09:15 〜 09:30
[SIT20-08] Permeability determination from multi-anvil experiments: Implications for the fluid flux in subduction zones
★Invited Papers
キーワード：serpentinite, multi-anvil, permeability
Dehydration of hydrous minerals and the subsequent transport of the released fluids are important for a number of subduction zone processes. The fluids carry dissolved element and are responsible for metasomatism of the overlying mantle wedge, which is an important stage in the geochemical cycle of many trace and volatile elements. Within the mantle wedge fluids raise the degree of melting. Furthermore, subduction zone fluids are often linked to the origin of deep focused earthquakes. These processes necessitate a significant fluid flux between serpentine layers, which are likely to be the main source of water in subduction zones, and the overlying mantle, which requires fluid permeability to be sufficiently high.
Shear deformation in subduction zones most likely causes strong foliation and preferred orientation of serpentine minerals. Previous experiments at low pressures (100 MPa) indicate the occurrence of permeability anisotropy in foliated serpentinites. Accordingly, fluids may preferentially migrate parallel to the foliation but then become channelized into deep-rooted fault zones.
However, so far permeability measurements were limited to low pressures (<0.5 GPa). Based on pressure-dependent changes in volumes of both solids and liquids it becomes questionable as to whether the results of such permeability measurements can be extrapolated to the significantly higher conditions of subduction zone dehydration.
We report a new experimental method, which allows the permeability in dehydrating mineral assemblages to be determined through the analysis of recovered samples. For this purpose, we performed high pressure multi-anvil experiments. Our assembly consists of an orientated serpentinite drill core embedded in an MgO sleeve. At 2 - 5 GPa antigorite dehydrates over a temperature interval of approximately 100 °C at temperatures <700 °C. The released fluid migrates into the MgO and reacts to form brucite. The fluid can leave the serpentine sample in a direction either parallel or perpendicular to the initial serpentine foliation. The analysis of the location and proportion of brucite formed allows the fluid discharge at the experimental conditions to be calculated over the experimental run time. We combined these results with numerical simulations and µCT-scans to estimate the porosity.
Our results show that serpentinites in subduction zones are expected to form an essentially impermeable layer prior to dehydration. As progressive dehydration occurs at temperatures exceeding 550 °C the permeability increases by 2 orders of magnitude. We also show that the dehydration reaction causes in a change in texture, so that fluid flow becomes isotropic. This should favour pervasive fluid flow rather than channelized flow.
Shear deformation in subduction zones most likely causes strong foliation and preferred orientation of serpentine minerals. Previous experiments at low pressures (100 MPa) indicate the occurrence of permeability anisotropy in foliated serpentinites. Accordingly, fluids may preferentially migrate parallel to the foliation but then become channelized into deep-rooted fault zones.
However, so far permeability measurements were limited to low pressures (<0.5 GPa). Based on pressure-dependent changes in volumes of both solids and liquids it becomes questionable as to whether the results of such permeability measurements can be extrapolated to the significantly higher conditions of subduction zone dehydration.
We report a new experimental method, which allows the permeability in dehydrating mineral assemblages to be determined through the analysis of recovered samples. For this purpose, we performed high pressure multi-anvil experiments. Our assembly consists of an orientated serpentinite drill core embedded in an MgO sleeve. At 2 - 5 GPa antigorite dehydrates over a temperature interval of approximately 100 °C at temperatures <700 °C. The released fluid migrates into the MgO and reacts to form brucite. The fluid can leave the serpentine sample in a direction either parallel or perpendicular to the initial serpentine foliation. The analysis of the location and proportion of brucite formed allows the fluid discharge at the experimental conditions to be calculated over the experimental run time. We combined these results with numerical simulations and µCT-scans to estimate the porosity.
Our results show that serpentinites in subduction zones are expected to form an essentially impermeable layer prior to dehydration. As progressive dehydration occurs at temperatures exceeding 550 °C the permeability increases by 2 orders of magnitude. We also show that the dehydration reaction causes in a change in texture, so that fluid flow becomes isotropic. This should favour pervasive fluid flow rather than channelized flow.