GeoPRISMS Themes - Geochemical Cycles
Geochemical Cycles
Elements cycle between the Earth's surface and interior at both rifting and subduction margins. The transfer and exchange of matter between Earth's oceans and atmosphere, subducting plates, asthenospheric and lithospheric mantle, and arc and continental crust ultimately control the composition and evolution of Earth's major near-surface solid and fluid reservoirs.
At subduction zones, the downgoing plate is enriched in volatiles through seafloor deformation and weathering processes and distributes this cargo to the overriding plate and mantle, selectively releasing volatile-rich fluids over a range of depths. This progressive devolatilization of the subducting plate creates a broad range of geochemical transformations in the overriding material and geological expressions at the surface, including forearc serpentinite diapirism and volatile-rich arc and back-arc magmatism, unique products of volatile transport. At rifts, volatiles bound in the pre-existing continental crust and lithosphere may be released to the atmosphere and oceans, through deformation and magmatism, or could be removed from the oceans and atmosphere by weathering processes. Additionally, alteration of exposed mantle along faults near the continental ocean transition may serve as a substantial volatile sink.
To date, most attention has been focused on the influence of H2O and CO2 on melting in subduction zones, but the cycling of other volatile species (S, N, rare gases, halides) at plate margins is also critical for large scale geochemical cycles and the importance of all of these volatiles goes beyond their influence on melting. For example, fluxes of volatiles between the surface and Earth's interior at plate margins have a first order influence on planetary climate on time scales ranging from years to billions of years. Storage and sequestration of volatiles by weathering, sedimentation, and subduction limits near-surface supplies of climate-influencing volatiles, whereas magmatism and the hydrologic cycle transport them back to the surface.
The extent to which oceanic plates entering subduction zones may be serpentinized may produce an important and unknown control on input budgets and fluxes of volatiles into the Earth's interior. Recent geophysical studies of oceanic plates suggest that faulting at the outer rise creates pathways by which low-temperature fluids circulate to up to 20 km into the oceanic lithosphere. The resulting hydration of the slab mantle could be a tremendous reservoir of water (and other volatiles) that can be transported to depth, given the high water content of serpentinite. Additionally, the cold corner of the mantle wedge may be serpentinized as slab-derived fluids flush through it, creating a large reservoir of H2O and other volatiles in the overriding fore-arc mantle. As yet, the processes that allow volatile fluxes out of this critical region are poorly understood. Hydrated and carbonated peridotite has emerged as a potential central control on the behavior of the subduction system at intermediate depths. New approaches are needed to quantify its abundance and total volatile budget in the mantle slab and forearc crust, and to assess how volatiles return to the Earth's surface. Finally, we still have a very poor understanding of the sources, sinks, and fluxes of volatiles in rift systems. Are rifts net sources of volatiles owing to mantle degassing, or sinks due to sequestration by weathering, hydrothermal alteration and sedimentation? Quantifying volatile fluxes at rift zones will be a critical new avenue of research in the Earth's geochemical cycles.
