Numerical Simualtions On The Evolution Of The Thermochemical Anomalies In The Lowermost Mantle

Multi-scales of heterogeneity exist in the mantle, especially in the lithosphere and in the D" zone (the lowermost part of the mantle). Difference between continental lithosphere and Oceanic lithosphere is the most prominent manifestation of the heterogeneity in the uppermost mantle. Heterogeneity in D" zone includes large scale superplume (or Large Low Velocity Zone, LLVZ), which is on the scale of thousands of kilometers, fluctuation of the top interface of D" zone, which is on the scale of hundreds of kilometers, and small scale ULVZ, which is on the scale of tens of kilometers. Continental lithosphere (especially the craton, which is the oldest part of the continental lithosphere) has lower intrinsic density, lower temperature and higher viscosity compared to that of oceanic lithosphere. Large scale superplume and small scale ULVZ may have chemical origin, medium-scale fluctuation of D" interface may origin from the Pv-pPv phase change. Because D" is the layer between mantle and out core, thermal heterogeneity may also play important role in the D" zone. It is thus very possible that thermal, phase, chemical heterogeneity may coexist in this region. The interaction between each factor lead to the complexity in this region revealed by seismic and geochemical observations.Studies on the origin and evolution of the heterogeneous in the lowermost mantle may help us to understand the evolution of the earth, the origin of geochemical heterogeneity and some other important problems in geoscience. Numerical simulation of mantle convection is now one of the most important method to investigate the evolution and origin of the heterogeneity in the lowermost mantle. We used numerical simulation of mantle convection in this paper to investigate the evolution of thermochemical piles in the lowermost mantle and its effect on the convective pattern of mantle. Main contributions of this paper are as follows.(1) Thermochemical Anomaly in the lowermost Mantle and its EvolutionThe large low shear wave velocity structure (LLSVPs) beneath Southern Africa is suggested to be a thermochemical anomaly generated early in the earth’s history. LIP (large igneous province) eruption sites of the last0.3Ga and most deep origin hotspots today is observed to correlate well with the boundary of this anomaly on CMB, which suggests the shape of this anomaly can remain largely unchanged for at least0.3Ga. We performed numerical models on thermochemical convection to study under which conditions can one thermochemical block in the lowermost mantle survive for4.5Ga and keep its shape largely unchanged for at least0.3Ga. Our main conclusions are:(1) Calculations in2D with a resolution of128is accurate enough to estimate the survival time of a dense chemical pile.(2) Chemical anomaly with higher viscosity ratio will stimulate plumes to generate from its boundary instead of its interior.(3) The entrainment rate of chemical anomaly is slow at the beginning and ending while much faster at intermediate stage. If we assume the volume of Africa anomaly has not changed too much during the last0.3Ga, it’s volume now should be about the same its volume when it is formed.(4) Chemical blocks with lower r|cl endured faster change in its morphology and location in its evolution. Thus high viscosity of the thermochemical anomaly in the lowermost mantle may serve as an explanation for its longevity and stability in shape and morphology in the past0.3billion years.(2) The interaction between continental lithosphere, surrounding mantle and thermochemical piles in theContinental lithosphere with low intrinsic density and high viscosity occupies about30%area of the Earth’s surface. Because of its special physical and chemical properties, the continental lithosphere does not actively take part in the convective mantle overturn. However it influences the convective flow and vice versa. Below central Pacific ocean and Southern Africa lie two high dense thermochemical piles, covering20%of Core Mantle Boundary(CMB) area in total. The structure of these thermochemical piles is influenced by convective flows and thus by the continental lithosphere. On the other hand, thermalchemical piles have important effect on the pattern of mantle convection. Thermochemical convection models including continental lithosphere and thermochemical piles with earthlike parameters are conducted to investigate the interaction between continental lithosphere, convective mantle flow and thermochemical piles. Our model results show that (1) Violent downwelling flow at continental margins, downward mantle vertical velocity under continental region, upward mantle vertical velocity under oceanic region, and compressive horizontal stress in continental lithosphere characterize the main feature of the mantle when size of the continental lithosphere is small. With the increase of continental size, downwelling flow at continental margins abates; mantle vertical velocity under continental/oceanic region reverses; horizontal stress in the continental lithosphere transforms to tensile.(2) Lithosphere-Asthenosphere Boundary(LAB) in continental region is deeper and colder than that in oceanic region. With the increase of continental size, difference of LAB depth and temperature between continental and oceanic region decrease or is even reversed.(3) The abundance of thermochemical piles in continental region is positively correlated with the size of continental lithosphere, i.e. low at small continent size and high at large continent size.(4) Surface heat flux is high and fluctuates with time in the oceanic region but is low and fluctuates little with time in the continental region.(5) CMB heat flux under thermochemical piles is lower than surrounding region.(6) Thermochemical piles in the lowermost mantle distorts under the viscous forces exerted by convecting mantle. Thermochemical plumes origin from the peaks of these thermochemical piles.(3) The effect of Pv-pPv phase change on the evolution of Large scale thermochemical piles and ultra-low velocity zoneD" zone in the lowermost mantle is the most complicated region in the mantle, except for lithosphere. Thermal, phase, chemical heterogeneity may coexist in this region. We conducted numerical simulations including Perovskite-post Perovskite (Pv-pPv) phase transition, thermochemical piles and Ultra Low Velocity Zone (ULVZ) in2D Cartesian coordinates to investigate the effect of Pv-pPv phase change on the evolution dense chemical piles and ULVZ. Our model results show that (1) Ultra low velocity zone prefer to reside at the boundary and interior of large scale thermochemical piles. The size of ULVZ at the boundary of large scale thermochemical piles is larger than that in the interior. As the location of the thermochemical piles changes, the location of ULVZ will also change to keep itself located in the interior or vicinity of the large thermochemical piles.(2) The appearance of Pv-pPv phase change will reduce the stability of thermochemical piles. If the viscosity in pPv phase is lower than that in Pv phase, the horizontal extent of pPv will be larger and this will reduce further the stability of thermochemical piles.(3) The appearance of Pv-pPv phase transition will increase the temperature in the upper mantle, but it has no significant effect on the temperature in the lowermost mantle.(4) The appearance of Pv-pPv phase transition will enhance the velocity rate in the mantle, especially in the lowermost mantle. If the viscosity in Pv phase is higher than that in pPv phase, this enhancement is even larger.(5) CMB topography in the downwelling region is negative, low viscosity pPv will reduce the amplitude of this negative topography while normal viscosity Pv will enhance it.(6) The appearance of Pv-pPv phase change make the fluctuation of the boundary of ULVZ more violent.(7) CMB topography in ULVZ region is lower compared with its adjacent region.