The Summary,Mechanism And Evolution History Of LLSVPs As Well As Their Possible Origins

Seismic tomography models have revealed that there are two large low shear velocity provinces(LLSVPs)beneath Africa and the Pacific Ocean in the Earth's lower mantle.Waveform modeling results further suggest that both LLSVPs have steep sharp side boundaries and high elevations which imply they are probably compositionally heterogeneous from the ambient mantle(Ni et al.,2002;He and Wen 2009).Understanding the formation and evolution of these two LLSVPs is one of the key issues for building a comprehensive picture of the whole Earth's evolutionary history.Thermochemical geodynamic modeling has been employed to study the evolution of the two LLSVPs in the lower mantle(McNamara and Zhong,2005;Zhang et al.,2010;Tan et al.,2011).However,it is rather subtle to produce a chemical block with structures similar to the seismic observations,i.e.structures with high elevations and steep side boundaries.In previous thermochemical mantle convection models,none of them considered a newly discovered electronic reconfiguration of iron under temperature-pressure conditions in the lower mantle:iron-spin transition.In chapter three,we included in our mantle convection model with the effects of iron-spin transition of ferropericlase which substantially changes the physical properties of the mantle.Our results show that the transition of iron in spin state plays a dominant role in controlling the structure and stability of LLSVPs.Large chemical blocks with steep side boundaries and high elevations up to?1200 km above core-mantle boundary(CMB)emerge in our models with?20%volume content of ferropericlase.Such blocks cause a shear wave velocity decrease of-3.5%which is consistent with the seismic observations.However,these LLSVPs-like blocks formed in models of chapter three are transient structures of the lower mantle,which can only last for a few hundred million years(Myr)before destroyed by large-scale mantle motion.This conflicts with the opinion that the LLSVPs should be long-term existent structures which is supported by geologic evidences.On the other hand,the model in chapter three can also not explain the origin of chemical layer which is place on the CMB in the start time of convection.Though there exists three possible explanations about the chemical source of LLSVPs including remnants of primordial dense materials formed in the early stage of the Earth(Labrosse et al.,2007),products of core-mantle differentiation(Kanda and Stevenson,2006;Hayden and Watson,2007),and accumulation of ecologitic components from recycled oceanic slabs(e.g.Christensen and Hofmann,1994;Tackley,2011)by far.Discuss all the possibilities in a single model would make the work too tedious,so we only discuss the last possibility from the view of geodynamics.In chapter four we show that when the effect of the iron-spin transition in ferropericlase is added into the geodynamic models,supercontinent cycle enhances the accumulation of the subducted oceanic crust above the core-mantle boundary and stability of formed thermochemical block,which solved the problem of unstable LLSVPs-like structures in chapter three.When applying the supercontinent cycle as the surface velocity boundary conditions,two thermo-chemical structures similar to the current LLSVPs in geometries formed in our models after?500 Myr of mantle convection.These two structures can keep stable for about 2.5 billion years(Gyr)with a preferred chemical density contrast of?2%and an averaged plate velocity of?7 cm/yr.The volume of the structures first increases then decreases with time,which leads to a maximum volume,?2 times of the current volume of LLSVPs at 1.5 Gyr?2.0 Gyr after convection starts.Our results suggest that the LLSVPs could be solely formed from the subducted oceanic crust,and that they probably have been there for more than 2.5 Gyr.Beside the long-term stability of LLSVPs,we also found that the variation in elevation of LLSVPs can be used to explain the periodical occurrence of superchrons in geomagnetism.