Core formation in terrestrial planets

Shaw, George H.

doi:10.1016/0031-9201(79)90106-7

Cited by 13 publications

(3 citation statements)

References 9 publications

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“…This melting could have resulted from the accretion of the Earth [Kaula, 1979;Abe and Matsui, 1986;Matsui and Abe, 1986;Zahnle et al, 1988;Ahrens, 1990;Agee, 1990], core formation [Birch, 1965;Solomon, 1978;Shaw, 1979], the impact formation of the moon [Bem et al, 1986[Bem et al, , 1987 Cam-MILLBR BT AL. : KOMATIITE PBTROGENESIS 11,855 eron and Benz, 1989;Stevenson, 1989], or from some combination of these mechanisms.…”

Section: Appucation To a Magma Ocean And The Evolution Of The Hadean mentioning

confidence: 99%

The equation of state of a molten komatiite: 2. Application to komatiite petrogenesis and the Hadean Mantle

Miller¹,

Stolper²,

Ahrens³

1991

J. Geophys. Res.

129

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New experimental data for the equation of state of a komatiitic liquid were used to model adiabatic melting in a peridotitic mantle. If komatiites are formed by > 30% partial melting of a peridotitic mantle, then komatiites generated by adiabatic melting come from source regions that began their unmelted ascent in the lower transition zone (:::500-670 km) or the lower mantle (>670 km). The great depth of incipient melting implied by this model suggests that komatiitic liquids may form in a pressure regime where they are denser than their coexisting crystals, possibly the bulk mantle. Although komatiitic magmas are thought to separate from residual crystals in the mantle at a temperature :::200"C greater than modem mid-()cean ridge basalts (MORBs), their ultimate sources are predicted to be diapirs that, if adiabatically decompressed from initially solid mantle, were more than 70Cf'C hotter than the sources of MORBs and were derived from great depth. We also studied the evolution of an initially molten mantle, i.e., a magma ocean. Our model considers the thermal structure of the magma ocean, density constraints on crystal segregation, and approximate phase relationships for a nominally chondritic mantle. Crystallization will begin at the core-mantle boundary. Perovskite buoyancy at >70 GPa may lead to a compositionally stratified lower mantle with iron-enriched magnesiowiistite content increasing with depth. Large convective velocities in the magma ocean would prohibit crystal-liquid fractionation by settling or flotation until quiescent boundary layers form. Such boundary layers could form when the crystal content of the magma reaches a critical value (near 44 vol %) and, in the late stages of crystallization, around unmelted blocks of foundered protocrust. Matrix compaction and diapirism could also lead to fractionation effects. The upper mantle could be depleted or enriched in perovskite components relative to the bulk mantle. Olivine neutral buoyancy may lead to the formation of a dunite septum in the upper mantle, partitioning the ocean into upper and lower reservoirs, but this septum must be permeable.

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Section: Appucation To a Magma Ocean And The Evolution Of The Hadean mentioning

confidence: 99%

The equation of state of a molten komatiite: 2. Application to komatiite petrogenesis and the Hadean Mantle

Miller¹,

Stolper²,

Ahrens³

1991

J. Geophys. Res.

129

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“…The details of the core-forming process depend in part on the amount of gravitational energy available. This energy is very small for a moon-sized body but already substantial (equivalent to a 500-1000 K temperature rise) for a Mercury-or Mars-sized body (Shaw 1979). It is important to stress, however, that provided the accretional energy retention is substantial, it will be the accretional time scale that dictates the core-forming time scale .…”

Section: H2mentioning

confidence: 99%

Planetary magnetic fields

1983

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As a consequence of the smallness of the electronic fine structure constant, the characteristic time scale for the free diffusive decay of a magnetic field in a planetary core is much less than the age of the Solar System, but the characteristic time scale for thermal diffusion is greater than the age of the Solar System. Consequently, primordial fields and permanent magnetism are small a;d the only means of providing a substantial planetary magnetic field is the dynamo process. This requires a large region which is fluid, electrically conducting and maintained in a non-uniform motion that includes a substantial RMS vertical component. The attributes of fluidity and conductivity are readily provided in the deep interiors of all planets and most satellites, either in the form of an Fe alloy with a low eutectic temperature (e.g. Fe-S-0 in terrestrial bodies and satellites) or by the occupation of conduction states in fluid hydrogen or 'ice' (H20-NH3-CH4) in giant planets. It is argued that planetary dynamos are almost certainly maintained by convection (compositional and/or thermal). If alternative mechanisms such as precessional torques work at all, they only work when they are not needed (i.e. when the core is neutrally or unstably stratified because of other larger energy sources). For any plausible convective vigour, it is possible to satisfy the sufficient conditions of dynamo onset (large magnetic Reynolds number, small Rossby number) for every planet and satellite. Estimates of convective vigour are obtained from estimates of likely energy fluxes and a consideration of the form of convective motions in a rotating fluid sphere. The reason that some planets and probably all satellites do not have dynamos is because the fluid regions of their cores are stably stratified and do not convect. Thermal evolution models indicate that any terrestrial body with an entirely fluid (iron alloy) core becomes stably stratified over geologic time and loses heat by conduction only at the present time. The core energy flux is then totally unavailable for dynamo generation. However, terrestrial bodies that nucleate an inner solid core (e.g. Earth) can usually continue to sustain a dynamo because of the resulting gravitational energy release and compositional buoyancy of convective motions. In contrast, giant planets can easily sustain a dynamo by gradual cooling alone.The field amplitudes of planetary dynamos are poorly understood. Existing attempts at 'scaling laws' are naive because they deny the diversity of planets, and poorly constrained because several 'laws' perform equally well. Nevertheless, it is possible to assess the likely presence or absence of a dynamo in each planet and satellite, primarily by an analysis of energetic considerations. An interpretation of each Solar-System body is offered and some testable predictions are given.

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“…The starting-point for the considerations is a fully differentiated planet. The energy release due to core formation contributes to the initial thermal state (for instance Shaw, 1979; for thermal profiles of Earth and Mars during and after the accretion see Coradini et al, 1983), so that it is justified to accept a hot thermal beginning. That means that the whole mantle is at Ehe solidus and therefore the core is superliquidus.…”

Section: Introductionmentioning

confidence: 99%

A new view of martian evolution

Franck

Orgzall

1987

Earth Moon Planet

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Conventional evolutionary models for Mars adopt a dry mantle solidus. Taking into account the condensation conditions in the preplanetary nebula in the accretion zone of Mars, it can be concluded that large amounts of water or hydrated silicates have condensed in those regions. Therefore, water influences significantly the melting behaviour and the viscosity of the silicatic material. A model for the calculation of the thermal history of a planet is constructed. On this basis, and use of water -saturated solidus -it is possible to derive that the core is not liquid, as given in models employing a dry mantle solidus, but solid to a large extent, which prevents the operation of a large-scale dynamo and explains in that way the lack of a magnetic field. With these assumptions one can construct a possible evolutionary scheme that covers earfy crust differentiation, a hot thermal past and the missing magnetic field at present.

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Core formation in terrestrial planets

Cited by 13 publications

References 9 publications

The equation of state of a molten komatiite: 2. Application to komatiite petrogenesis and the Hadean Mantle

The equation of state of a molten komatiite: 2. Application to komatiite petrogenesis and the Hadean Mantle

Planetary magnetic fields

A new view of martian evolution

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