Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p44a..07l&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P44A-07
Physics
[8124] Tectonophysics / Earth'S Interior: Composition And State, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [8125] Tectonophysics / Evolution Of The Earth, [8147] Tectonophysics / Planetary Interiors
Scientific paper
The boundary between Earth’s crystalline rock mantle and molten iron alloy core at a pressure of 135 GPa and temperature around 4000 K separates thermo-chemical boundary layers that have profound influence on thermal evolution and dynamics of the planet. The compositional change causes the largest density contrast in the interior, and the transition from solid to molten material produces the largest internal viscosity contrast. These conditions favor accumulation of density stratification on either side of the boundary. Seismological evidence indicates distinctive elastic wave properties in both boundary layers. On the mantle side, large-scale chemical heterogeneities appear to be accumulated in large piles and there is evidence for the presence of a melt component in the basal boundary layer which is likely to have distinct chemistry. On the core side, slight reduction of the P-wave velocity relative to a homogeneous medium suggests compositional stratification of the outermost outer core. These compositional anomalies probably represent long histories of chemical differentiation. Similar attributes may hold for other planetary interiors. The specific P-T conditions in the Earth fortuitously position a phase transition for the lower mantle’s primary mineral (Mg,Fe)SiO3 magnesium silicate perovskite within the lowermost mantle. The proximity of this phase change to the core-mantle boundary and the particular properties of the high-pressure post-perovskite phase appear to contribute to the observed seismological structure of the lowermost mantle and likely augment the dynamical instability of the thermal boundary layer. Experimental calibration of the phase change and seismological detection of the phase boundary enables a deep mantle thermometer, providing unprecedented bounds on the thermal structure. The phase boundary appears to modulate in depth laterally and even to undergo reversal in a steep thermal gradient above the core-mantle boundary, allowing estimates of temperature structure and heat flow to be made. The proximity of a major phase change to the core-mantle boundary may be a unique attribute of Earth.
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