This paper presents a study on the effects of phase transitions on the mantle convection of Venus in a three-dimensional(3D)spherical shell domain.Our model includes strong depth-and temperature-dependent viscosity and exothermic phase change from olivine to spinel as well as endothermic phase change from spinel to perovskite.From extensive numerical simulations of the effects of Rayleigh number(Ra),and the Clapeyron slopes and depths of phase changes,we found the following:(1)The endothermic phase change prevents mass flow through the interface.Increasing the absolute value of the Clapeyron slopes decreases radial mass flux and normalized radial mass flux at the endothermic phase boundary,and decreases the number of mantle plumes.In other words,mass flow through the phase boundary decreases.The inhibition influence of phase changes increases,as do convective wavelengths.(2)Increasing Ra also increases the convective wavelength and decreases the number of mantle plumes,but it has less influence on the mass exchange.As Ra increases,the convective vigor increases along with the radial mass flux and the mass flow through the phase boundary;however,the normalized mass flux through the phase boundary varies little with Ra,which is different from the conclusion that increasing Ra will greatly increase the inhibition of mass flow through the phase boundary based on two-dimensional(2D)modeling.(3)Increasing the depth of endothermic phase change will slightly decrease the number of mantle plumes,but has little effect on the mass flow through the phase boundary.Consistent with previous studies,our results show that the phase change from spinel to perovskite could inhibit the mass flow through the phase boundary,but they also show that the buildup of hot materials under the endothermic phase boundary in the 3D model could not be so large as to cause strong episodic overturns of mantle materials,which is quite different from previous 2D studies.Our results suggest that it is difficult for phase changes to cause significant m
