11:45 〜 12:00
[SCG48-10] 中央海嶺-トランスフォーム断層系における枯渇度の空間分布
キーワード：中央海嶺、トランスフォーム断層、枯渇度、数値モデリング
Partial melting occurs beneath a mid-ocean ridge, which creates a chemically depleted layer in the uppermost mantle. This depleted layer has a lower density than the unmelted mantle due to the exhaustion of dense minerals and the extraction of iron into the melt. It may also have a higher viscosity compared with the unmelted mantle because of dehydration associated with partial melting. It is therefore important to constrain the detailed spatial distributions of the degree of depletion beneath a mid-ocean ridge to better understand the structure and dynamics of the oceanic plate. Previous studies have mainly considered the formation of the depleted layer in a two-dimensional mid-ocean ridge system. However, the formation of this layer is expected to be more complicated due to the presence of transform faults, which produce three-dimensional (3-D) temperature and rock velocity fields. This study investigates the effects of rheology, the half-spreading rate, and transform fault length on spatial distributions of the degree of depletion in a mid-ocean ridge-transform fault system.
Steady-state 3-D temperature and rock velocity fields are obtained by solving the equations that describe the conservation of mass, momentum, and energy. The degree of depletion is defined as the maximum degree of melting that each passive tracer has experienced along the trajectory, where the degree of melting is predicted based on temperature, pressure, bulk water content, and modal clinopyroxene.
The degree of depletion generally increases with increasing half-spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle, which is consistent with the results of previous studies. Lateral differences in the degree of depletion in the ridge-parallel direction are reduced when plastic yielding is included. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate. These findings may at least partly explain the observed global variations in abyssal peridotite compositions. The present results show that the oceanic plate is chemically highly heterogeneous. It is therefore critical to investigate how this heterogeneity affects the structure and dynamics of the oceanic plate in a quantitative way.
Steady-state 3-D temperature and rock velocity fields are obtained by solving the equations that describe the conservation of mass, momentum, and energy. The degree of depletion is defined as the maximum degree of melting that each passive tracer has experienced along the trajectory, where the degree of melting is predicted based on temperature, pressure, bulk water content, and modal clinopyroxene.
The degree of depletion generally increases with increasing half-spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle, which is consistent with the results of previous studies. Lateral differences in the degree of depletion in the ridge-parallel direction are reduced when plastic yielding is included. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate. These findings may at least partly explain the observed global variations in abyssal peridotite compositions. The present results show that the oceanic plate is chemically highly heterogeneous. It is therefore critical to investigate how this heterogeneity affects the structure and dynamics of the oceanic plate in a quantitative way.