Experimental constraint on grain-scale fluid connectivity in subduction zones

The dihedral angle (θ) between olivine and aqueous fluid is a critical parameter in identifying the grain-scale fluid connectivity which controls the distribution and migration of aqueous fluids in subduction zones besides physical properties of mantle wedges. Recent magnetotelluric observations have suggested the occurrence of significant fluid circulation in deep fore-arc regions, which can be explained by infiltration of saline fluid with a low θ in the mantle wedge (Huang et al., 2019). Along with the salt component, non-polarized gas such as CO_2, is a crucial constituent of subduction zone fluids. CO₂ is known to increase the olivine–fluid θ under conditions in which the olivine does not react with CO₂, which is in contrast to the effect of NaCl on θ. For a better understanding of the connectivity of multicomponent fluid in the mantle wedge, we experimentally constrained θ in olivine + H₂O–CO₂ fluid and olivine + H₂O–CO₂–NaCl (multicomponent) fluid systems at 1–4 GPa and 800–1100 °C. For the H₂O–CO₂ system, we found that CO₂ tends to increase θ at 1 GPa and 800–1100 °C and at 2 GPa and 1100 °C. In contrast, CO₂ reduced θ even below 60° at relatively high-pressure (P) and low-temperature (T) conditions, in which the olivine partly reacts with CO₂ to form magnesite and orthopyroxene (opx). The consumption of non-wettable CO₂ components in aqueous fluid alone cannot explain θ lower than those in a pure H₂O system. Additional experiments on olivine–magnesite + H₂O and olivine–opx + H₂O systems showed that the presence of magnesite or opx decreased the olivine–fluid θ, which implies that coexisting minerals affect the olivine–fluid interfacial energy by changing the fluid chemistry. The results of the multicomponent system showed that the effect of NaCl on θ is much more significant than that of CO₂. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems. Our results suggest that slab-derived fluid can infiltrate into the deep fore-arc mantle wedge through an interconnected network even in a CO₂-bearing multicomponent system at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation. The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity in a wide condition may allow for the possibility of mapping partial melt in the mantle wedge.