11:30 AM - 11:45 AM
[SCG48-09] Onset of small-scale convection beneath the oceanic plate: effects of melt depletion
Keywords:oceanic plate, small-scale convection, composition, density, viscosity
It has been known for long time that the observed seafloor subsidence and surface heat flow deviate from the predictions of half-space cooling models at old plate ages. To explain this discrepancy plate models are proposed, where the temperature at the bottom of the oceanic plate is assumed to be constant. Many previous studies argue that small-scale convection beneath the oceanic plate can be a physical mechanism to explain plate models, although most of these studies focus only on temperature effects. However, the density and viscosity of the mantle also depend on composition: the depleted mantle has a lower density and higher viscosity compared with the unmelted mantle. It is therefore expected that the behavior of small-scale convection changes by the presence of depleted mantle. This study investigates the effects of melt depletion on the onset time of small-scale convection beneath the oceanic plate by using numerical modeling.
Two-dimensional cross section perpendicular to the plate motion direction is considered, and the initially hot mantle is cooled from the top. The uppermost, depleted mantle is assumed to have a lower density and/or higher viscosity compared to those of the unmelted mantle. Spatiotemporal variations in temperature and rock velocity are obtained by solving the equations describing the conservation of mass, momentum, and energy. The onset time of small-scale convection is determined based on the difference between the predicted temperature and that expected from half-space cooling models.
The results show that the onset of small-scale convection is delayed by including the depletion effects, and the effects become larger when a higher mantle temperature is assumed. The small-scale convection occurs earlier for higher mantle temperature even when the effects of melt depletion are included. A previous study has proposed based on the observations of seafloor bathymetry and Rayleigh wave phase velocity and a stability analysis that the small-scale convection occurs earlier for lower mantle temperature when the depleted mantle is considered. It differs from the present results, which may indicate that the stability analysis cannot fully capture complex behavior of the thermochemical convection. It is also found that horizontal variation in the thickness of the depleted mantle disappears quickly if there is no viscosity change due to melt depletion.
Two-dimensional cross section perpendicular to the plate motion direction is considered, and the initially hot mantle is cooled from the top. The uppermost, depleted mantle is assumed to have a lower density and/or higher viscosity compared to those of the unmelted mantle. Spatiotemporal variations in temperature and rock velocity are obtained by solving the equations describing the conservation of mass, momentum, and energy. The onset time of small-scale convection is determined based on the difference between the predicted temperature and that expected from half-space cooling models.
The results show that the onset of small-scale convection is delayed by including the depletion effects, and the effects become larger when a higher mantle temperature is assumed. The small-scale convection occurs earlier for higher mantle temperature even when the effects of melt depletion are included. A previous study has proposed based on the observations of seafloor bathymetry and Rayleigh wave phase velocity and a stability analysis that the small-scale convection occurs earlier for lower mantle temperature when the depleted mantle is considered. It differs from the present results, which may indicate that the stability analysis cannot fully capture complex behavior of the thermochemical convection. It is also found that horizontal variation in the thickness of the depleted mantle disappears quickly if there is no viscosity change due to melt depletion.