FLUID COMPOSITION IN SUBDUCTION ZONES AND IMPLICATIONS FOR GEOCHEMICAL CYCLING

Fluids contained in the subducting plate and overlying sediment are expelled at various levels with increasing pressure and temperature. The fluid compositions are chemically and isotopically very different from that of seawater (SW), and the rates of flow, although as yet not fully constrained, are sufficient to have profound consequences on chemical and isotopic global geochemical cycling; it is estimated that the entire volume of the ocean is recycled through the earth's subduction zones (SZ) in ~ 300 my. Fluid expulsion is mostly focused along pathways that have structural and lithological constraints. Some of the fluid is involved in arc magma generation. If the efficiency of fluid expulsion is incomplete, some must be cycled back into the mantle; knowing how much is critical for mass balances. The subducted volatiles, in particular H₂O, CO₂, and CH₄ impact biogeochemical, diagenetic, and metamorphic processes.

So far Li, Sr, and Cl concentrations and stable isotopes provide the best constraints on cycling in SZs. Based on Li concentration and fluid flux data, the net Li flux into the ocean is estimated to be ~1 x 10¹⁰ moles/yr, similar to the hydrothermal and river fluxes. The δ⁷Li values of the fluids are lower than the SW value but higher than the mantle value. There also is a net flux of Sr into the ocean. Although the ⁸⁷Sr/⁸⁶Sr values range greatly, between 0.7060-0.7100, most values fall between 0.7065-0.7085, thus are less radiogenic than the modern SW (0.70916) and the average river (0.7116) values. Thus, the ⁸⁷Sr/⁸⁶Sr flux from SZs reduces the recently calculated missing hydrothermal input of ~6 x 10⁹ mol/yr of ⁸⁷Sr/⁸⁶Sr at 0.7037, and with the flux from ridge flanks may balance the oceanic ⁸⁷Sr/⁸⁶Sr budget. The expelled ubiquitous low-Cl fluids that originate at greater depths in SZs are also depleted in δ³⁷Cl relative to the SW value, indicating that Cl is not conservative in SZs. The large Cl isotopic fractionation in the pore fluids, implies that with no compensating fluxes, the δ³⁷Cl of SW should have decreased by ≤5‰ in ~ 300 my, not observed in marine evaporites over 200 Ma. The nearly constant SW δ³⁷Cl thus implies either a compensating flux of isotopically enriched Cl into the ocean, may be from the mantle at ridge-crests, or efficient recycling of SW-derived Cl via volatilization associated with arc magma genesis.