The connectivity of multicomponent fluids in subduction zones

Lithospheric plates collide at convergent boundaries. Here, the less dense oceanic lithosphere subducts below the continental plate, and releases an abundance of water due to a progressive metamorphic reaction at high pressure and high temperature. The released water can infiltrate into the mantle wedge which lies between the subducting oceanic lithosphere and the continental crust.

Fluids that circulate in subduction zones have a significant effect on magma genesis, global material exchange between the Earth's interior and surface, and seismicity. The dihedral angle (θ) -- the angle between two intersecting planes -- holds the key to revealing the fluid connectivity and migration regime for a fluid-bearing, deep-seated rock in the Earth's interior known as pyrolite -- a rock mainly composed of olivine.

Although H₂O is the predominant composition of subduction-zone fluids, minor components in the fluid can have a dramatic impact on the wetting properties of olivine. This is evidenced in the dihedral angle between olivine and fluid.

Salt (NaCl) and non-polarized gases such as CO₂ are two crucial components of subduction-zone fluids that significantly affect the dihedral angle between olivine and fluid. CO₂ is known to increase the olivine-fluid θ under conditions in which the olivine does not react with CO₂. Whereas, a recent study showed that NaCl can effectively decrease the olivine-fluid θ even with a low NaCl concentration. NaCl and CO₂ have opposite effects on the olivine-fluid θ, and this factor has inhibited researchers in their understanding of fluid migration in subduction zones.

Clarifying the competing effects of NaCl and CO₂ on θ in an olivine + multicomponent (H₂O-CO₂-NaCl) fluid system can help researchers understand the connectivity of aqueous fluid with more realistic compositions of the mantle wedge; thus making mapping of fluid distribution easier.

To do this, doctoral student Yongsheng Huang, professor Michihiko Nakamura, and postdoctoral researcher Takayuki Nakatani from Tohoku University worked alongside professor Catherine McCammon from the University of Bayreuth. The research team sought to constrain θ in olivine + H₂O-CO₂ fluid and olivine + H₂O-CO₂-NaCl (multicomponent) fluid systems at 1-4 GPa and 800-1100 °C.

The results in the H₂O-CO₂ system showed that CO₂ tends to increase θ at 1 GPa and 800-1100 °C and at 2 GPa and 1100 °C. In contrast, CO₂ reduced the θ to below 60° at relatively high-pressure and low-temperature conditions. Here, the olivine partly reacts with CO₂ to form magnesite and orthopyroxene (opx).

Additional experiments on olivine-magnesite + H₂O and olivine-opx + H₂O systems showed magnesite or opx decreased the olivine-fluid θ. This implies that coexisting minerals affect the olivine-fluid interfacial energy by changing fluid chemistry. The multicomponent system results showed that the effect of NaCl on θ is much more significant than CO₂. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems.

"Our study has revealed that CO₂-bearing multicomponent aqueous fluid can infiltrate the overlying plate through an interconnected network at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation and confirms the origin of the high electrical conductivity anomalies detected in the fore-arc mantle wedge," said Nakamura.

The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity may allow for mapping partial melt in the mantle wedge.