Thermal regime in the earth's core and lower mantle

Current views favour the presence of sulphur in the core, giving a composition of Fe + FeS. It is argued that the core composition is close to the eutectic and that this eutectic composition is Fe₂S. The consequences for the thermal regime in the core are examined in terms of the most likely properties of the Fe₂S eutectic. This leads to much lower temperatures than would be expected for an iron or FeSi core.Consideration of the thermal regime in the mantle and the probable thermal properties of lower-mantle assemblages leads to a similar low temperature for the core-mantle boundary. These temperatures require a temperature gradient near the adiabatic in the mantle, implying a convective thermal history.     

Noble gases in the earth's mantle

The abundances and isotopic compositions of noble gases in two samples from ultramafic xenoliths in alkali basalt, a young kaersutitic amphibole separated from a peridotite xenolith from Dish Hill, California and an ancient whole-rock lherzolite xenolith from Baja California, are reported and compared with the results of analyses on other mantle samples. In addition to previously recognized excesses of ³He and ¹²⁹Xe, our results indicate that ambient gases in the mantle show a general enrichment of the lighter-mass nonradiogenic isotopes of Ar, Kr and Xe, and Ar with ⁴⁰Ar/³⁶Ar = 3 · 10².     

A note on convection in the earth's mantle

In the present note a boundary-layer model of thermal convection throughout the mantle is outlined. It is shown that recent criticisms of mantle-wide convection by A.E. Ringwood do not apply to this model. The phase transitions discussed by Ringwood are consistent with the model, and in fact provide an additional driving force for the convective motion. It is further noted that the model offers explanations of the core-mantle coupling hypothesized by R. Hide from consideration of correlations between the earth's magnetic and gravity fields, and of the appearance in several parts of the world of pairs of trenches separated by distances of the order of 2000 km.     

Structural superplastic creep and linear viscosity in the earth's mantle

The rheology of dry polycrystalline olivine is examined by adopting a hyperbolic sine flow law (which reduces to a power law below 3 kbars) for high stress behavior, and a model for diffusion accommodated, coherent, grain boundary sliding (structural superplastic creep) for low stress behavior. The model for superplastic creep gives a linear relation between stress and strain rate and is consistent with the behavior of polycrystalline olivine during ductile faulting experiments (Post, 1973). For any given stable grain size, linear superplastic creep is promoted by relatively low stress and temperature. For a 1 -cm grain size and a homologous temperature between 0.6 and 0.8, superplastic creep dominates below transition stresses between 402 and 25 bars, respectively. Transition stresses are higher for smaller grain size and lower temperature. If grain size is stress dependent, superplastic creep is non-linear and dominates above a stress of 300 bars. Below that stress, relatively lower temperatures promote superplastic creep. Grain size may be stabilized by either physical or kinetic inhibition of grain growth, thereby allowing linear superplastic creep in the mantle. Results suggest that superplastic creep can dominate in most of the upper mantle except possibly for the asthenosphere where homologous temperatures are maximal and hyperbolic sine law creep can dominate. Mantle diapirism is at least in part accomplished by superplastic flow above and along the margins of the rising diapir.     

Pressure dependence of the thermal Grüneisen parameter,with application to the earth's lower mantle and outer core

By considering high-temperature (classical) thermal oscillations of atoms in certain simple crystal structures with purely central interatomic forces, the treatment of anharmonic oscillations is generalised to random three-dimensional motion, yielding the Vashchenko and Zubarev relationship for the Grüneisen ratio γ at any pressure. If one-dimensional atomic oscillations only are considered the equation reduces to the Dugdale-MacDonald expression. To account for non-central forces additional terms must be introduced, giving:

$\text{γ=}\text{1}\text{2}\text{d}\text{K}\text{d}\text{P}\text{?}\text{5}\text{6}\text{+}\text{2}\text{9}\text{P}\text{K}\text{?}\text{f}\text{18K}\text{+}\text{1}\text{6}\text{d}\text{f}\text{d}\text{P}\text{1?}\text{4}\text{3}\text{P}\text{K}\text{+}\text{f}\text{3K}$

where f = 0 for purely central forces. Calculations of f in terms of the Poisson ratio for different crystal structures have not been made, but for many materials the central-force approximation suffices. This is believed to be true both for the outer core $\text{(}\text{γ}\text{≈1.4)}$ and for the close-packed structures of the lower mantle $\text{(}\text{γ}\text{≈1.0)}$. For the upper mantle non-central atomic forces are important and we have no estimate of ($\text{γ}$ independently of laboratory values for plausible minerals which suggest γ ≈ 0.8.     

The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO₃)-pyrope (3 MgSiO₃ · Al₂O₃) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO₃ and 95% MgSiO₃ · 5% Al₂O₃ are especially remarkable. The phase assemblages β-Mg₂SiO₄ + stishovite and γ-Mg₂SiO₄ (spinel) + stishovite displayed by MgSiO₃ were not observed in 95% MgSiO₃ · 5% Al₂O₃, and the garnet phase, which was observed in 95% MgSiO₃ · 5% Al₂O₃ at high pressure, was not detected in MgSiO₃. These results suggest that the high-pressure phase transformations found in pure MgSiO₃ would be inhibited under mantle conditions by the presence even of small amounts of Al₂O₃ (?4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO₃ · 10% Al₂O₃ and Mg₂SiO₄. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg₂SiO₄, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO₃ · 10% Al₂O₃) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg₂SiO₄ and garnet → ilmenite in 90% MgSiO₃ · 10% Al₂O₃. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO₃ · 10% Al₂O₃ is expected.     

Earth's magnetic field modelling and earth's structure and evolution

The advantages of the approximation of the Earth's magnetic field by means of the field of the so-called natural magnetic sources are discussed. The shifting of these natural magnetic sources, determined for different epochs, is used to forecast the Earth's magnetic field and to draw conclusions about the motion of the corresponding part of the Earth. On the basis of the representation of the Earth's magnetic field from several past geological epochs as a field of one optimum dipole a new theory about the Earth's evolution is proposed.     

Oceanic area's influence on the range of multi-annual variations of the earth's magnetic field in France

The pluri-annual variations of the earth's magnetic field in France increase their range of influence from East to West. This leads to presume a discontinuity of electric conductivities at the transition from the continental to the oceanic area.     

The mechanical and electrical properties of earth's asthenosphere

An adequate theory of continental drift can be based on heat transfer theory, but it does demand the acceptance of a large downward revision of traditional estimates of average upper mantle temperatures and a consistent understanding of lithosphere and asthenosphere in terms of a difference in rheological behaviour under prolonged non-hydrostatic stress. The recognition that an extremely viscous average state of the upper mantle is self regulating both requires and permits an explanation of magma generation at a strictly limited rate (when averaged for the whole planet over a few years) in terms of unsteady and local deformational heating.The activity of water as a reducer of silicate creep resistance is used to develop the hypothesis that water produced by an amphibole dehydration has been effectively trapped in the Earth and is the underlying cause of a low seismic Q ～ 50 and an electrical conductivity 10^?2 ?10^?1 ohm^?1 m^?1, at depths of ～ 100 km. At the predicted low horizontally-avera temperatures, the conductivity contrast of rock and aqueous solutions is very large, and mantle electrical conductivity studies now look best-suited to explore this trapping process, and the distinctly recognisable possibility that the uptake of ocean water in the subduction process exceeds the rate of loss that can be explained purely through magmatic activity.     

Early Archaean rocks and geochemical evolution of the earth's crust

Remnants of Early Archaean rocks (>FX3000 m.y. old) are reported from most continents. A critical review of the radiometric data shows that few of these are well authenticated and most are very limited in extent. The oldest are predominantly plutonic gneisses of tonalitic-to-granitic composition (e.g., the basement gneisses of West Greenland, Labrador, Rhodesia and South Africa). In all cases there are inclusions of meta-volcanic and sedimentary rocks with greenstone belt affinities which probably represent crust into which the igneous parents of the gneisses were intruded.The trace element chemistry of these very old rocks is reviewed in an attempt to establish the mechanism of formation of early crust and place constraints on the chemical evolution of the earth's mantle. “Mantle-type” Sr isotope compositions show that the sialic members of both early gneisses and greenstone belts were not derived from much older crustal differentiates, either at 3800 or at 2800 m.y. ago. However, trace element ratios such as K/Rb and Sr/Ba, and rare earth element abundances, are not consistent with direct derivation of the plutonic suite from the upper mantle and also rule out a common parentage for the tonalites and granites. An origin by partial melting of metamorphosed juvenile crust with a composition range equivalent to that represented by the greenstone belts is preferred. Tonalites resulted from high-pressure melting of mafic garnet-amphibolite and at least some of the granites from low-pressure melting of more felsic (possibly even sedimentary) material.The trace element chemistry of the greenstone belt volcanics is thought to characterize the composition of early mantle melts, although the best preserved and best documented cases are about 500–1000 m.y. younger than the oldest known gneisses. The dominant type is tholeiite with low incompatible element contents and light-depleted or essentially flat rare earth patterns, features even more marked in the ultramafic komatiites which represent large degrees of melting. More evolved calc-alkaline rocks with relative incompatible and light rare earth element enrichment are also important. With the exception of the ultramafic lavas, all these types can be matched by the chemistry of present-day oceanic volcanism.It is concluded that the range of trace element variations in the earth's mantle was comparable in early Archaean times to that at the present. This is supported by mass balance calculations for the lithophile elements which have been preferentially extracted into the crust. Thus the isotope and trace element evidence of the oldest rocks argues against primary differentiation of the crust either during accretion of the earth or during its first 500 m.y. as a solid body. Crust formation has probably occurred continuously, although worldwide evidence for magmatism at around 2800 m.y. ago probably marks a particularly active period.     

Latent-heat driven convection in the earth's core: Numerical results

The paper presents a numerical model of a slowly cooling Earth's core. On the boundary conditions selected, cooling alone is too slow to effect convection. Convective motions arise only by the additional release of latent heat of crystallization owing to the growth of the inner core. A fundamental feature of the model is the choice of a subadiabatic initial temperature distribution.This is the reason why the outer core acts as a heat reservoir, that slows down the growing rate of the inner core on an acceptable size. For the whole time convection covers only the lower part of the outer core, the upper part remains stably stratified.     

DC resistivity methods for determining resistivity in the earth's crust

The use of Schlumberger and dipole arrays for crustal-scale resistivity soundings is considered. Advantages and disadvantages of the two methods are described. The depth to which resistivity may be determined from field measurements is discussed as well as the determination from the sounding curves of various parameters associated with layered structure. The interpretation of experimental data using reference curves as well as two approaches used in computer assisted interpretation are discussed.     

P-wave velocities in the earth's core: An interim report

Recent observations of core phases made at conventional stations and seismograph arrays are summarized and evaluated to produce a series of conclusions concerning the P-wave velocity structure of the earth's core. Limits are suggested for allowable variations in P velocity in various parts of the core. The prime conclusion is that observations that previously demanded velocity discontinuities in the lower part of the outer core may now be explained adequately on a scattering hypothesis, and that in models where parametric simplicity is desired, the earth's core may be approximated by a two-layered model, with the P-wave velocity varying continuously in each layer.     

Gas-mineral equilibria and a possible geochemical model of the earth's interior

The study of the thermodynamic regime of metamorphism and magmatism has been based on mineralogical thermometry and barometry and calculations of the oxidation-reduction, hydration-dehydration and carbonatization-decarbonatization reactions. The origin of the fluids is considered in connection with the hydride-carbide-oxide structure of the earth's interior.     

Behaviour of the earth's magnetic field as recorded in the sediment of Loch Lomond

A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from¹⁴C analyses on the Lomond sediment and comparison with other¹⁴C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations.The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced.The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole.     

On the activation volume for creep and its variation with depth in the Earth's lower mantle

This activation volume ΔV for creep may be derived from Keyes's elastic strain energy model or from Weertman's empirical relationship between viscosity and the melting temperature. These formulations are shown to be equivalent if the anharmonic Grüneisen parameters γ of all acoustic modes are equal and if the pressure dependence of the melting temperature follows Lindemann's law, both of which assumptions are valid for the close-packed mineral structure of the lower mantle. The pressure derivative of ΔV depends only on the bulk modulus and the acoustic γ, both of which are directly available from seismic models. Using the data of Brown and Shankland, we show that ΔV decreases by almost 50% between the top and the bottom of the lower mantle, which makes it easier to maintain a constant viscosity in this region. The isoviscous temperature profile can be adiabatic in the deep lower mantle only below 1700 km depth; it is super-adiabatic in the top 1000 km of the lower mantle.     

The thermodynamic properties of the earth's lower mantle

The thermodynamic properties of the lower mantle are determined from the seismic profile, where the primary thermodynamic variables are the bulk modulus K and density ρ. It is shown that the Bullen law (K ∝ P) holds in the lower mantle with a high correlation coefficient for the seismic parametric Earth model (PEM). Using this law produces no ambiguity or trade-off between ρ₀ and K₀, since both K₀ and K′₀ are exactly determined by applying a linear K?ρ relationship to the data. On the other hand, extrapolating the velocity data to zero pressure using a Birch-Murnaghan equation of state (EOS) results in an ambiguous answer because there are three unknown adjustable parameters (ρ₀, K₀, K′₀) in the EOS.From the PEM data, K = 232.4 + 3.19 P (GPa). The PEM yields a hot uncompressed density of 3.999 ± 0.0026 g cm^?3 for material decompressed from all parts of the lower mantle. Even if the hot uncompressed density were uniform for all depths in the lower mantle, the cold uncompressed mantle would be inhomogeneous because the decompression given by the Bullen law crosses isotherms; for example, the temperature is different at different depths. To calculate the density distribution correctly, an isothermal EOS must be used along an isotherm, and temperature corrections must be placed in the thermal pressure P_TH.The thermodynamic parameters of the lower mantle are found by iteration. Values of the three uncompressed anharmonic parameters are first arbitrarily selected: α₀ (hot), the coefficient of thermal expansion; γ₀, the Grüneisen parameter; and δ, the second Grüneisen parameter. Using γ₀ and the measured ρ₀ (hot) and K₀ (hot), the values of θ₀ (Debye temperature) and q = dlnγ/dlnρ are found from the measured seismic velocities. Then from (αK_T)₀ and q the thermal pressure P_TH at all high temperatures is found. Correlating P_TH against T to the geotherm for the lower mantle, P_TH is found at all depths Z. The isothermal pressure, along the 0 K isotherm, at every Z is found by subtracting P_TH from the measured P given by the seismic model. Using the isothermal pressure at depth Z, the solution for the cold uncompressed density ρ_0C and the cold uncompressed bulk modulus, K_T0 is found as a trace in the K_T0?_ρ0C plane. A narrow band of solutions is then found for ρ_0C and K_T0 at all depths.The thermal expansion at all T is found from [ρ_0C ? ρ₀ (hot)/ρ_0C. From Suzuki's formula, the best fit to the thermal expansion determines γ₀ and α₀ (hot). When the values of these two parameters do not agree with the original assumptions, the calculation is repeated until they do agree. In this way all the important thermodynamic parameters are found as a self-consistent set subject only to the assumptions behind the equations used.     

Phase relations in the system LiFMgF2 at elevated pressures: Implications for the proposed mixed-oxide zone of the earth's mantle

Solvi and liquidi for various LiFMgF₂ mixtures have been determined at pressures up to 40 kbar by differential-thermal-analysis in a piston-cylinder high-pressure device. The melting curves of pure LiF and MgF₂ were also studied and the initial slopes (dT_m/dP)_{P = 0} were found to be 11.2 and 8.3°C/kbar, respectively. The eutectic composition (LiF)_0.64(MgF₂)_0.36 is independent of pressure to 35 kbar and the eutectic temperature rises approximately 6.3°C per kbar. Initial slopes of 11°C/kbar and 35°C/kbar are inferred for the melting curves of MgO and SiO₂ (stishovite) respectively, on the basis of data for their structural analogue compounds. The observed solid solution of LiF in MgF₂ and other evidence suggest the possibility of solid solution in the system (Mg,Fe)OSiO₂ (stishovite) under mantle conditions which may have important consequences for the elastic properties of a “mixed-oxide” zone of the earth's mantle.