10:15 〜 10:30
[SIT17-06] The passage of water through mantle assemblages under subduction zone conditions
★Invited Papers
キーワード：subduction, serpentine, high pressure
Fluids produced through dehydration reactions in subducting lithosphere are involved in melting and metasomatism in the sub-arc. The nature of the fluid flow mechanisms that allow transport from the subducting slab to the mantle wedge are, however, poorly understood. These processes have implications for the geochemistry of the fluids, as channelized flow will leach elements from much smaller volumes of the slab compared to pervasive flow. It is also important to understand the factors that control the timescale of fluid transport because if it is too slow, fluids will tend to be subducted with the slab rather than being released into the overlying mantle.
We have designed a type of high-pressure experiment to explore fluid migration time scales. A sintered polycrystalline core of olivine or orthopyroxene is produced with a particular average grain size. The core is then sandwiched between a fluid source, either serpentine or aluminium hydroxide Al(OH)3, and a fluid sink of MgO. The experiments are performed for durations of 6-144 hours. At the experimental conditions, the source dehydrates, inducing an overpressure and resulting in a fluid flow through the dense sintered core. The fluid can then be captured in the MgO fluid sink on the other side of the core through the formation of brucite (Mg(OH)2). This method allows the rate of water transport through the bulk sample, including along the grain boundaries, to be determined and does not require spectroscopic methods or an ion probe to determine the amount of water transport, which is often challenging for samples with small grain sizes. The experiments are performed at a range of temperatures and at pressures consistent with sub-arc conditions.
The results show that while it is possible for water to pass through a mineral core of orthopyroxene on a reasonable experimental time scale when using Al(OH)3 as the water source, this is not possible on similar timescales when serpentine is used as the source. This implies that the fluid overpressure, which is much higher in the case of Al(OH)3, is playing a role in the rate of water transport. The results also show that at conditions of serpentinite dehydration, water diffusivities are relatively low and similar to those expected for grain boundary diffusion. When these diffusivities are extrapolated to grain sizes of the upper mantle, they appear to be too slow to allow the transport of water into the overlying mantle wedge. Further experiments to examine the transport of other species such as CO2 have also been performed using different sources and sinks.
We have designed a type of high-pressure experiment to explore fluid migration time scales. A sintered polycrystalline core of olivine or orthopyroxene is produced with a particular average grain size. The core is then sandwiched between a fluid source, either serpentine or aluminium hydroxide Al(OH)3, and a fluid sink of MgO. The experiments are performed for durations of 6-144 hours. At the experimental conditions, the source dehydrates, inducing an overpressure and resulting in a fluid flow through the dense sintered core. The fluid can then be captured in the MgO fluid sink on the other side of the core through the formation of brucite (Mg(OH)2). This method allows the rate of water transport through the bulk sample, including along the grain boundaries, to be determined and does not require spectroscopic methods or an ion probe to determine the amount of water transport, which is often challenging for samples with small grain sizes. The experiments are performed at a range of temperatures and at pressures consistent with sub-arc conditions.
The results show that while it is possible for water to pass through a mineral core of orthopyroxene on a reasonable experimental time scale when using Al(OH)3 as the water source, this is not possible on similar timescales when serpentine is used as the source. This implies that the fluid overpressure, which is much higher in the case of Al(OH)3, is playing a role in the rate of water transport. The results also show that at conditions of serpentinite dehydration, water diffusivities are relatively low and similar to those expected for grain boundary diffusion. When these diffusivities are extrapolated to grain sizes of the upper mantle, they appear to be too slow to allow the transport of water into the overlying mantle wedge. Further experiments to examine the transport of other species such as CO2 have also been performed using different sources and sinks.