Volatiles and Fluids in Subduction Zones

Material metabolism at subduction zones

The material transported on the incoming crust, i.e. the down-going oceanic plate, consists of pelagic and hemi-pelagic sediments overlying altered oceanic crust and a slice of the upper mantle. In addition, large quantities of terrigenous sediments are carried down-slope at continental margins, deposited in the deep-sea trenches, and thereby incorporated into the subducting material (von Huene and Scholl, 1991). The rapid sedimentation and high accumulation rate of these trench deposits buries organic matter from primary production as an important volatile ingredient. Processes inside the subduction zone, analogous to "metabolism" in an organism, transform these materials into volatile reservoirs, which are either ejected into the exosphere through the upper plate, accreted to the leading edge of the continental plate, or are transferred into the lower mantle. These volatile reservoirs consist of aqueous fluids, gases, mud diapirs, as well as components of continental and back-arc oceanic crusts. A largely unknown part of volatile reservoirs descends into the mantle and may rise again in the form of mantle plumes.

Climate-active volatiles are transported with sediments and altered crust in the form of carbonates, organic matter, sulfurous and hydrous minerals, and aqueous and gaseous fluids (Alt, 1995; Staudigel et al., 1996). The diagenetic and metamorphic processing of these materials, here referred to as subduction-zone metabolism, generates gas hydrates, carbonates, serpentized rocks and other phases, which regulate the volatile input into the deeper melting zones (Iwamori, 1998; Schmidt and Poli, 1998). Due to the rapid burial of the organic matter, methane formation and fixation as gas hydrates at continental margins is intensified and accelerated (Suess et al., 1999; von Huene and Pecher, 1999). Gas hydrates which form in the fore-arc -often associated with mud diapirs- are of special interest, as they influence fluid and heat fluxes, affect the stability of the slope and can release enormous quantities of methane when decomposing. At depths of approx. 50-150 km, CO₂-rich aqueous fluids and melts are transferred from the down-going plate into the overlying mantle wedge. Here, melting is induced by ascending aqueous fluids, which lower the melting temperatures. The climatically effective volatiles are transported to the surface either with the magma or as a separate fluid phase (Fischer et al., 1998; Giggenbach, 1996).

The return flow or ejection of volatile metabolic products takes three dominant transport paths: those of the subduction zone vents, the mud diapirs, and the explosive volcanoes. The most rapid return flow is through subduction zone vents caused by the dewatering of the accreted and subducted sediments at the deformation front of colliding plates. Hereby a series of subparallel ridges develops with backthrusts and overthrusts opening up conduits for fluid escape to release the overpressure. The escape sites, called cold vents, are colonized by chemosynthetic vent fauna which represent a unique deep-sea ecosystem involved in further metabolising vent volatiles (Suess et al., 1985; Wallmann et al., 1997). Another transport path is through mud diapirism. Mass transport by ascending mixed sediments and fluids is being increasingly recognized as an important transport path in its own right. For example, the Mariana fore-arc serpentinite mud diapirs are spectacular features, transporting fluids from up to 50 km of depth to the surface (Mottl, 1992). Other well-known subduction-related mud diapirs are found off the Indonesian island arc, Barbados, Costa Rica, Pakistan, and on the Mediterranean Ridge (Brown, 1994; Henry et al., 1996; Robertson & Kopf, 1998 and others). The most obvious transport path, however, is through explosive volcanoes, whereby enormous quantities of volatile-rich magmas are returned to the surface and the volatiles being released into the atmosphere (Allard, 1992; Sano and Williams, 1996). The mass transfer from the down-going plate to the mantle wedge and the deeper melting zones beneath the volcanoes -as metamorphic transformation- is most difficult to assess, but is essentially also determined by fluids (Schmidt and Poli, 1999). Thus, the role of fluids is not restricted to the fore-arc region but regulates the material recycling throughout the entire subduction zone.