‐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 H 2 O and influences a range of chemical and isotopic compositions; (2) organic matter regeneration, which increases alkalinity, NH 4 + , and Br − and decreases SO 4 2− concentrations; (3) cation exchange with clay minerals, which increases Na + and decreases NH 4 + concentrations; (4) carbonate recrystallization and precipitation, which decrease Ca 2+ but increase Sr 2+ concentrations; and (5) conversion of volcanic ash to clay minerals, which utilizes H 2 O and decreases Mg 2+ concentrations. The geochemistry of the pore fluids suggests that these reactions occurred at temperatures ranging from ∼75° to 115°C. Thus at the regional geothermal gradients of ∼27 to 40°C/km, these temperatures indicate that the source regions are at ∼2 to 4.5 km depth.