Experimental constraint on grain-scale fluid connectivity in subduction zones

The dihedral angle (theta) 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 theta in the mantle wedge (Huang et al., 2019). Along with the salt component, non-polarized gas such as CO2, is a crucial constituent of subduction zone fluids. CO2 is known to increase the olivine-fluid theta under conditions in which the olivine does not react with CO2, which is in contrast to the effect of NaCl on theta. For a better understanding of the connectivity of multicomponent fluid in the mantle wedge, we experimentally constrained theta in olivine + H2O-CO2 fluid and olivine + H2O-CO2-NaCl (multicomponent) fluid systems at 1-4 GPa and 800-1100 degrees C. For the H2O-CO2 system, we found that CO2 tends to increase theta at 1 GPa and 800-1100 degrees C and at 2 GPa and 1100 degrees C. In contrast, CO2 reduced theta even below 60 degrees 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 theta 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 theta, 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 theta is much more significant than that of CO2. Strikingly, theta was smaller than 60 degrees 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. (C) 2020 Elsevier B.V. All rights reserved.