Pressure-release Partial Melting Research Articles

Boundary-layer model of mantle plumes with thermal and chemical diffusion and buoyancy

SUMMARY We present a boundary-layer model for mantle plumes driven by thermal and chemical diffusion and buoyancy. The problem is solved for a Boussinesq, Newtonian fluid with infinite Prandtl number and constant physical properties. We focus on axisymmetric mantle plumes, but also solve 2-D plumes due to line-sources for comparison. The results show that chemical plumes are much thinner than thermal plumes because of small chemical diffusivity in the mantle. When pressure-release partial melting occurs in a thermal-chemical plume, at least two mantle components may be involved: one from the chemical plume and one from the ambient mantle. A buoyant chemical boundary layer in the plume source region tends to cause narrow and strong plumes. A dense chemical source would have the opposite effect. The effects of chemical buoyancy diminish as the Lewis number, the ratio of thermal to chemical diffusivity, increases. For fully developed mantle plumes, the effects of chemical buoyancy may be insignificant. The physical parameters of mantle plumes may be estimated using surface information deduced from swell models. The total heat input from the Hawaiian plume source is about 1.3 x lo1' W, nearly 5-10 per cent of the total heat loss from the core. The depth of the Hawaiian plume source is constrained to be near the core-mantle boundary. Our results show that 2-D plumes are generally stronger than axisymmetric plumes.

On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth

Important constituents of Archean cratons, formed in the early and hot history of the Earth, are Tonalite–Trondhjemite–Granodiorite (TTG) plutons and greenstone belts. The formation of these granite–greenstone terrains is often ascribed to plate-tectonic processes. Buoyancy considerations, however, do not allow plate tectonics to take place in a significantly hotter Earth. We therefore propose an alternative mechanism for the coeval and proximate production of TTG plutons and greenstone-like crustal successions. That is, when a locally anomalously thick basaltic crust has been produced by continued addition of extrusive or intrusive basalts due to partial melting of the underlying convecting mantle, the transition of a sufficient amount of basalt in the lower crust to eclogite may trigger a resurfacing event, in which a complete crustal section of over 1000 km long sinks into the mantle in less than 2 million years. Pressure release partial melting in the complementary upwelling mantle produces large volumes of basaltic material replacing the original crust. Partial melting at the base of this newly produced crust may generate felsic melts which are added as intrusives and/or extrusives to the generally mafic crustal succession, adding to what resembles a greenstone belt. Partial melting of metabasalt in the sinking crustal section produces a significant volume of TTG melt which is added to the crust directly above the location of ‘subduction’, presumably in the form of a pluton. This scenario is self-consistently produced by numerical thermochemical mantle convection models, presented in this paper, including partial melting of mantle peridotite and crustal (meta)basalt. The metamorphic p, T conditions under which partial melting of metabasalt takes place in this scenario are consistent with geochemical trace element data for TTGs, which indicate melting under amphibolite rather than eclogite facies. Other geodynamical settings which we have also investigated, including partial melting in small scale delaminations of the lower crust, at the base of a anomalously thick crust and due to the influx of a lower mantle diapir fail to reproduce this behavior unequivocally and mostly show melting of metabasalt in the eclogite stability field instead.

Plate tectonics on the terrestrial planets

Plate tectonics is largely controlled by the buoyancy distribution in oceanic lithosphere, which correlates well with the lithospheric age. Buoyancy also depends on compositional layering resulting from pressure release partial melting under mid-ocean ridges, and this process is sensitive to pressure and temperature conditions which vary strongly between the terrestrial planets and also during the secular cooling histories of the planets. In our modelling experiments we have applied a range of values for the gravitational acceleration (representing different terrestrial planets), potential temperatures (representing different times in the history of the planets), and surface temperatures in order to investigate under which conditions plate tectonics is a viable mechanism for the cooling of the terrestrial planets. In our models we include the effects of mantle temperature on the composition and density of melt products and the thickness of the lithosphere. Our results show that the onset time of negative buoyancy for oceanic lithosphere is reasonable (less than a few hundred million years) for potential temperatures below ∼ 1500 ° C for the Earth and ∼ 1450 ° C for Venus. In the reduced gravity field of Mars a much thicker stratification is produced and our model indicates that plate tectonics could only operate on reasonable time scales at a potential mantle temperature below about 1300–1400 °C.

Interaction between small‐scale mantle diapirs and a continental root

A possible mechanism for adding material to a continental root is by means of upwellings from the convecting mantle subject to pressure release partial melting. We present results of numerical modeling of the interaction of melting diapirs with continental roots in an Archean setting characterized by a mantle potential temperature of 1750°C in a two‐dimensional (2‐D) Cartesian geometry. In an extension of earlier work [de Smet et al., 2000b] we have investigated the influence of mantle rheology on the behavior of diapirs. We have in particular looked at the difference in behavior of diapirs using a composite rheology combining both grain size sensitive diffusion creep and dislocation creep mechanisms. We have used the grain size, here taken to be uniform, as a control parameter to obtain model cases with varying contribution from the two creep mechanisms. The diapirs in the composite rheology model rise much faster than they do in a purely Newtonian model. Observed diapiric ascent times from 230 km depth to the top of the ascent path at about 80 km depth are approximately 1 Myr for a Newtonian model (averaged 14 cm/yr) compared to about 50 thousand years for a composite rheology model (averaged 3 m/yr) with the same parameters for the Newtonian component. This clearly indicates the large impact of the dislocation creep component of the viscous deformation process. We have also investigated the effect of an increase in the viscosity due to dehydration during partial melting. This increase has a strong influence on the development of rising diapirs. The ascent velocity and lateral spreading of the diapirs at the end of their ascent are effectively reduced when a viscosity increase by a factor of 10 is applied, and the effect becomes stronger for larger factors. Average vertical velocities range from 1.4 cm/yr for a factor 10 to 2 mm/yr for a factor 200. The most striking result of the viscosity increase due to dehydration is the reduction of the ascent velocity, thereby stretching the characteristic timescale of the diapiric intrusion process to a value between 5 and 50 Myr for dehydration viscosity prefactor values of 10 and 200, respectively. In contrast with the strong difference between the Newtonian and the composite rheology models, small differences are found in the overall dynamics between the composite rheological models, characterized by different values of the uniform grain size. The composite rheological models exhibit self‐regulating behavior where substantial differences between the relative contributions of the two creep components result in very similar effective viscosities, due to a local dominance of dislocation creep at high stresses and corresponding similar flow dynamics. Stress levels and P,T paths in the modeling results are consistent with estimates obtained from Precambrian peridotite bodies which are interpreted to have originated from asthenospheric diapirism.

Mantle wedge control on back-arc crustal accretion.

At mid-ocean ridges, plate separation leads to upward advection and pressure-release partial melting of fertile mantle material; the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away. In back-arc basins, the subducting slab is an important control on the pattern of mantle advection and melt extraction, as well as on compositional and fluid gradients. Modelling studies predict significant mantle wedge effects on back-arc spreading processes. Here we show that various spreading centres in the Lau back-arc basin exhibit enhanced, diminished or normal magma supply, which correlates with distance from the arc volcanic front but not with spreading rate. To explain this correlation we propose that depleted upper-mantle material, generated by melt extraction in the mantle wedge, is overturned and re-introduced beneath the back-arc basin by subduction-induced corner flow. The spreading centres experience enhanced melt delivery near the volcanic front, diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away. Our model explains fundamental differences in crustal accretion variables between back-arc and mid-ocean settings.

Coupled magmatism–mantle convection system with variable viscosity

Numerical models are presented for the thermal and chemical evolution of the mantle when it is controlled by both magmatism and mantle convection. The mantle is modeled as a fluid with temperature- and pressure-dependent viscosity in a two-dimensional square box heated by incompatible radioactive elements that decay with time. The influence of phase transitions at 660 km between the upper and lower mantle is also included. Magmatism is modeled as permeable flow of magma produced upon pressure release partial melting of the convecting mantle materials. It is shown that the mantle may exist in one of two regimes. At first, when the internal heating rate is higher than a threshold, the mantle is on a regime where episodic, but active, magmatism takes place to make the mantle chemically stratified with the upper mantle occupied by magma residue (harzburgite) and the deeper part of the lower mantle occupied by basaltic materials. There is a chemical discontinuity along the 660 km phase boundary. Mantle convection occurs as a layered convection punctuated by flushing events and is sluggish except at the time of flushing events owing to the compositional buoyancy from the chemical stratification. The flushing events induce vigorous magmatic activity. As the internal heating rate falls below the threshold owing to the decay of radioactive elements, the thermal and chemical state jumps to another regime. In this regime, magma does not erupt, and mantle convection occurs as layered thermal convection. In spite of the stirring due to the thermal convection, the chemical discontinuity and a portion of the chemical stratification formed in the earlier regime remains for billions of years. The numerical models suggest that (a) mantle convection was much less active than expected from the strong internal heating, and chemically distinct reservoirs are formed in the early Earth, and once formed (b), the reservoirs have survived convective stirring for billions of years.

Early formation and long-term stability of continents resulting from decompression melting in a convecting mantle

The origin of stable old continental cratonic roots is still debated. We present numerical modelling results which show rapid initial formation during the Archaean of continental roots of ca. 200 km thick. These results have been obtained from an upper mantle thermal convection model including differentiation by pressure release partial melting of mantle peridotite. The upper mantle model includes time-dependent radiogenic heat production and thermal coupling with a heat reservoir representing the Earth's lower mantle and core. This allows for model experiments including secular cooling on a time-scale comparable to the age of the Earth. The model results show an initial phase of rapid continental root growth of ca. 0.1 billion year, followed by a more gradual increase of continental volume by addition of depleted material produced through hot diapiric, convective upwellings which penetrate the continental root from below. Within ca. 0.6 Ga after the start of the experiment, secular cooling of the mantle brings the average geotherm below the peridotite solidus thereby switching off further continental growth. At this time the thickness of the continental root has grown to ca. 200 km. After 1 Ga of secular cooling small scale thermal instabilities develop at the bottom of the continental root causing continental delamination without breaking up the large scale layering. This delaminated material remixes with the deeper layers. Two more periods, each with a duration of ca. 0.5 Ga and separated by quiescent periods were observed when melting and continental growth was reactivated. Melting ends at 3 Ga. Thereafter secular cooling proceeds and the compositionally buoyant continental root is stabilized further through the increase in mechanical strength induced by the increase of the temperature dependent mantle viscosity. Fluctuating convective velocity amplitudes decrease to below 10 mma −1 and the volume average temperature of the sub-continental convecting mantle has decreased ca. 340 K after 4 Ga. Surface heatflow values decrease from 120 to 40 mW m −2 during the 4 Ga model evolution. The surface heatflow contribution from an almost constant secular cooling rate was estimated to be 6 mW m −2, in line with recent observational evidence. The modelling results show that the combined effects of compositional buoyancy and strong temperature dependent rheology result in continents which overall remain stable for a duration longer than the age of the Earth. Tracer particles have been used for studying the patterns of mantle differentiation in greater detail. The observed ( p, T, F, t)-paths are consistent with proposed stratification and thermo-mechanical history of the depleted continental root, which have been inferred from mantle xenoliths and other upper mantle samples. In addition, the particle tracers have been used to derive the thermal age of the modelled continental root, defined by a hypothetical closing temperature.

Continental emergence and growth on a cooling earth

Isostasy considerations are connected to a 1-D model of mantle differentiation due to pressure release partial melting to obtain a model for the evolution of the relative sea level with respect to the continent during the earth secular cooling. In this context, a new mechanism is derived for the selective exhumation of exposed ancient cratons. The model results in a quantitative scenario for sea-level fall due to the changing thicknesses of the oceanic basaltic crust and its harzburgite residual layer as a function of falling mantle temperature. It is also shown that the buoyancy of the harzburgite root of a stabilized continental craton has an important effect on sea-level and on the isostatic readjustment and exhumation of exposed continental surface during the earth's secular cooling. The model does not depend on the usual assumption of constant continental freeboard and crustal thickness and its application is not restricted to the post-Archaean. It predicts large-scale continental emergence near the end of the Archaean and the early Proterozoic. This provides an explanation for reported late Archaean emergence and the subsequent formation of late Archaean cratonic platforms and early Proterozoic sedimentary basins. For a period of secular cooling of 3.8 Ga, corresponding to the length of the geological record, the model predicts a fall of the ocean floor of some 4 km or more. For a constant ocean depth, this implies a sea-level fall of the same magnitude. A formula is derived that allows for an increasing ocean depth due to either the changing ratio of continental with respect to oceanic area, or to a possible increase of the oceanic volume during the geological history. Increasing ocean depth results in a later emergence of submarine ancient geological formations compared to the case when ocean depth is constant. Selective exhumation is studied for the case of constant ocean depth. It is shown that for this case, early exposed continental crust can be exhumed to a lower crustal depth, which explains the relative vertical displacement of low-grade- with respect to high-grade terrain. Increasing ocean depth is not expected to result in diminished exhumation.

Thermochemical regime of the early mantle inferred from numerical models of the coupled magmatism‐mantle convection system with the solid‐solid phase transitions at depths around 660 km

A numerical model is presented for the coupled magmatism‐mantle convection system in the entire mantle to study how the coupled system controls the thermochemical state of the mantle under the influence of the solid‐solid phase transitions at depths around 660 km depending on the internal heating rate. The solid‐state convection in the mantle is modeled by a convection of a binary eutectic material with constant viscosity in a two‐dimensional square box uniformly heated by an internal heat source. One of the end‐members of the material stands for an olivine‐rich material in the uppermost mantle and is transformed into its high‐pressure phase at depths greater than a threshold around 660 km. The phase boundary has a negative Clausius‐Clapeyron slope. Another end‐member stands for a garnet‐rich material in the uppermost mantle and is gradually transformed into its high‐pressure phase with increasing depth in a depth range around 660 km. The density of the material depends on its composition, phase, melt content, and temperature. Magmatism is modeled by a permeable flow of melt generated by a pressure release partial melting of the material. The permeable flow is driven by the buoyancy of the melt. There are two regimes in the thermochemical state of the mantle regardless of the strength of the barrier against mass exchange between the upper mantle and the lower mantle due to the solid‐solid phase transitions. On one regime called the TC branch, magmatism occurs only mildly at most, the mantle remains chemically homogeneous as a whole, and the solid‐state convection occurs dominantly as a thermal convection. The convective circulation is mantle‐wide or layered depending on the strength of the barrier due to the solid‐solid phase transitions. The TC branch is stable only when the internal heating rate is lower than a threshold. A bifurcation occurs on the TC branch at the threshold, and the thermochemical state falls on another regime called the CS branch above the threshold. An episodic magmatism actively occurs to make the mantle chemically stratified with the upper mantle largely occupied by olivine‐rich residual materials and the deeper part of the lower mantle occupied by magmatic products of basaltic composition on the CS branch. The solid‐state convection occurs as a whole mantle convection when the barrier is weak, while it occurs as a layered convection punctuated by flushing events that induce particularly vigorous magmatic activities when the barrier due to the phase transition of the garnet‐rich material is sufficiently strong. The overall features of the thermochemical state on the CS branch mesh in many observations from the Archean and early Proterozoic continents.

A bifurcation in the coupled magmatism-mantle convection system and its implications for the evolution of the Earth's upper mantle

A numerical model is presented for the coupled magmatism-mantle convection system in the upper mantle to study how the coupled system controls the thermochemical state of the upper mantle depending upon the internal or basal heating rate. The solid-state convection in the upper mantle is modeled as a convection of a binary eutectic material with constant viscosity in a two-dimensional rectangular box uniformly heated from the bottom boundary or by an internal heat source. The density of the material depends on the composition and melt-content as well as temperature. Magmatism is modeled as a permeable flow of the melt generated by a pressure-release partial melting of the material; the permeable flow is driven by the buoyancy of the melt. There are two branches in the thermochemical state controlled by the coupled system for both of the basal and internal heating cases. On one branch called the TC-branch, the solid-state convection occurs dominantly as a thermal convection, the box remains chemically homogeneous as a whole, and magmatism occurs only slightly at most. The TC-branch is stable only when the heating rate is lower than a threshold; a bifurcation occurs on the TC-branch at the threshold and the thermochemical state falls on another branch called the CS-branch above the threshold. An episodic magmatism actively occurs, a chemically stratified structure develops well in spite of the homogenizing effect of convective stirring, the temperature becomes as high as the solidus temperature at depth, and the convection is strongly affected by the buoyancy of melt and the chemical buoyancy that accompanies the chemical stratification on the CS-branch. The CS-branch is stable even at heating rates slightly lower than the threshold; a hysteresis in the thermochemical state occurs between the two branches as the heating rate changes around the threshold. The observations of tectosphere suggest that the thermochemical state on the CS-branch occurred in the upper mantle in the Archean and early Proterozoic when the mantle was strongly heated by radioactive elements and that the thermochemical state jumped to the TC-branch at the end of the early Proterozoic.

Thermal history of the Moon: Implications for an early core dynamo and post-accertional magmatism

Thermal evolution models for the Moon starting from a hot and differentiated initial state are presented. The main effect of the cooling is the growth of the thermal lithosphere to a thickness of about 700 km while the lower mantle and core cool relatively little. The heat flow from the core decreases substantially during the first bilion years to a value at which the core cools conductively. This allows a core dynamo to operate during the time before roughly 3 Ga b.p. as is suggested by paleomagnetic data. The dynamo is then shut off. During the first 2 billion years hot upwelling plumes from the lower mantle cause pressure release partial melting in the mid to upper mantle. This is in accord with evidence for post-accretional magmatism.

Effects of chemical fractionation of heat-producing elements on mantle evolution inferred from a numerical model of coupled magmatism-mantle convection system

A numerical model of a coupled magmatism-mantle convection system has been developed to study how a strong chemical fractionation of heat-producing elements as a result of mantle magmatism influences the evolution of the uppermost mantle. Mantle convection is modeled by a convection of a binary eutectic material A ξ B 1− ξ with a Newtonian rheology in a two-dimensional rectangular box; A stands for olivine and B stands for a mixture of pyroxene and garnet. The viscosity depends strongly on temperature and the density increases with decreasing ξ and temperature. The material contains decaying heat-producing elements. Mantle magmatism is modeled by an extraction of melt generated at depth by a pressure-release partial melting and a deposit of the extracted melt in a thin crustal layer at the top of the box. The melt extraction and deposit give rise to heterogeneity in the distribution of heat-producing elements as well as in ξ distribution. A dense, low-ξ magmatic product is more enriched in heat-producing elements than is a less dense, high-ξ residual material. The positive thermal buoyancy that a low-ξ material gains owing to its enrichment with heat-producing elements causes a catastrophic change in the chemical structure of the uppermost mantle with time when the viscosity in the top thermal boundary layer (lithosphere) is sufficiently high. A chemically stratified structure with a high-ξ residual material in the upper part and a low-ξ material in the lower part of the box develops at the beginning, when active magmatism occurs owing to strong internal heating. As the heat-producing elements decay and the magmatism becomes weaker, a steep gradient or a discontinuity in ξ distribution develops between the high-ξ part and the low-ξ part of the box, and the convection occurs temporarily as a layered convection. The chemically layered structure is, however, catastrophically destroyed and the box suddenly becomes more chemically homogeneous at a subsequent time when the positive thermal buoyancy of the low-ξ material at depth becomes stronger than the chemically induced negative buoyancy of the low-ξ material. The catastrophic change in chemical structure induces a drop in temperature at depth and a sudden enhancement of mantle magmatism. When the viscosity in the lithosphere is sufficiently low, in contrast, the chemically layered structure is stable and the catastrophic change from a chemically layered structure to a more chemically homogeneous structure does not occur. A catastrophic destruction of a chemically layered structure in the upper mantle as a result of strong fractionation of heat-producing elements may have occurred in the early Earth.

Mantle differentiation and the crustal dichotomy of Mars

Two thermal evolution models for Mars with crust formation and mantle differentiation are compared. In the first model—termed the homogeneous differentiation model—we assume that a basaltic crust has grown steadily in 4.5 Ga as a consequence of pressure-release partial melting of mantle rock. The second model—termed the early differentiation model—incorporates the dichotomy and an early differentiation event. This event is assumed to have resulted in a mantle depleted of radioactive elements and a primordial enriched southern highland crust. We assume that the primordial crust acts as an efficient thermal blanket on the southerly hemisphere mantle. In a second stage of differentiation, a secondary basaltic crust in the northerly hemisphere is produced by pressure-release partial mantle melting. Our calculations suggest that the homogeneous differentiation model cannot explain the isotopic characteristics of the SNC-meteorites, the concentration of 40Ar in the present Martian atmosphere, the dichotomy, and the long-term stability of the northerly hemisphere volcanism. The early differentiation model has the required geochemical reservoirs (depleted mantle, enriched crust and, possibly, subcrustal mantle layer) and the calculated volume of the secondary crust is consistent with the concentration of 40Ar in the atmosphere. The thickness of the secondary crust is between 10 and 40 km. It depends mainly on the amount of mantle depletion and the crust production efficiency, but little on the amount of thermal blanketing of the mantle by the primordial crust. The lithosphere thicknesses in the two hemispheres, on the contrary, depend to a large extent on the amount of thermal blanketing and little on the other two of the above parameters. The present lithosphere thicknesses are roughly 150–200 km in the northerly hemisphere and 350–500 km in the southerly hemisphere. The present-day surface heat flow in the southerly hemisphere may be about 15 mW m −2 smaller than in the northerly hemisphere. Mantle temperature decreases with the amount of depletion of the mantle and increases with the amount of thermal blanketing, but differs by no more than about 100 K from mantle temperatures calculated in thermal evolution models that neglect differentiation. Therefore, earlier core evolution and magnetic field generation models [Stevenson et al., Icarus 54, 466–489 (1983); Schubert and Spohn, J. geophys. Res. 95, 14,095–14,104 (1990)] calculated without allowing for mantle differentiation remain essentially valid.

A Numerical Model of a Coupled Magmatism—Mantle Convection System in Venus and the Earth's Mantle beneath Archean Continental Crusts

A numerical model of a coupled magmatism-mantle convection system has been developed to study how a surface-temperature T srfc of a terrestrial planet influences the thermal state and the chemical structure of its mantle. Mantle convection is modeled by a convection of a binary eutectic material A ζB 1-ζ in an internally heated two-dimensional rectangular box; A stands for olivine, B stands for a mixture of pyroxene and garnet, and the eutectic composition (ζ = 0.1) corresponds to a basaltic composition. Initially, the box is chemically homogeneous with ζ = 0.64. The theology of the material is Newtonian with a strongly temperature-dependent viscosity. The density of the material decreases linearly with increasing ζ, temperature, and melt content except in a thin crustal layer at the top of the box. Mantle magmatism (i.e., an upward migration of magma generated at depth in the mantle) is modeled by an extraction of the end member B-rich melt generated at depth by a pressure-release partial melting and the deposit of the extracted melt in the crustal layer at the top of the box. (a) When T srfc is 447°C, a layer of high-ζ, low-density material produced as a residue of melt develops in the upper part of the box and a layer of a dense low-ζ material, which is a mixture of the undifferentiated material and a magmatic product, develops in the lower part of the box. A sharp chemical discontinuity develops between the two layers and the convection occurs us a layered convection. The temperature in the upper layer is significantly lower than expected from the high surface temperature because of the chemical layering. (b) When T srfc is 27°C, a layer of high-ζ material develops in the upper part of the box and a layer of dense low-ζ material develops in the lower part of the box only at the beginning when the temperature is sufficiently high at depth and an active magmatism occurs to produce a large amount of new additions of residue and magmatic product. As time goes, the temperature at depth decreases, the magmatism becomes less active, and the box becomes more and more chemically homogeneous because of the mixing due to convection. Layered convection is unstable and chemical discontinuity does not develop well when T srfc is 27°C. A further calculation at T srfc = 447°C with higher viscosity at the surface reveals that whether or not a chemical discontinuity develops well and a layered convection occurs stably is controlled by the viscosity at the surface, which depends strongly on T srfc. The surface temperature of a terrestrial planet controls the chemical structure of its mantle indirectly through the strong temperature-dependence of the viscosity of mantle material. The qualitative feature of the coupled magmatism-mantle convection system is consistent with many observations of Venus and the Earth's Archean continental crusts.

Crustal evolution of the early earth: The role of major impacts

Impact cratering was an important — even dominant — process affecting the crustal evolution of the small terrestrial planets. The fundamental highlands/maria dichotomy of the Moon's surface can be traced to a late heavy bombardment by basin-forming, asteroid-sized bodies which produced not only a topographic division in the lunar crust but also localized the later eruptions of mare basalts. Major impact basins with diameters in excess of 200 km are recognized throughout the inner solar system from Mars to Mercury. Similar craters must have formed on the Earth prior to 4 Ga ago, and the minimum number of such basin-forming impacts can be calculated by scaling from the observed (minimum) number preserved on the Moon. When allowance is made for differences in impact velocity, gravitational cross-section and the effects of gravity on crater diameter, it is found that at least 50% of a presumed global sialic crust would have been converted into impact basins by 4 Ga ago. Among the effects resulting from the impact of an asteroidal object on the early crust were: (a) establishment of a topographic dichotmy of 3–4 km (after isostatic adjustment), (b) pressure-release partial melting of the upper mantle and rapid flooding of the basin floor by basalt, and (c) enhancement of thermal gradients in the sub-basin lithosphere and upper asthenosphere. Comparative planetary data such as impact scaling can be used as important constraints on models of the early terrestrial crust. For example, the topography resulting from impact bombardment produced discrete oceans and dry land by 4 Ga ago, making unreasonable models of a globe-encircling ocean on the Earth after that time.