Fluid Origins, Thermal Regimes, and Fluid and Solute Fluxes in the Forearc of Subduction Zones

Abstract:

An in-depth analysis and synthesis of published and newly acquired data on the chemical and isotopic composition of forearc fluids, fluid fluxes, and the associated thermal regimes in five well-studied, representative erosional and accretionary subduction zone (SZ) forearcs will be presented. Evidence of large-scale fluid flow, primarily focused along faults, is manifested by widespread seafloor venting, associated biological communities, authigenic carbonate formation, chemical and isotopic anomalies in pore-fluid depth-profiles, and thermal anomalies. The nature of fluid venting seems to differ at the two types of SZs. At both, fluid and gas venting sites are primarily associated with faults. At accretionary SZs, the décollement and underthrust coarser-grained stratigraphic horizons are the main fluid conduits, whereas at non-accreting and erosive margins, the fluids from compaction and dehydration reactions are to a great extent partitioned between the décollement and focused conduits through the prism.

The measured fluid output fluxes at seeps are high, ~15-40 times the amount that can be produced through local steady-state compaction, suggesting additional fluid sources or non-steady-state fluid flow must be involved. Recirculation of seawater must be an important component of the overall forearc output fluid flux.

The most significant chemical and isotopic characteristics of the expelled fluids relative to seawater are: Cl dilution, sulfate, Ca and Mg depletions, and enrichments in Li, B, Si, Sr, alkalinity, and hydrocarbon concentrations; they often have distinctive δ¹⁸O, δD, δ⁷Li, δ¹¹B, and δ³⁷Cl values, and variable Sr isotope ratios. These characteristics provide key insights on the source of the fluid and the temperature at the source.

Using our best fluid output flux estimate and considering an ocean volume of 1340 × 10⁶ km³, the global ocean residence time in SZs is ~100 Myr. This value is five times faster than previous estimates for SZs and is more similar to the estimates of the residence time in the global ridge crest that range from 20-90 Myr. Based on this extrapolated fluid reflux to the global ocean, SZs are an important source and sink for several elements and isotopic ratios, in particular an important sink for seawater sulfate, Ca, Mg, and sulfate, and an important source of B and Li.