4:15 PM - 4:30 PM
[SCG62-14] Effects of viscosity and density changes of the mantle associated with melt depletion on the onset time of small-scale convection beneath the oceanic plate
Keywords:oceanic plate, small-scale convection, composition, density, viscosity
Small-scale mantle convection beneath the oceanic plate (SSC) is thought to explain various observations, including surface heat flow and seafloor subsidence. It is therefore essential to understand physical conditions under which SSC occurs. Previous numerical studies on the onset of SSC have mainly focused on temperature effects, while the effects of composition have been little investigated. However, it has been suggested that the viscosity and density of the mantle which has experienced partial melting and melt extraction beneath a mid-ocean ridge (we call it melt-depleted mantle) can differ significantly from those of the unmelted mantle. To investigate possible effects of viscosity and density changes associated with melt depletion on the onset time of SSC, we conducted two-dimensional numerical simulations.
When we assumed the depleted mantle with constant thickness, the onset of SSC was delayed by the depletion effects, particularly when higher mantle temperatures (which result in a thicker depleted mantle) were assumed. However, even when the depletion effects were considered, the onset time remained longer for lower temperatures, contradicting predictions from previous studies. The viscosity change associated with the depletion had a larger effect on the onset of SSC relative to that of density.
We also performed calculations assuming that the thickness of the depleted mantle varies horizontally with given wavelengths. When only depletion-related density change was considered the onset time showed little dependence on the wavelength, because the initially assumed depleted mantle quickly deformed to a nearly uniform thickness. When only depletion-related viscosity change was considered, the onset time first decreased and then became almost constant as the wavelength increased. When both viscosity and density changes due to depletion were taken into account, the minimum value of the onset time were seen at intermediate wavelengths. These results suggest that the presence and lateral heterogeneity of the depleted mantle are important factors in better understanding the onset time of SSC.
When we assumed the depleted mantle with constant thickness, the onset of SSC was delayed by the depletion effects, particularly when higher mantle temperatures (which result in a thicker depleted mantle) were assumed. However, even when the depletion effects were considered, the onset time remained longer for lower temperatures, contradicting predictions from previous studies. The viscosity change associated with the depletion had a larger effect on the onset of SSC relative to that of density.
We also performed calculations assuming that the thickness of the depleted mantle varies horizontally with given wavelengths. When only depletion-related density change was considered the onset time showed little dependence on the wavelength, because the initially assumed depleted mantle quickly deformed to a nearly uniform thickness. When only depletion-related viscosity change was considered, the onset time first decreased and then became almost constant as the wavelength increased. When both viscosity and density changes due to depletion were taken into account, the minimum value of the onset time were seen at intermediate wavelengths. These results suggest that the presence and lateral heterogeneity of the depleted mantle are important factors in better understanding the onset time of SSC.