Outer core density heterogeneity and the discrepancy between PKP and PcP travel time observations
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Abstract:
We derive 3‐D tomographic maps of the Earth's mantle, CMB and outer core, from seismic P, PcP, PKPbc, PKPdf travel time data, based on the bulletins of the International Seismological Centre (1964–1995), after source relocation by Antolik et al. [2001] and phase re‐identification by Engdahl et al. [1998] . Maps of the CMB derived independently from core‐reflected (PcP) or core‐refracted (PKP) phases are not well correlated. We attempt to explain this discrepancy, and study the radial coherence of whole‐Earth tomographic images, to identify possible trade‐offs between CMB undulations and velocity anomalies in the mantle or outer core. Imaged velocity anomalies in the lowermost mantle are anticorrelated with the topography of the CMB; likewise, imaged lateral heterogeneities in the outer core are correlated with the topography of the CMB. This, together with the study of Piersanti et al. [2001] , suggests that the core anomalies might not be entirely fictitious.Keywords:
Outer core
Core–mantle boundary
Seismic Tomography
Summary. The phase diagram of iron up to 330 GPa is solved using the experimental data of static high pressure (up to 11 GPa) and the experimental data of shock wave data (up to 250 GPa). A solution for the highest triple point is found (P= 280 GPa and T= 5760 K) by imposing the thermodynamic constraints of triple points. This pressure of the triple point is less than the pressure of the inner core–outer core boundary of the Earth. These results indicate that the density of iron at the inner core–outer core boundary pressure is close to 13 g cm−3, which lies close to the seismic solutions of the Earth at that pressure. It is thus concluded that the Earth's inner core is very likely to be virtually pure iron in its hexagonal close packed (hcp) phase.
It is shown that four properties of the Earth's inner core determined from seismology are close in value to the corresponding properties of hcp iron at inner core conditions: density, bulk modulus, longitudinal velocity, and Poisson's ratio. The density–pressure profile of hcp iron at inner core conditions matches the density–pressure profile of the inner core as determined by seismic methods, within the spread of values given by recent seismic models.
This indicates that the Earth is slowly cooling, the Earth's inner core is growing by crystallization, and the impurities of the core are concentrated in the outer core. The calculated temperature at the Earth's centre is 6450 K.
Outer core
Core–mantle boundary
Triple point
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The phase diagram of iron up to 330 GPa is solved using the experimental data of static high pressure (up to 11 GPa) and the experimental data of shock wave data (up to 250 GPa). A solution for the highest triple point is found (P= 280 GPa and T= 5760 K) by imposing the thermodynamic constraints of triple points. This pressure of the triple point is less than the pressure of the inner core—outer core boundary of the Earth. These results indicate that the density of iron at the inner core—outer core boundary pressure is close to 13 g cm−3, which lies close to the seismic solutions of the Earth at that pressure. It is thus concluded that the Earth's inner core is very likely to be virtually pure iron in its hexagonal close packed (hcp) phase. It is shown that four properties of the Earth's inner core determined from seismology are close in value to the corresponding properties of hcp iron at inner core conditions: density, bulk modulus, longitudinal velocity, and Poisson's ratio. The density—pressure profile of hcp iron at inner core conditions matches the density—pressure profile of the inner core as determined by seismic methods, within the spread of values given by recent seismic models. This indicates that the Earth is slowly cooling, the Earth's inner core is growing by crystallization, and the impurities of the core are concentrated in the outer core. The calculated temperature at the Earth's centre is 6450 K.
Outer core
Core–mantle boundary
Triple point
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Core–mantle boundary
Outer core
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The aim of this paper is to give an overview of important historical results and review current knowledge of the Earth's core, as well as to discuss prospects for seismological studies of the core. Although the properties of the core of the Earth can only be determined indirectly, there has been considerable progress in elucidating its structure. The iron-rich core is dense but has lower P-wave speed than the mantle above; the solid inner core has fewer light constituents than the fluid outer core. The density contrast at the inner-core boundary is too large for just a phase transition. The fluid outer core is well stirred by the convective flows associated with the generation of the geodynamo and is expected to have a nearly adiabatic profile. Only inside the tangent cylinder defined by the presence of the inner core might there be some seismic heterogeneity in the bulk of the outer core. Some variability along the underside of the core – mantle boundary due to selective separation of lighter material is suggested by some observations. By comparison, the inner core is rather complex with heterogeneous and anisotropic structures that appear to have hemispherical differences. Significant attenuation occurs just below the inner-core boundary, probably due to a mushy zone associated with the growth of the inner core. A variety of seismic observations help to define inner-core structures, but it is important to take account of the influence of the complex structure at the base of the mantle. A slightly different zone has been suggested around the centre of the Earth, although it is difficult to get good control on this region.
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