| Seismology observation is an important approach of exploring the internal properties of the earth.The information of the density,sound velocity and phase transition of minerals given by high pressure and temperature experiments is of great improtance for us to understand the velocity density changes and anomalies of the mantle as well.In this paper,we use laser-heated diamond anvil cells together with synchrotron X-ray diffraction technique to explore the phase boundary and equations of state is some major minerals in the lower mantle subducted oceanic ridge basalt.We first measured the thermal equations of state of CaSiO3 perovskite,which is the third most abundant mineral in the lower mantle.Combining the literature results of bridgmanite and ferropericlase,we concluded that the Earth's lower mantle should be in pyrolite composition.Then,by studying the phase transition and equations of state of the Fe-rich bridgmanite,we concluded that pyrolitic lowermost mantle should be highly heterogeneous in composition and temperature laterally to match the observed variations in the depth and seismic signatures of the D" discontinuity.Finally,we explore the properites and phase stabilities of SiO2 under lower mantle station.Our results provide better constraints for the thermodynamic equations of the various mantles of the lower mantle,as well as for the lower mantle.Research on composition and speed anomalies provides better support.CaSiO3 perovskite is the third most abundant mineral in Earth's lower mantle.In the subducted oceanic ridge basalt,the content of CaSiO3 perovskite could reach as high as 22-25%.In this thesis,complimentary synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell were conducted up to 156 GPa between 1200 K and 2600 K to provide more reliable constraints on the thermal EoS parameters of CaSiO3 perovskite.We further re-calibrate the pressures of bridgmanite and ferropericlase in different studies.We have modeled ?,V?,and dlnp/dln V?profiles of the lower mantle with representative pyrolitic and chondritic compositional models in which the effect of Fe spin transition in ferropericlase is also taken into account.Our modeling results show that a pyrolitic lower mantle with an aggregate mineralogy of 75 vol.%bridgmanite,17 vol.%ferropericlase,and 8 vol.%CaSiO3-perovskite produces ? and V? profiles in better agreement with PREM than a lower mantle with a chondritic composition.The modeled ?,V?,and dlnp/dln V? are mainly affected by the relative ratio of bridgmanite and ferropericlase but are not sensitive to the variation of the CaSiO3-perovskite content.In addition,the spin crossover of Fe in ferropericlase can greatly raise the value of dlnp/dln V? in the mid-lower mantle,which is useful to detect the presence of ferropericlase in the region.We further explore the stability of both tetragonal and cubic structure CaSiO3-perovskite up to 203 GPa.Meanwhile,we have redefined the structure of the tetragonal CaSiO3-perovskite,which is important for us to understand the formation condition of diamond inclusions containing CaSiO3-perovskite.We further determined the phase boundary between Mg0.735Fe0.2iAl0.07Si0.965O3-Bm and PPv and the thermal equations of state of both phases up to 202 GPa and 2600 K using synchrotron X-ray diffraction in laser heated diamond anvil cells.Our experimental results have shown that the combined effect of Fe and Al produces a wide two-phase coexistence region with a thickness of 26 GPa(410 km)at 2200 K,and addition of Fe lowers the onset transition pressure to 98 GPa at 2000 K,consistent with previous experimental results.Furthermore,addition of Fe was noted to reduce the density(?)and bulk sound velocity(V?)contrasts across the Bm-PPv phase transition,which is in contrast to the effect of Al.Using the obtained phase diagram and thermal equations of state of Bn and PPv,we have also examined the effect of composition variations on the ? and V? profiles of the lowermost mantle.Our modeling results have shown that the pyrolitic lowermost mantle should be highly heterogeneous in composition and temperature laterally to match the observed variations in the depth and seismic signatures of the D" discontinuity.Normal mantle in a pyrolitic composition with?10%Fe and Al in Bm and PPv will lack clear seismic signature of the D" discontinuity because the broad phase boundary could smooth the velocity contrast between Bm and PPv.On the other hand,Fe-enriched regions close to the cold slabs may show a seismic signature with a change in the velocity slope of the D" discontinuity,consistent with recent seismic observations beneath the eastern Alaska.Only regions depleted in Fe and Al near the cold slabs would show a sharp change in velocity.Fe in such regions could be removed to the outer core by strong core-mantle interactions or partitions together with Al to the high-pressure phases in the subduction mid ocean ridge basalts.Our results thus have profound implication for the composition of the lowermost mantle.We last investigated the properties and phase diagram of SiO2,which is the major component of subduction slabs in lower mantle.The phase transition between the CaCl2-type SiO2 and a-PbO2-type SiO2 and their thermal equations of state were measured from 55 to 147 GPa up to 3500 K using X-ray diffraction in laser-heated diamond anvil cells.Together with previous experimental results,?-PbO2-type SiO2 which coexists with the CaCl2-type SiO2 at 108-130 GPa upon laser heating above 2000 K should be metastable.A single-phase ?-PbO2-type SiO2 can be synthesized from both silica and the CaCl2-type SiO2 at 132-136 GPa and 1400 K.The phase boundary between the CaCl2-type SiO2 and ?-PbO2-type SiO2 was determined to be at 132 GPa and 2000 K with a Clapeyron slope of 6.8 MPa/K.In the pure-SiO2 system and geological time scale,CaCl2-type structure should be the dominant SiO2 phase in the lower mantle,and seifertite is unlikely to be present as a free silica phase unless along a super-cold slab geotherm.Using the obtained thermal equations of state,we have found that the phase transition from stishovite to the CaCl2-type SiO2 does not cause noticeable change in density but lowers the bulk sound velocity(V?)by?10%.The modeled V? profiles of SiO2 indicated that the transition in the recycled oceanic crust can cause ??2%discontinuity in F?,which could be related to the observed seismic velocity anomalies at depths of 1400-1800 km.With high pressure and temperature results,we modelled the density and bulk sound velocity of different compositions along different mantle geotherms.Combined with the results of seismology,we provide a possible explanation for the existence of velocity density anomalies and global components in the lower mantle in this thesis. |
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