Japan Geoscience Union Meeting 2019

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-CG Complex & General

[S-CG51] Role of volatiles on Earth and planetary dynamics

Thu. May 30, 2019 1:45 PM - 3:15 PM A11 (TOKYO BAY MAKUHARI HALL)

convener:Takayuki Ishii(Bavarian Research Institute, University of Bayreuth), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Michihiko Nakamura(Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University), Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington), Chairperson:Michihiko Nakamura, Eiji Ohtani(Tohoku University)

2:15 PM - 2:30 PM

[SCG51-03] Fluid circulation in the forearc mantle wedge inferred from the dihedral angle in an olivine-H2O-NaCl system

*Yongsheng HUANG1, Takayuki Nakatani1, Michihiko Nakamura1, Catherine McCammon2 (1.Graduate School of Science, Tohoku University, 2.Byerisches Geoinstitut, University of Bayreuth)

Keywords:dihedral angle, NaCl-bearing, fluid network, mantle wedge, subduction zone

In subduction zones, the circulation of slab-released aqueous fluids is crucial to understand the generation of arc magma and recycle of materials, and further to constrain the global geodynamics. The infiltration of aqueous fluids plays an important role in controlling geophysical properties of the mantle wedge such as elasticity, permeability and electrical conductivity. Recently, magnetotellulic studies found a high electrical conductor beneath the fore-arc region which was interpreted as a storage of voluminous slab-derived fluid, implying the presence of permeable mantle wedge at fore-arc depths. By contrast, experimental constraints on dihedral (wetting) angle (θ) in the olivine-H2O system indicated the fluid is once trapped in isolated pores (θ> 60°) within the down-dragging mantle matrixat fore-arc depths and finally released just beneath the arc volcano where θ< 60°. Here we present the precisely determined dihedral angles in the olivine and NaCl-bearing aqueous fluid (5–27.5 wt% NaCl) system at 1–4 GPa and 800–1100°C to reveal the influence of NaCl on the fluid connectivity because the NaCl is now recognized to be an important constituent of subduction zone fluids. We found that NaCl significantly decreases θdown to below 60° at all the investigated temperature and pressure conditions even at 5 wt% NaCl. Our results suggest that released NaCl-bearing aqueous fluid begins to form an interconnected network in the mantle at relatively shallow depths of ~80 km and can partly penetrate the fore-arc crust without causing mantle melting, which could account for the high electrical conductivity anomalies beneath the for-are region. Our model also emphasizes that partial melting of peridotite triggered by the addition of aqueous fluid can occur more widely than that previously thought, and implies that the release depth of aqueous fluid is not a primary control on the volcanic front position. This work was supported by the JSPS Japanese-German Graduate Externship.