Internal constitution and evolution of the moon

The composition, structure and evolution of the moon's interior are narrowly constrained by a large assortment of physical and chemical data. Models of the thermal evolution of the moon that fit the chronology of igneous activity on the lunar surface, the stress history of the lunar lithosphere implied by the presence of mascons, and the surface concentrations of radioactive elements, involve extensive differentiation early in lunar history. This differentiation may be the result of rapid accretion and large-scale melting or of primary chemical layering during accretion; differences in present-day temperatures for these two possibilities are significant only in the inner 1000 km of the moon and may not be resolvable. If the Apollo 15 heat-flow result is representative of the moon, the average uranium concentration in the moon is 0.05–0.08 p.p.m.Density models for the moon, including the effects of temperature and pressure, can be made to satisfy the mass and moment of inertia of the moon and the presence of a low-density crust inferred from seismic refraction studies only if the lunar mantle is chemically or mineralogically inhomogeneous. The upper mantle must exceed the density of the lower mantle at similar conditions by at least 5%. The average mantle density is that of a pyroxenite or olivine pyroxenite, though the density of the upper mantle may exceed 3.5 g/cm³. The density of the lower mantle is less than that of the combined crust and upper mantle at similar temperature and pressure, thus reinforcing arguments for early moon-wide differentiation of both major and minor elements. The suggested density inversion is gravitationally unstable and implies stresses in the mantle 2–5 times those associated with the lunar gravitational field, a difficulty that can be explained or avoided by: (1) adopting lower values for the moment of inertia and/or crustal thickness, or (2) postulating that the strength of the lower mantle increases with depth or with time, either of which is possible for certain combinations of composition and thermal evolution.A small iron-rich core in the moon cannot be excluded by the moon's mass and moment of inertia. If such a core were molten at the time lunar surface rocks acquired remanent magnetization, then thermal-history models with initially cold interiors strongly depleted in radioactive heat sources as a primary accretional feature must be excluded. Further, the presence of ～||pre|40 K in a FeFeS core could significantly alter the thermal evolution and estimated present-day temperatures of the deep lunar interior.     

Velocity-density systematics,mean atomic weight,and the interior of the moon

Mean atomic weight profiles for the lunar mantle have been calculated from velocity-density systematic relations using lunar density and seismic velocity models. Despite large variability among the models, the calculation including Poisson's ratio yields a range of mean atomic weight values between 22 and 23 g mol^?1 below 150 km. A similar calculation for the Earth's mantle produces a mean atomic weight of 21.1 ±0.4 g mol^?1. This suggests that the Moon cannot be derived directly from the Earth's mantle, or that it has had a differentiation history different from the Earth's. The lunar $\text{m}\text{'}\text{s}$ require an Fe mole fraction between 0.25 and 0.33 for a pure olivine mantle, or between 0.33 and 0.45 for pure pyroxene.The present profiles are 0.5–3.0 g mol^?1 higher than those calculated from lunar compositional models based on lunar rock compositions and petrology and assumed lunar histories, indicating inadequacies in either the seismic or compositional models, or in both. The mean atomic weight approach provides a method of comparing the consistency of seismic and compositional models of planetary interiors.     

Implications of correlated Nd and Sr isotopic variations for the chemical evolution of the crust and mantle

Published data showing a linear correlation between initial Nd and Sr isotope compositions in young basalts indicate the existence of a spectrum of isotopically distinct reservoirs in the mantle which represent either (1) mixtures of two homogeneous endmember reservoirs, one of which may be undifferentiated material or (2) fractionated reservoirs all derived from a homogeneous initial reservoir with the same ratio of enrichment factors for Sm/Nd and Rb/Sr. The slope of the correlation, which can be described approximately by (⁸⁷Sr/⁸⁶Sr) = ?3.74114 (¹⁴³Nd/¹⁴⁴Nd) + 2.61935orε_Nd = ?2.7 ε_Sr, places constraints on the origin of these reservoirs and hence on the chemical evolution of the crust-mantle system. The reservoirs could be residual regions of the mantle left after ancient partial melting events. If so, the requirement of constant relative fractionation of Sm/Nd and Rb/Sr in refractory residues is a strong constraint on partial melting models. Calculations suggest that batch melting models are more compatible with this constraint than are fractional melting models, but models incorporating currently accepted distribution coefficients and residual phase assemblages cannot reproduce the observed isotope effects except under highly specific conditions. The slope of the correlation is not consistent with the hypotheses that chemical structure in the mantle is due to accretional heterogeneity or variable loss of elements to the core. If the mantle reservoirs are complementary in composition to the continental crust, and if the crust + mantle has ε_Nd = 0andε_Sr = 0 and chondritic Sr/Nd, then Rb/Sr in the crust is calculated to be less than 0.10, suggesting that the crust may be more mafic in composition and contain a smaller proportion of the earth's Rb and heat-producing elements than previously estimated.     

On the frozen flux velocity field at the surface of earth's core necessary to account for the poloidal main magnetic field and its secular variation

An expression for the inviscid horizontal velocity field at the surface of the Earth's core necessary to account for the poloidal main magnetic field and its secular variation seen at the Earth's surface is derived for an insulating mantle in the limit of infinite core conductivity. The starting point of derivation is Ohm's law rather than the magnetohydrodynamic induction equation. Maps of the resulting motion for epoch 1965.0 at different truncation levels are presented and discussed.     

Observations of multiple core reflections of the PnKP and SnKP type and regional variations at the base of the mantle

Seismological results interpreted as evidence for large inhomogeneities near the base of the Earth's mantle below Hawaii have recently been published. It is possible to place constraints on the magnitude of such heterogeneities by identifying seismic phases multiply reflected within the Earth's core. The value of such a simple technique is illustrated by using array recordings of P and S5KP waves that have traversed the bottom of the mantle beneath Hawaii to show that there is no clear evidence for the unusual physical properties attributed to this region of the Earth. Identification of the phase S7KP is also reported.     

CO2- and LREE-rich mantle below eastern Australia: a REE and isotopic study of alkaline magmas and apatite-rich mantle xenoliths from the Southern Highlands Province,Australia

Alkaline magmatism in the Southern Highlands Province, New South Wales, Australia is associated with continental rifting. Near-primary liquids have a wide range in Nd and Sr isotope composition that indicates gross isotopic and chemical heterogeneities in a mantle source region depleted in light rare earth elements (LREE) for much of Earth's history. The large-ion lithophile element and LREE-enriched nature of the primary lavas ((Ce)_N = 95–182 and (Yb)_N = 8.5–13.3) is consistent with an enriched mantle source region. This elemental enrichment may be accomplished by veining of the subcontinental mantle with volatile-rich phases like amphibole, apatite and carbonate which provide the volatile flux necessary to trigger anatexis.Degassing of mantle CO₂ has led to migration of LREE-enriched fluids and local transformation of the lherzolitic mantle to pyroxenite veined by apatite ± kaersutite ± mica ± diopside. The mantle veining event may be related to upwelling of silica-undersaturated incompatible element-enriched magmas similar to the host magma of the Kiama xenoliths. In a relatively short period of time (100 m.y.), the Sr and Nd isotopes in essentially LREE-depleted mantle have evolved in response to low Sm/Nd and low Rb/Sr ratios, and now define a near-vertical vector on a isotope-isotope plot. From this rather unique signature we can infer that CO₂- and LREE-rich, Rb-poor mantle is a potentially suitable mantle source region for the genesis of alkali-potassic volcanic rocks characterized by a narrow range in⁸⁷Sr/⁸⁶Sr ratio and a wide range in¹⁴³Nd/¹⁴⁴Nd ratio (e.g. Leucite Hills).     

Corrugations on the core boundary interfaces due to constitutional supercooling and effects on motion in a predominantly stratified liquid core

With the notion that interface and boundary layer phenomena play an important part in those geophysical processes which, by observation appear to be related to the earth's internal boundaries between the solid and liquid phases of its core and mantle, constitutional supercooling suggests itself as a mechanism capable of generating and maintaining inhomogeneities in concentration and density at the boundaries of the liquid core. The mechanism of constitutional supercooling requires a slow overgrowth of mantle and core, and, it implies that this growth process is associated with a selective partitioning of certain impurities shared in different concentrations by the liquid core and the solid phases of mantle and inner core. It can lead to the formation of regular (quasi-periodic) corrugations of the core-mantle and the inner-outer core boundaries with amplitudes of the order of 1 km. Mass redistributions, off-setting continually regenerated concentration and density inhomogeneities, provide a mechanism for core motion in the form of concentration currents. A regular distribution of corrugations or humps may give rise to (zonal) patterns of closed loops of concentration currents either in layers adjacent to the solid-liquid interfaces, or in loops extending through the entire outer core. The development of regular flow patterns should be enhanced if, referable to one particular constituent of the liquid phase, some parts of the solid-liquid interfaces acted as sources, others as sinks.     

Trace elements in shergottite meteorites: Implications for the origins of planets

The average concentrations of 19 siderophile and volatile elements in shergottite meteorites differ from those in terrestrial basalts by less than a factor of ten. This observation undermines claims that the abundances of siderophile and volatile elements in the Earth's upper mantle are uniquely terrestrial. Claims that similarities in the Moon's siderophile element pattern imply a terrestrial origin for the Moon are also weakened. The implication that basalt source regions on the asteroidal parent body of the shergottites resembled the terrestrial upper mantle constrains models of planetary formation and evolution. Heterogeneous accretion models may explain many of the similarities between these planets. Alternatively, separation of sulfide from basaltic magmas or their source regions on the Earth and the shergottite parent body may explain some of these similarities.     

Thermal histories of the core and mantle

Recognition that the cooling of the core is accomplished by conduction of heat into a thermal boundary layer (D″) at the base of the mantle, partly decouples calculations of the thermal histories of the core and mantle. Both are controlled by the temperature-dependent rheology of the mantle, but in different ways. Thermal parameters of the Earth are more tightly constrained than hitherto by demanding that they satisfy both core and mantle histories. We require evolution from an early state, in which the temperatures of the top of the core and the base of the mantle were both very close to the mantle solidus, to the present state in which a temperature increment, estimated to be ～ 800 K, has developed across D″. The thermal history is not very dependent upon the assumption of Newtonian or non-Newtonian mantle rheology. The thermal boundary layer at the base of the mantle (i.e., D″) developed within the first few hundred million years and the temperature increment across it is still increasing slowly. In our preferred model the present temperature at the top of the core is 3800 K and the mantle temperature, extrapolated to the core boundary without the thermal boundary layer, is 3000 K. The mantle solidus is 3860 K. These temperatures could be varied within quite wide limits without seriously affecting our conclusions. Core gravitational energy release is found to have been remarkably constant at ～ 3 × 10¹¹ W. nearly 20% of the core heat flux, for the past 3 × 10⁹ y, although the total terrestrial heat flux has decreased by a factor of 2 or 3 in that time. This gravitational energy can power the “chemical” dynamo in spite of a core heat flux that is less than that required by conduction down an adiabatic gradient in the outer core; part of the gravitational energy is used to redistribute the excess heat back into the core, leaving 1.8 × 10¹¹ W to drive the dynamo. At no time was the dynamo thermally driven and the present radioactive heating in the core is negligibly small. The dynamo can persist indefinitely into the future; available power 10¹⁰ y from now is estimated to be 0.3 × 10¹¹ W if linear mantle rheology is assumed or more if mantle rheology is non-linear. The assumption that the gravitational constant decreases with time imposes an implausible rate of decrease in dynamo energy. With conventional thermodynamics it also requires radiogenic heating of the mantle considerably in excess of the likely content of radioactive elements.     

Time-dependent electromagnetic core-mantle coupling

The magnetic field in the Earth's mantle is computed using a depth-dependent electrical conductivity, of form σ = σ_a(r/a)^?α, and an approximation scheme in which the electromagnetic time constant of the mantle is assumed small compared with the time scales of the secular variation, and in which the induced currents and fields are obtained iteratively. We first associate the toroidal fields in the mantle with motions at the core surface (r = a) which create the observed geomagnetic field by flux rearrangement, and compute the resulting couple, Γ, parallel to the geographical axis. Using only zonal core motions, and values σ_a = 3 × 10³ω^?1m^?1, α = 30 for the conductivity profile, we find that the toroidal induced fields create a couple, Γ_T, that over most of this century has been roughly ten times greater than the poloidal part, Γ_S, of Γ, and has the same sign. The total couple, Γ, has fluctuations of order 10¹⁸ Nm as required for the observed decade fluctuations in the length of the day. Its average is ～ ?1.5 × 10¹⁸ Nm, i.e., it is too large to remain unbalanced. We suppose that an equally important couple in the opposite sense is created by flux leakage from the core, and we estimate the necessary gradient of toroidal field in the core to be of order ?0.5 Gs km^?1 at the core surface. During the course of the data analysis needed for the present work, we found some evidence for a torsional wave in the Earth's core with a period of ～ 60 y.     

Osmium isotopes as petrogenetic and geological tracers

Using terrestrial osmium-enriched samples of known ages, we have shown that¹⁸⁷Os/¹⁸⁶Os varies with time in result of the¹⁸⁷Re β^? radioactivity. Such a variation in the earth's mantle can be fitted by a straight line corresponding to¹⁸⁷Re/¹⁸⁶Os = 3.15 for the mantle, comparable to C1 carbonaceous chondrites. Using this result and the Re and Os contents of various rocks, several theoretical considerations and predictions can be deduced for the chemical evolution of the earth, such as a method for distinguishing between different processes of development of the continental crust. The special result of¹⁸⁷Os/¹⁸⁶Os from Bushveld is discussed with respect to the possible existence of an “enriched” subcontinental mantle.     

Heat transport by variable viscosity convection and implications for the Earth's thermal evolution

The case is presented that the efficiency of variable viscosity convection in the Earth's mantle to remove heat may depend only very weakly on the internal viscosity or temperature. An extensive numerical study of the heat transport by 2-D steady state convection with free boundaries and temperature dependent viscosity was carried out. The range of Rayleigh numbers (Ra) is 10⁴?10⁷ and the viscosity contrast goes up to 250000. Although an absolute or relative maximum of the Nusselt number (Nu) is obtained at long wavelength in a certain parameter range, at sufficiently high Rayleigh number optimal heat transport is achieved by an aspect ratio close to or below one. The results for convection in a square box are presented in several ways. With the viscosity ratio fixed and the Rayleigh number defined with the viscosity at the mean of top and bottom temperature the increase of Nu with Ra is characterized by a logarithmic gradient β = ?ln(Nu)/? ln(Ra) in the range of 0.23–0.36, similar to constant viscosity convection. More appropriate for a cooling planetary body is a parameterization where the Rayleigh number is defined with the viscosity at the actual average temperature and the surface viscosity is fixed rather than the viscosity ratio. Now the logarithmic gradient β falls below 0.10 when the viscosity ratio exceeds 250, and the velocity of the surface layer becomes almost independent of Ra. In an end-member model for the Earth's thermal evolution it is assumed that the Nusselt number becomes virtually constant at high Rayleigh number. In the context of whole mantle convection this would imply that the present thermal state is still affected by the initial temperature, that only 25–50% of the present-day heat loss is balanced by radiogenic heat production, and the plate velocities were about the same during most of the Earth's history.     

The Chandler annual resonance and its possible geophysical significance

The secular decrease of the earth's rotation rate has caused the Chandler and annual modes of the earth's wobble to resonate in the past. The physics of the resonance is discussed as well as its thermal impact on the mantle. Energy of the order of 10²⁶ erg/year were dissipated in the upper mantle during the resonance. Estimates of the epoch and duration of the resonance place them at about 185 m.y. ago and about 10 m.y., respectively. The possibility of a causal connection between the resonance and the onset of continental drift is suggested.     

A note on short-term core-mantle coupling,geomagnetic secular variation impulses,and potential magnetic field invariants as Lagrangian tracers of core motions

This note summarizes recent studies of atmospheric excitation of short-term changes in the length of the day and polar motion which set useful limits on the timescales associated with angular momentum transfer between the Earth's core and mantle. It also speculates about the nature of the recently-discovered phenomenon of “impulses” or “jerks” in the geomagnetic secular variation, proposing that they might be manifestations of “loop” instability of the magnetic field within the core. Finally, it outlines novel properties of high magnetic Reynolds number flows that bear on the inverse problem of deducing core motions from geomagnetic secular variation data.     

Core motions,electromagnetic core-mantle coupling and variations in the earth's rotation: New constraints from geomagnetic secular variation impulses

Recently observed secular acceleration impulses (SAI) of the geomagnetic field are interpreted in terms of organized motions of the outer core layers. Such motions have planetary dimensions (5000 km) and a large amplitude (3 × 10^?4 m s^?1) and are established in very short times (less than one year). The correlation of SAI observed in the Northern Hemisphere with minima in the Earth's rotation rate (around 1840, 1905 and 1970) is shown to be consistent with a simple model involving electromagnetic coupling of the weakly conducting (of the order of 100 ω^?1 m^?1) mantle, of a coherent outer core layer (thickness 100 to a few hundred kilometres) and of the rest of the core. The magnitude of the torque which acts suddenly on both parts of the core at the time of the impulses is estimated.     

Inferences on the lunar temperature from gravity,stress state and flow laws

Inferences on the lunar temperature regime are made from the inversion of gravity for density anomalies and the stress-state of the Moon's interior, and by comparing these results with flow laws and estimates of likely strain-rates.The nature of the spectrum of the lunar gravitational potential indicates that the density anomalies giving rise to the potential are mainly of near-surface origon. The average stress-differences in the lunar mantle required to support these density anomalies are of the order of a few tens of bars and have persisted for more than 3 · 10⁹ years. If current flow laws for dry olivine can be extrapolated to the conditions of the lunar mantle, and the selenotherms based on electrical conductivity models are valid, the strain rates are too high to explain the preservation of the lateral near-surface density anomalies. We suggest that the present temperatures in the Moon are relatively low, of the order of 800°C or less, at a depth of about 300 km. This compares with 1100°C based on electrical conductivity models and is near the lower limit predicted by Keihm and Langseth (1977) from lunar heat-flow observations.     

Origin of Deccan Trap lavas: evidence from combined trace element and Sr-, Nd- and Pb-isotope studies

Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar Formations can be shown to reflect three distinct end-members which provide new constraints for petrogenetic models. Pb-isotope data for selected basalts exhibit a wide range with²⁰⁶Pb/²⁰⁴Pb= 16.87–22.45, and a linear correlation on a Pb—Pb diagram. The least contaminated Ambenali basalts plot within the Pb-array, and interaction with mantle lithosphere involves a shift to less radiogenic Pb whereas contamination with crust is characterised by more radiogenic Pb. Unlike the Karoo and Parana continental flood basalt provinces only four flow units within the southern Deccan appear to contain a significant contribution from mantle lithosphere. The Mahabaleshwar and Ambenali Formation basalts exhibit a striking negative Pb—Sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle. It further suggests that basalts from the Walvis Ridge, Kerguelen and Ninetyeast ridge all remobilised such shallow level material, and that the Deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to T-type MORB, but related to the Reunion hotspot, and continental mantle lithosphere of the Indian plate.     

A gradational density model for gravity anomalies over oceanic ridges

The long-wavelength gravity anomalies observed over oceanic ridges have been interpreted in terms of horizontal slabs with lateral variation of density. The location of such a slab in the earth's interior is estimated to be between the depths of 350 and 430 km, which defines the boundaries of the upper phase-transition zone of the mantle. A total density contrast, between the end planes of the horizontal slab, of 0.3 g/cm³ appears to be satisfactory for the interpretation. This remarkable coincidence in depth and density contrast associated with the pyroxene-garnet transformation process is considered to suggest that this process may possibly be: (1) taking place laterally; and (2) generating the gradational density contact which is reflected in the gravity anomalies. In turn, the mechanism for this lateral phase transformation may ultimately be attributable to the convection currents in the asthenosphere.     

Mantle plumes control magnetic reversal frequency

Magnetic reversal frequency correlates inversely with mantle plume activity for the past 150 Ma, as measured by the volume production rate of oceanic plateaus, seamount chains, and continental flood basalts. This inverse correlation is especially striking during the long Cretaceous magnetic normal “superchron”, when mantle plume activity was at a maximum. We suggest that mantle plumes control magnetic reversal frequency by the following sequence of events. Mantle plumes rise from theD″ seismic layer just above the core/mantle boundary, thinningD″ to fuel the plumes. This increases core cooling by allowing heat to be conducted more rapidly across the core/mantle boundary. Outer core convective activity then increases to restore the abnormal heat loss, causing a decrease in magnetic reversal frequency in accord with model predictions for bothα² andαω dynamos. When core convective activity increases above a critical level, a magnetic superchron results. The pulse of plume activity that caused the Cretaceous superchron resulted in a minimum increase in core heat loss of about 1200 GW over the present-day level, which corresponds to an increase in Joule heat production of about 120 GW within the core.