Silicate dissolution boosts the CO₂ concentrations in subduction fluids.

Area of Science:

• Geochemistry
• Experimental Petrology
• Subduction Zone Processes

Background:

• Estimates of dissolved carbon dioxide (CO2) in subduction zone fluids traditionally rely on limited experimental data and thermodynamic models.
• Understanding CO2 solubility in deep fluids is crucial for modeling carbon cycles and mantle dynamics.

Purpose of the Study:

• To experimentally determine the volatile and solute content of graphite-saturated fluids interacting with silicates at subduction zone conditions.
• To investigate the influence of silicate interactions on CO2 solubility in deep aqueous fluids.

Main Methods:

• Experimental determination of fluid composition in the systems COH, SiO2-COH, and MgO-SiO2-COH at 1-3 GPa and 800°C.
• Utilized graphite-saturated conditions to simulate deep Earth environments.
• Analyzed fluid volatiles and solute contents, including CO2 and water activity.

Main Results:

• Fluids interacting with silicates showed up to 30 mol% higher CO2 content compared to pure COH systems.
• A decrease in water activity, likely due to the formation of Si-O-C and Si-O-Mg organic complexes, was observed.
• Experimental data contradicts sparse previous estimates, highlighting higher CO2 solubility.

Conclusions:

• The interaction between deep aqueous fluids and silicates represents a significant, previously unrecognized mechanism controlling subduction zone fluid composition.
• This interaction enhances the transfer of deep carbon from the slab-mantle interface to the overlying mantle wedge, particularly in fluid-rich environments.
• Novel insights into deep carbon cycling and mantle metasomatism are provided.