Magazine article Oceanus

Fluid Composition in Subduction Zones

Magazine article Oceanus

Fluid Composition in Subduction Zones

Article excerpt

Evidence for large-scale fluid flow and fluid expulsion at subduction zones includes several observations:

* the porosity of the originally water-rich sediments of accretionary complexes is rapidly reduced by tectonic forces,

* heat flow is regionally variable,

* depth profiles have characteristic temperature and pore-fluid chemical and isotopic anomalies that can only be maintained by rapid and rather recent fluid flow, and

* diffusive and/or channelized fluid venting is widespread.

The latter occurs along sedimentary and structural-tectonic conduits such as unconformities, faults, and the decollement (the prominent boundary between the overriding and underthrusting plates shown in the figure on page 87) as well as through mud volcanoes.

These fluids sustain prolific benthic biological communities and cause widespread carbonate deposition as cement, vein filling, crusts, or chimneys, mostly from oxidation of microbially or thermogenically derived methane. The fluids also play an important role in the deformational, thermal, and chemical evolution of subduction zones, and enhance sediment diagenesis and rock metamorphism. Fluids released from these reactions transport dissolved components into the ocean, some of which may be important for global geochemical budgets. At greater depths (more than 80 kilometers), released fluids, especially water from the subducted sediments and altered oceanic basement and carbon dioxide from methane oxidation and decarbonation reactions, may expedite partial melting processes in the overlying mantle wedge, leading to arc volcanism.

The presence of sediment-derived isotopes and trace elements, especially cosmogenic beryllium 10 (half life 1.5 million years) in arc lavas, provides evidence for sediment recycling in some subduction zones. Global estimates of sediment contribution to arc lavas range from a few to 20 percent of the subducted sediments.

The total volume of the internally available fluid sources in subduction zones through steady-state processes has been estimated to be 1 to 2 cubic kilometers per year. These estimates, however, do not account for the 2 to 6 order of magnitude larger than predicted fluid-flow rates measured at numerous channelized fluid venting sites, for example, at the Barbados, Nankai, and Cascadia accretionary complexes. This discrepancy in fluid volumes suggests either that the channelized fluid flow is transient in nature and/or that a major external fluid source exists. Meteoric water (rain or snow) is the most likely external source, but how it might be transported to the subduction zones is yet unknown.

Geochemistry of the Fluids

Detailed studies of the chemical and isotopic compositions, mostly of the pore fluids obtained through drilling and of the channelized venting fluids obtained with submersibles and conventional coring, indicate that the chemical and isotopic characteristics of the expelled fluids differ markedly from seawater, the original pore fluid. Of particular interest are the ubiquitous fresher-than-seawater fluids often found in accretionary complexes and associated with fluid conduits such as faults, the decollement, or mud volcanoes. Seawater chloride dilution of 10 to 64 percent has been recorded. Unraveling the origin of fresher-than-seawater fluids is of great importance to understanding subduction zone hydrogeochemistry. The only internal sources and processes that may provide water for the formation of the low-chloride fluids are: 1) Dehydration or breakdown of hydrous minerals, particularly clay minerals, amorphous opal (opal-A), and zeolites in the accretionary complex and of minerals such as talc, phengite, serpentine, and amphiboles in the oceanic basement, 2) Dissociation of gas hydrates (clathrates), ice-like crystalline compounds whose expanded ice-lattice forms cages that contain gas molecules (mostly methane hydrate has been recovered from several accretionary complexes, and geochemical and geophysical evidence for the presence of gas hydrates has been observed at most of them), and 3) Clay membrane ion filtration: Geochemical evidence for the occurrence or importance of the latter process in clay-rich subduction zones is yet unavailable. …

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