Scripps Institution of Oceanography, University of California, San Diego
Ecole Normale Supérieure, Département de Géologie, Paris
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°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 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 decrease Ca2+ but increase 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°
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.