Thermoelastic Properties At High P-T Conditions And Geophysical Implication Of Mantle Minerals In MASH System

Seismic studies provide crucial constraints on the velocity profiles of the Earth's deep interior.The composition of the earth's interior can be deduced by comparing the measured sound velocity of major mantle minerals at high pressures and temperatures with the obtained seismic data.Thus,it is important to conduct in situ meaesurements about the density and elasticity of mantle minerals which are needed to calculate the sound velocities at simultaneously high P-T conditions.Minerals in the MgO-Al2O3-SiO2-H2O(MASH)system are important Mg-Al and hydrogen bearing silicates.In this dissertation,we investigated the thermel elasticity of MgAl2O4-spinel and ?-AlOOH in MASH system with Diamonds anvil cells,brillouin scattering,X-ray diffraction and laser heating system.Results revealed the influence of spinel on the velocity structure of the upper mantle and the stability field,thermal equation of state and seismic feature of ?-AlOOH.MgAl2O4-spinel is one of the aluminum bearing phase in the top upper mantle,it also con-stitutes about 20 vol.%of the MORB layer in the subduction slab under lower mantle condition with the calcium ferrite structure which plays an important role in transporting aluminum into the deep earth.We measured the three independent elastic constants of MgAl2O4-spinel ranges from 300 K,1 bar to 1000 K,10.9 GPa,then fitting them with the 3rd order finite strain equation of state,results shows that the three elastic constants increase almost linearly as the pressure increases under all experimental temperature.Comparing with other main rock forming miner-als in the top upper mantle,MgAl2O4-spinel exhibits the greatest Vp,Vs and VP/VS.The top upper mantle comprises dominant spinel peridotite,we depicted the trend of Vp,Vs and Vp/Vs along both cold and hot slab geotherms with the data of the main minerals in the peridotite,and further estimated the potential effect of mineral composition variation on the velocity of the peridotite.Assessment demonstrate that variation of mineral composition leads to about 2.1%,1.3%increment based on each lower bound in terms of Vp,Vs,respectively.The peri-dotite that comprises more spinel or olivine possess faster velocities.It is acknowledged that aluminum bearing phase in the upper mantle undergoes spinel-garnet transition,we evaluated the consequence of this transition and found there's no remarkable change of Vp,Vs.?-AlOOH located at the sedimentary layer in the subducting slab.We conducted exper-iments to investigate the stability field of ?-AlOOH over the P-T range cover the whole mantle conditions,and gained the thermal equation of state(EoS)togather with the thermal pa-rameters at the first time.?-AlOOH has vast stability field that can extent to the core-mantle boundary(CMB)conditions,the dehydration line of ?-AlOOH nearly coincides with the normal mantle geotherm.With the thermal parameters obtained by fitting thermal EoS,we modeled the density(p)and bulk sound velocity(V?)as a function of pressure along the subduction slab geotherm.Comparing with other main minerals in the lower mantle,?-AlOOH characterizes with the samllest p and p/V?,fastest V? which is a critical constraint for detecting ?-AlOOH in the lower mantle by seismic methods.The process that phase Egg decomposes into an aggregate of ?-AlOOH and stishovite in the cold,hot slabs conduce to increment in ? of?2.1%,?2.5%and in V? of?19.7%,?20.4%,respectively.Supposing that ?-AlOOH is carried to CMB,it decomposes due to the dramatic temperature increment and release water which may trigger partial melting in the surrounding rocks.Furthermore,water can react with iron from the outer core and form FeO2Hx which is has slow velocities.In these two studies,we derived the thermoelastic properties of one of the aluminus bearing phase in the upper mantle and aluminus,hydrogen bearing phase in the subducting slab.Elas-ticity determination of MgAl2O4-spinel under high P-T provide a significant reference model for researches of the upper mantle via seismological techniques.While,?-AlOOH offer us an excellent carrier to transport water into the deep earth and an significant key to decipher the the super low velocity anomalyies above the CMB.