Experimental constraints on the aqueous fluid connectivity and electrical conductivity in the mantle wedge

概要

In subduction zones, slab-derived aqueous fluids circulate through a mantle wedge and play a key role in arc magma genesis and global material cycling. Their presence can be detected by geophysical observations because of their remarkable influence on the physical properties of overlying plate. Recently, high electrical conductors have been detected beneath some fore-arcs and are believed to store voluminous slab-derived fluids, which accounts for some of the missing fluid flux in subduction zones. This implies that the mantle wedge at fore-arc depths is permeable for aqueous fluids. In this doctoral research, I experimentally constrain dihedral angle in olivine–H2O–NaCl–CO2 systems to understand the effect of salt and nonpolar gas on the connectivity of aqueous fluids in the mantle wedge and propose a new fluid circulation model in fore-arc regions based on the dihedral angle data. Furthermore, I measure the electrical conductivity in forsterite–H2O–NaCl systems under conditions of textural equilibrium to quantitatively understand the fluid distribution in the fore-arc mantle wedge via interpreting the electrical conductivity anomalies, which could provide an insight into fluid fraction in forearc regions.

In part 1, I precisely determine the dihedral angle in an olivine–H2O–NaCl system at 1–4 GPa and 800–1100 °C to assess the effect of salinity on the fluid connectivity. The results show that NaCl significantly decreases dihedral angle to below 60° in all of the investigated temperature and pressure conditions at NaCl concentrations above 5.0 wt% and, importantly, even at 1.0 wt% at 2 GPa. Combining my experimental results with the previous geodynamic simulation, I find that the slab- released saline fluid forms an interconnected network in the mantle at relatively shallow depths of ~80 km and can partly reach the fore-arc crust without causing melting and serpentinization of the mantle. Fluid transport through this permeable window of mantle wedge accounts for the location of the high electrical conductivity anomalies detected in fore-arc regions.

In part 2, I further examine the effect of non-polar gas, namely CO2, on the fluid connectivity in an olivine–H2O–CO2 system and finally evaluate the fluid connectivity in a multicomponent (olivine–H2O–NaCl–CO2) system at 1–4 GPa and 800–1100 °C. Contrary to NaCl, CO2 tends to increase the dihedral angle at 1 GPa and 800–1100 °C, and 2 GPa and 1100 °C. However, it can reduce the dihedral angle even below 60° at relatively high pressure and low temperature conditions where olivine partly reacts with CO2 to form magnesite and orthopyroxene. This result sheds a new light on the effect of CO2 on fluid connectivity and supports the fluid circulation model I proposed above. Results in the multicomponent system suggest that the effect of NaCl on dihedral angle is much more significant than that of CO2. In addition, strikingly, the dihedral angles in the multicomponent system are the smallest among all investigated systems under the presence of magnesite and orthopyroxene. Therefore, the fluid circulation model presented above can persist even in the more realistic multicomponent system.

In part 3, I present the measurement results of electrical conductivity of texturally equilibrated brine-bearing forsterite aggregate with various fluid fractions at 1 GPa and 800°C. I find that the electrical conductivity nonlinearly increases with increasing fluid fraction, and the data fit the Archie’s law model very well with parameters C = 1.00 and m = 1.90. Three-dimensional microstructure of interstitial pores determined by synchrotron X-ray computed micro-tomography shows a change in fluid distribution from isolated pockets to interconnected networks with increasing fluid fraction, accounting for the nonlinear increase in the electrical conductivity. My results provide the direct evidence that >2 vol% aqueous fluid with moderate salinity (5.0 wt%) can explain the high conductivity anomalies detected in fore-arc regions.