Composition and State of the Core

Hirose, Kei; Labrosse, S.; Hernlund, J. W.

doi:10.1146/annurev-earth-050212-124007

Cited by 285 publications

(264 citation statements)

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“…Figure 3 shows predicted inner core age and CMB temperature (T 3.5Ga ) and CMB heat flow (Q 3.5Ga ) at 3.5 Ga, the time of the earliest paleomagnetic measurement 1 . The influence of radiogenic heating is demonstrated by adding 300 ppm of 40 K at the present day, which likely represents an extreme scenario 44,63 . The shaded temperature range of 4150 ± 150 K corresponds to present estimates of the lower mantle solidus temperature 67 ; core temperatures exceeding this range suggest partial melting in past.…”

Section: Geophysical Implications Of Revised Core Propertiesmentioning

confidence: 96%

“…however, recent experiments found that adding up to 10% of Ni does not change the hexagonal close-packed crystal structure of the solid 42 , while ab initio calculations suggest that at high T the seismic properties of Fe-Ni alloys are almost indistinguishable from those of pure iron 43 . Recent studies of core composition [44][45][46] given in section 1 of Table 1; each model is named after the corresponding molar concentration.…”

Section: Materials Properties For Earth's Corementioning

confidence: 99%

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Constraints from material properties on the dynamics and evolution of Earth’s core

et al. 2015

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The Earth's magnetic field is powered from energy supplied by slow cooling and freezing of the liquid iron core. Core thermal history calculations have been hindered in the past by poor knowledge of the properties of iron alloys at the extreme pressures and temperatures pertaining in the core. This obstacle is now being overcome by developments in high pressure experiments and computational mineral physics. Here we review the relevant properties of iron alloys at core conditions and discuss their uncertainty and geophysical implications.Powerful constraints on core evolution are now possible, due partly to recent factor 2-3 upward revisions of the all-important electrical and thermal conductivities. This has dramatic implications for the thermal history of the entire Earth, not just the core: the inner core is very young, the core is cooling quickly, and was so hot in the past that the lowermost mantle

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Section: Geophysical Implications Of Revised Core Propertiesmentioning

confidence: 96%

Section: Materials Properties For Earth's Corementioning

confidence: 99%

Constraints from material properties on the dynamics and evolution of Earth’s core

et al. 2015

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“…Unfortunately, large discrepancies exist between the different studies on the topic Cao and Romanowicz, 2004;Koper and Dombrovskaya, 2005;Hirose et al, 2013, for a review) and the uncertainty in this key parameter directly affects the compositional energy. In this paper, I choose to use the value from Masters and Gubbins (2003) (see table 2) because it derives from a normal mode analysis and is therefore more likely to represent a global estimate, compared to wave reflection studies that can be influenced by the local interface state at the ICB.…”

Section: Parametermentioning

confidence: 99%

Thermal evolution of the core with a high thermal conductivity

Labrosse

2015

Physics of the Earth and Planetary Interiors

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The rate at which heat is extracted across the core mantle boundary (CMB) is constrained by the requirement of dynamo action in the core. This constraint can be computed explicitly using the entropy balance of the core and depends on the thermal conductivity, whose value has been revised upwardly. A high order model (fourth degree polynomial of the radial position) for the core structure is derived and the implications for the core cooling rate and thermal evolution obtained, using the recent values of the thermal conductivity. For a thermal conductivity increasing with depth as proposed by some of these recent studies, a CMB heat flow equal to the isentropic value (13.25TW at present) leads to a 700 km thick layer at the top of the core where a downward convective heat flow is necessary to maintain an isentropic and well mixed average state. Considering a CMB heat flow larger than the well mixed isentropic value leads to an inner core less than 700 Myr old and the thermal evolution of the core is largely constrained by the conditions for dynamo action without an inner core. Analytical calculations for that period show that a CMB temperature larger than 7000 K must have prevailed 4.5 Gyr ago if the geodynamo has been driven by thermal convection for that whole time. This raises questions regarding the onset of the geodynamo and its continuous operation for the last 3.5 Gyr. Implications regarding the evolution of a basal magma ocean are also considered.

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“…A recent (upward) revision in the rate of inner-core growth (e.g. Hirose et al, 2013) lowers the required viscosity, but not by more than a factor of four. Consequently, the viscosity needed to produce crystal alignment may be too large to favor viscous flow as the primary relaxation mechanism.…”

Section: Comparison With Viscous Relaxation Timementioning

confidence: 99%

The fluid dynamics of inner-core growth

Buffett

Matsui

2015

Physics of the Earth and Planetary Interiors

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a b s t r a c tAspherical growth of the inner core has been suggested as a mechanism to produce seismic anisotropy through alignment of crystal lattices. This mechanism is viable if the response to aspherical growth occurs by slow viscous deformation. The inner core can also respond by melting and solidification at the boundary if flow in the liquid core can redistribute latent heat over the surface. We use a numerical geodynamo model to quantitatively assess the process of melting and solidification, and find that the response to aspherical growth occurs primarily through phase transitions when the viscosity of the inner core is 10 21 Pa s or higher. A lower inner-core viscosity favors viscous adjustment, but the associated stresses may be too low to produce substantial crystal alignment. Independent of the primary relaxation mechanism, we expect a persistent and large-scale flow of the liquid core over the surface of the inner core. The predicted flow should be large enough to affect the crystal orientation of hcp-iron alloys during solidification, yet the absence of detectable seismic anisotropy in the top 60-80 km is suggestive. Either the mechanism of flow-induced alignment does not apply in the core or the intrinsic anisotropy of hcp iron at inner-core conditions is weak. Future seismological modeling using the predicted distribution of lattice preferred orientation might establish whether this texture is detectable with current observations.

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Composition and State of the Core

Cited by 285 publications

References 204 publications

Constraints from material properties on the dynamics and evolution of Earth’s core

Constraints from material properties on the dynamics and evolution of Earth’s core

Thermal evolution of the core with a high thermal conductivity

The fluid dynamics of inner-core growth

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