Mantle Overturn Research Articles

MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars

A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency's Mars Express spacecraft is providing new constraints on the martian subsurface. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over approximately 14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.

Archean cratons and mantle dynamics

The apparent stability of Archean cratons and cratonic keels for billions of years is a difficult observation for geodynamic modeling to explain. While it may be straight-forward to assert that chemical buoyancy and high viscosity are needed to stabilize cratons, there are many questions regarding craton formation, variability (both in space and time), and evolution that remain unanswered. In numerical studies, strong and buoyant cratonic keels survive relatively undeformed for several mantle overturn times (the equivalent of several hundred million years); extending this to several billion years remains a challenge. The strength required to stabilize keels in some of these numerical experiments exceeds reasonable estimates of the laboratory measurements of strength of mantle materials (including both the effects of temperature and melt-depletion). In addition, the most common explanation of keel formation, vertical stacking of subducted plate, requires the keel material to be deformable at the time of formation and soon afterward the keel material becomes strong enough to resist shearing. The extent to which cratonic keels interact with and influence the pattern of mantle convection, by nucleating small-scale edge-driven convection or by coupling plate motions to deeper mantle flow, remains an open question.

Numerical analysis of geodynamic evolution of the Earth based on a thermochemical model of mantle convection: 3-D model

Evolution of the Earth’s mantle from a hot initial state is modelled in the Boussinesq approximation within the framework of a thermochemical model. The spatial dynamics of substance within a spherical layer is illustrated by video records. Introduction of a chemical component into intermittent (with regard for an endothermic phase transition) thermal convection facilitates the overcoming of the phase barrier, enhances nonlinearity of the dynamic process, and promotes the formation of new cycles in the evolutionary process. As a result, avalanches become more numerous in the thermochemical variant and reduce to a regional scale; therefore, it is more natural to associate them with Bertrand (rather than Wilson) cycles. A basically new result of the numerical modelling is that convection developing from an unstable hot initial state gives rise to global mantle overturns (see Animations 2, 3, 4) decaying with cooling material and remarkably correlating with data of historical geology on Wilson cycles. The inferred spatial patterns of overturns are represented by a single funnel-shaped sink and a few uprising superplumes, accounting for the origins of supercontinents, opening of oceans, and the observed asymmetry of the planet.

TTGs and adakites: are they both slab melts?

Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings. Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 °C. Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni. These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth.

Earth's heat flux revised and linked to chemistry

Recent estimates of the heat flux from the oceanic crust rest on the validity and accuracy of the half-space cooling (HSC) model. The known discrepancies between calculated and measured heat fluxes are not due to hydrothermal circulation, as commonly assumed, because the magmatic source provides too little energy, and the Rayleigh number is too small to foster vigorous convection on an oceanic scale. The half-space cooling model errs in assuming constant thermal conductivity ( k), but more importantly, provides infinite flux along the ridge centers over all time for any form for k. This unrealistic global singularity strongly impacts the derived mean flux. We develop three independent methods to ascertain Earth's mean oceanic heat flux directly from compiled heat flow data. The results are congruent, insensitive to uncertainties in the dataset, corroborate previous spherical harmonic analyses, provide the same average flux as from the continents, and constrain the global power as 31±1 TW. Geological observations, inferred mantle overturn rates, estimated mantle cooling rates, and recent geodynamic models independently suggest that neither delayed secular cooling nor primordal heat are currently significant sources, necessitating that current heat production predominately originates in radioactive decay and is quasi-steady-state. Models of Earth's bulk composition based on CI chondritic meteorites provide an unrealistically low radioactive power of ∼20 TW, whereas enstatite chondrites are sufficiently radioactive to supply the observed heat flux, contain enough iron metal to account for Earth's huge core, and have the same oxygen isotopic ratios as the bulk Earth. We devise a method to obtain K/U/Th ratios for the Earth and other planetary bodies from their power, including secular delays, and use this to constrain Earth's cooling rate.

Major episodic increases of continental crustal growth determined from zircon ages of river sands; implications for mantle overturns in the Early Precambrian

In order to better understand continental growth history, we have determined the U–Pb age of about 1500 detrital zircons from river sands in three major rivers in North America and South America, and have estimated the rate of continental growth in order to discuss the linkage between continental growth and the thermal evolution of the mantle. The method for estimating continental growth from the age population of a significant number of detrital zircons in river sands has three advantages; (1) the almost even collection of zircons from sedimentary rocks and granitic basements in the hinterlands of the major rivers, (2) determination of the age of juvenile crust due to the high closure temperature of the U–Pb systematics of zircons, and the removal of the influence of recycling of crustal materials within the continental crust, and (3) direct determination of the age of continental formation. The age population of zircons in the river sands of the Mississippi River shows four peaks at 2.6–2.8, 1.7–1.8, 1.1–1.5 and 0.2–0.5 Ga, the peaks corresponding to periods of major crustal formation of the Ouachita orogenic belt, the Grenville orogenic belt, the Yavapai-Mazatzal orogenic belt and Wyoming province in North America. The areal distribution of each orogenic belt is 11, 20, 33, and 14%, respectively, and this agrees with the age population of the zircons. This fact confirms the validity of the calculation of continental formation from the age population of zircons in river sands. In contrast, the age population of zircons in river sands of the Mackenzie and Amazon Rivers displays major peaks at 2.4–2.7, 1.0–1.9 and 0.2 Ga, and at 2.4–2.7, 1.9–2.2 and 1.0–1.5 Ga, respectively. However, the age distribution of the surface rocks in the drainage basins of the Mackenzie and Amazon Rivers also comprises widely-distributed young orogenic belts. This discrepancy suggests that the young orogenic belts were formed through recycling of older materials, and that essentially no juvenile continental crusts grew at those times. Our new continental growth curve estimated from the age populations of zircons in the river sands of the Mississippi, Mackenzie and Amazon Rivers indicates continuous growth of continental crust since the Archean and major abrupt increases in the Late Archean and early Proterozoic. The continental growth curve reflects episodic increase of the volume of continental crust at 1.9–2.3 and 2.5–2.8 Ga. The mantle overturn model provides the best explanation for the episodic growth of the continents. Accordingly, our data suggest that mantle overturn occurred at 1.9–2.3 and 2.5–2.8 Ga. If the mantle temperature were high enough in the Archean, indicated by geologic records to be ca. 150 K higher than that of modern MORB-source mantle (e.g., [Lithos 37 (1996) 199; Phys. Earth Planet. Int., in this issue]), a consequent higher Rayleigh number than at present would have favored double-layered mantle convection. If this were the case, the upper mantle would have behaved as a stagnant lid. On the other hand, it was also responsible for the production of large quantities of MORB that were later subducted beneath island arcs. The subducted material accumulated on the boundary between the upper and lower mantle, and consequently caused catastrophic downwelling into the lower mantle. The catastrophic sinking of the material induced a new influx of recycled material from the lower mantle, resulting in mantle overturn. As a consequence, superplumes and plumes ascended from the core–mantle boundary and the deep mantle, to give rise to large igneous provinces, flood basalts and oceanic plateaus. Episodic continental growth resulted from the mantle overturn because the influx of hotter and fertile materials from the lower mantle revitalized the upper mantle to produce large quantities of MORB and subsequent sialic continental crust through slab-melting.

Chemical stratification in a two‐dimensional convecting mantle with magmatism and moving plates

A self‐consistent numerical model is presented for magmatism in a convecting mantle with moving plates. Mantle convection is modeled as a thermal‐chemical convection of binary eutectic material with Newtonian rheology in a two‐dimensional rectangular box internally heated by radioactive elements. Viscosity is assumed to depend on stress history as well as temperature and pressure to self‐consistently reproduce moving plates. The barrier effect of the solid‐solid phase transitions at depths around 660 km to convective flow across the 660 km phase boundary is also taken into account. Magmatism is modeled as a permeable flow of basaltic melt produced upon decompression melting of the convecting material through the coexisting matrix. Magmatism makes mantle chemically stratified with the shallower part occupied by chemically buoyant residue of magma and the deeper part occupied by hot but chemically dense materials enriched in basaltic component even under the influence of moving plates and subducting slabs. The magmatism is induced by moving plates (ridge volcanism), mantle overturn, and hot plumes uprising from the 660 km phase boundary or the lower mantle, depending upon the internal heating rate and the strength of the 660 km barrier. A weaker 660 km barrier leads to more continuous plate tectonics. When plates move, subducting slabs penetrate deep into the lower mantle and induce broad thermal and chemical heterogeneity at depth in the lower mantle. The chemically stratified mantle with moving plates is compared to the Earth's mantle.

The origin and evolution of Archean lithospheric mantle

Abstract The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model” of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere.

Longevity and stability of cratonic lithosphere: Insights from numerical simulations of coupled mantle convection and continental tectonics

The physical conditions required to provide for the tectonic stability of cratonic crust and for the relative longevity of deep cratonic lithosphere within a dynamic, convecting mantle are explored through a suite of numerical simulations. The simulations allow chemically distinct continents to reside within the upper thermal boundary layer of a thermally convecting mantle layer. A rheologic formulation, which models both brittle and ductile behavior, is incorporated to allow for plate‐like behavior and the associated subduction of oceanic lithosphere. Several mechanisms that may stabilize cratons are considered. The two most often invoked mechanisms, chemical buoyancy and/or high viscosity of cratonic root material, are found to be relatively ineffective if cratons come into contact with subduction zones. High root viscosity can provide for stability and longevity but only within a thick root limit in which the thickness of chemically distinct, high‐viscosity cratonic lithosphere exceeds the thickness of old oceanic lithosphere by at least a factor of 2. This end‐member implies a very thick mechanical lithosphere for cratons. A high brittle yield stress for cratonic lithosphere as a whole, relative to oceanic lithosphere, is found to be an effective and robust means for providing stability and lithospheric longevity. This mode does not require exceedingly deep strength within cratons. A high yield stress for only the crustal or mantle component of the cratonic lithosphere is found to be less effective as detachment zones can then form at the crust‐mantle interface which decreases the longevity potential of cratonic roots. The degree of yield stress variations between cratonic and oceanic lithosphere required for stability and longevity can be decreased if cratons are bordered by continental lithosphere that has a relatively low yield stress, i.e., mobile belts. Simulations that combine all the mechanisms can lead to crustal stability and deep root longevity for model cratons over several mantle overturn times, but the dominant stabilizing factor remains a relatively high brittle yield stress for cratonic lithosphere.

Mantle circulation models with variational data assimilation: inferring past mantle flow and structure from plate motion histories and seismic tomography

SUMMARY Mantle convection models require an initial condition some time in the past. Because this initial condition is unknown for Earth, we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent Mesozoic and Cenozoic geological history of our planet. Here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation. We derive the generalized inverse of mantle convection and explore the initial condition problem in high-resolution, 3-D spherical mantle circulation models for a time period of 100 Myr, roughly comparable to half a mantle overturn. We present a synthetic modelling experiment to demonstrate that mid-Cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling, assuming present-day mantle structure is well-known, even if an initial first guess assumption about the mid-Cretaceous mantle involved only a simple 1-D radial temperature profile. We also demonstrate that convecting present-day mantle structure back in time by reversing the time-stepping of the energy equation is insufficient to model the mantle structure of the past. The difficulty arises, because such backward convection calculations ignore thermal diffusion effects, and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time. Inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle.

Mantle circulation models with variational data assimilation: inferring past mantle flow and structure from plate motion histories and seismic tomography

SUMMARY Mantle convection models require an initial condition some time in the past. Because this initial condition is unknown for Earth, we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent Mesozoic and Cenozoic geological history of our planet. Here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation. We derive the generalized inverse of mantle convection and explore the initial condition problem in high-resolution, 3-D spherical mantle circulation models for a time period of 100 Myr, roughly comparable to half a mantle overturn. We present a synthetic modelling experiment to demonstrate that mid-Cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling, assuming present-day mantle structure is well-known, even if an initial first guess assumption about the mid-Cretaceous mantle involved only a simple 1-D radial temperature profile. We also demonstrate that convecting present-day mantle structure back in time by reversing the time-stepping of the energy equation is insufficient to model the mantle structure of the past. The difficulty arises, because such backward convection calculations ignore thermal diffusion effects, and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time. Inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle.

Mesozoic crust-mantle interaction beneath the North China craton: A consequence of the dispersal of Gondwanaland and accretion of Asia

We present evidence from zircons entrained within lower-crustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.

Mantle-circulation models with sequential data assimilation: inferring present-day mantle structure from plate-motion histories.

Data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow. Such an approach is essential in mantle circulation models, where we seek to constrain an unknown initial condition some time in the past, and thus cannot hope to use first-principles convection calculations to infer the flow history of the mantle. One of the most important observables for mantle-flow history comes from models of Mesozoic and Cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle-buoyancy forces due to subducted oceanic slabs. Here we present five mantle circulation models with an assimilated plate-motion history spanning the past 120 Myr, a time period for which reliable plate-motion reconstructions are available. All models agree well with upper- and mid-mantle heterogeneity imaged by seismic tomography. A simple standard model of whole-mantle convection, including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation, reveals downwellings related to Farallon and Tethys subduction. Adding 35% bottom heating from the core has the predictable effect of producing prominent high-temperature anomalies and a strong thermal boundary layer at the base of the mantle. Significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep, factor 100, viscosity rise from the upper to the lower mantle results in substantial transition-zone heterogeneity, enhanced by the effects of trench migration implicit in the assimilated plate-motion history. An expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km, which is influenced by Jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the Cretaceous. This result implies that sequential assimilation of past plate-motion models is ineffective in studying the temporal evolution of core-mantle-boundary heterogeneity, and that a method for extrapolating present-day information backwards in time is required. For short time periods (of the order of perhaps a few tens of Myr) such a method exists in the form of crude 'backward' convection calculations. For longer time periods (of the order of a mantle overturn), a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model.

Did global tectonics drive early biosphere evolution? Carbon isotope record from 2.6 to 1.9 Ga carbonates of Western Australian basins

The δ 13C carb record of well preserved carbonates in outcrop and core is here examined from the 2.6 to 1.9 Ga old basins of Western Australia. These data, which are constrained by a well defined stratigraphic and tectonic framework, and by U–Pb zircon ages, provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere. In the latest Archaean (ca. 2.6 Ga) the secular δ 13C carb curve is flat much like that seen in the later Palaeoproterozoic basins of Northern Australia (<1.8 Ga). This implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable. In the early Palaeoproterozoic, beginning after 2.5 Ga and continuing until at least 1.9 Ga, the δ 13C carb the curve is much more dynamic, with significant positive and negative excursions, including a major positive excursion (+9‰ pdb) close to 2.2 Ga. These excursions can be correlated with the Lomagundi event identified in Africa, Europe and North America. Previously published studies of the overlying Meso- to Palaeoproterozoic Bangemall basin and of 1.8–1.5 Ga old basins in northern Australia suggest that the δ 13C carb curve became relatively monotonic again after ca. 1.8 Ga and remained so for most of the following Mesoproterozoic. Comparisons with data from other ancient cratons, especially Africa, suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the Neoproterozoic. When the Western Australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest-Archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins. The data are consistent with an earth in which the carbon mass balance was in equilibrium. The δ 13C carb curve began to oscillate following the onset of glaciation as the Pilbara and Yilgarn Cratons began to converge during the Capricorn Orogeny suggesting periods of rapid carbon burial during continental dispersal. However, the major positive excursion is preserved in carbonates from back-arc basins formed as the ocean closed and subduction began. Since similar tectonic processes can be recognised, not only in Northern Australia but also on other early cratons, it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development. We explain this coincidence of carbon isotopic excursions and tectonism by the sequestration of carbon during ocean closure with organic-rich passive margin sediments containing isotopically light carbon subducted into and stored in the lower crust and mantle. The global ocean thereby became enriched in isotopically heavy carbon, releasing oxygen to the atmosphere. A second stepwise increase in atmospheric oxygen in the Neoproterozoic may also have been connected with the assembly of Rodinia. This second event has been associated with the development of multicellular life and the evolutionary ‘Big Bang’. Between the two events the carbon cycle, and to some extent the biosphere, appear to have entered a period of prolonged evolutionary stasis. This implies that the evolution of both the atmosphere, and the biosphere, may have been driven forward by planetary evolution, implying that biospheric evolution has largely been driven by the dynamo of earth's tectonism and its long term survival depends upon these endogenic (thermal) energy resources. If this is so it has fundamental implications, not only for life on earth, but for the more general problems surrounding the likelihood of life having evolved on other planetary bodies. Small planets with insufficient endogenic energy resources to sustain the crust/mantle interactions of plate tectonics (such as Mars) seem to us unlikely to have allowed evolution beyond single-celled life forms. Once a planet's energy resources are expended the biosphere would most likely enter a prolonged stasis and ultimately face extinction.

Archaean tectonics: a review, with illustrations from the Slave craton

Abstract The tectonic evolution of Archaean granite-greenstone terranes remains controversial. Here this subject is reviewed and illustrated with new data from the Slave craton. These data show that a thick, c. 2.7Ga, pillow basalt sequences extruded across extended sialic basement of the Slave craton at a scale comparable with that of modern large igneous provinces. The pillow basalts do not represent obducted oceanic allochthons. Basement-cover relationships argue for autochthonous to parautochthonous development of the basaltic greenstone belts of the west-central Slave craton, an interpretation that is further supported by geochemical and geochronological data. Similar data exist for several other cratons and granite-greenstone terrains, including the Abitibi greenstone belt of the Superior craton, where stratigraphic and subtle zircon inheritance data are equally incompatible with accretion of oceanic allochthons. Many classical granite-greenstone terrains, including most well-documented komatiite occurrences, thus appear to have formed in extensional environments within or on the margins of older continental crust. Closest modern analogues for such basalt-komatiite-rhyolite-dominated greenstone successions are rifts, marginal basins and volcanic rifted margins. Indeed, these environments have high preservation potential compared with fully oceanic settings. Collapse and structural telescoping of these highly extended volcano-sedimentary basins would allow for the complex structural development seen in granite-greenstone terrains while maintaining broadly autochthonous to parautochthonous tectonostratigraphic relationships. Seismic reflection profiles cannot discriminate between these telescoped autochthonous to parautochthonous settings and truly allochthonous accretionary complexes. Only carefully constructed structural-stratigraphic cross-sections, allowing some degree of palinspastic reconstruction, and underpinned by sufficient U-Pb zircon dating, can address the degree of allochthoneity of greenstone packages. Furthermore, seismic reflection profiles are essentially blind for the steep structures produced by multiple phases of upright folding and buoyant rise of mid- to lower-crustal, composite, granitoid and gneiss domes. Such structures are ubiquitous in granite-greenstone terrains and, indeed, most of these terrains appear to have experienced at least one phase of convective overturn to re-establish a stable density configuration, irrespective of the complexities of the pre-doming structural history. Buoyant rise of mid- to lower-crustal granitoid and gneiss domes can explain the typical size and spacing characteristics of such domes in granite-greenstone terranes, and the coeval deposition of late-kinematic, ‘Timiskaming-type’ conglomerate-sandstone successions in flanking basins. The extensional and subsequent contractional evolution of granite-greenstone terrains may have occurred in the overall context of a plate tectonic regime (e.g. volcanic rifted margins, back-arc basins) but highly extended, intraplate, rift-like settings seem equally plausible. Explaining the evolution of the latter in terms of Wilson cycles is misguided. Periods of intense rifting and flood volcanism (e.g. 2.73–2.70 Ga) may have been related to increased mantle plume activity or perhaps catastrophic mantle overturn events. Although there is evidence for plate-like lateral movement in late Archaean time (e.g. lateral heterogeneity of cratons, arc-like volcanism, cratonscale deformation patterns, strike-slip faults, etc.), the details of how these plate-like crustal blocks interacted and how they responded to rifting and collision appear to have differed significantly from those in Phanerozoic time. The most productive approach for Archaean research is probably to more fully understand and quantify these differences rather than the common emphasis on the superficial similarities with modern plate tectonics.

Rotund versus skinny orogens: Well-nourished or malnourished gold?

Orogenic gold vein deposits require a particular conjunction of processes to form and be preserved, and their global distribution can be related to broad-scale, evolving tectonic processes throughout Earth history. A heterogeneous distribution of formation ages for these mineral deposits is marked by two major Precambrian peaks (2800‐2555 Ma and 2100‐1800 Ma), a singular lack of deposits for 1200 m.y. (1800‐600 Ma), and relatively continuous formation since then (after 600 Ma). The older parts of the distribution relate to major episodes of continental growth, perhaps controlled by plume-influenced mantle overturn events, in the hotter early Earth (ca. 1800 Ma or earlier). This worldwide process allowed preservation of gold deposits in cratons, roughly equidimensional, large masses of buoyant continental crust. Evolution to a less episodic, more continuous, modern-style plate tectonic regime led to the accretion of volcano-sedimentary complexes as progressively younger linear orogenic belts surrounding the margins of the more buoyant cratons. The susceptibility of these linear belts to uplift and erosion can explain the overall lack of orogenic gold deposits at 1800‐600 Ma, their exposure in 600‐50 Ma orogens, the increasing importance of placer deposits back through the Phanerozoic since ca. 100 Ma, and the absence of gold deposits in orogenic belts younger than ca. 50 Ma.

Rise of atmospheric oxygen and the “upside‐down” Archean mantle

The establishment of an oxygen‐rich atmosphere dramatically altered the evolution of life on Earth. Most of the recent discussion of the topic has been focused on the timing of the event rather than on its mechanism. Here we draw upon recent developments in the understanding of Earth's interior to propose that the rise of oxygen followed a geologically abrupt period of mantle overturn and/or intense plume activity near the Archean‐Proterozoic transition, 2470–2450 million years ago. The magmatic event has already been linked to the widespread deposition of oxide‐facies banded iron formation, and the rise of oxygen has been implicated as the trigger for iron deposition and Earth's first major glaciation. We argue that these events are all related to a change in redox state of volcanic gases brought about by deep‐seated Late Archean and earliest Paleoproterozoic magmatism.

The 2.7–2.0 Ga volcano-sedimentary record of Africa, India and Australia: evidence for global and local changes in sea level and continental freeboard

The 2.7–2.0 Ga volcano-sedimentary records of the African, Indian and Australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas, concomitant with lowered freeboard. Carbonate-banded iron formation (BIF) platforms characterised the first such event, at ca 2.6–2.4 Ga (Africa and Australia) to 2.7 Ga (India). These earlier globally enhanced sea levels are ascribed to increased mid-ocean ridge activity, possibly related to breakup of a postulated Late Archaean ‘southern’ supercontinent. Alternatively, a transition from global-scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the Late Archaean (Nelson, 1998. Earth Planet. Sci. Lett. 158, 109–119). Both explanations of increased mid-ocean ridge activity are compatible with significant Early to Middle Archaean crustal growth (Armstrong, 1981. Phil. Trans. R Soc. London A 301, 443–472), with the emergent high freeboard cratons being subjected to aggressive weathering and erosion. Enhanced continental crustal growth near the Archaean–Proterozoic boundary (McLennan and Taylor, 1982. J. Geol. 90, 347–361), related to the development of significant island arc complexes, would have resulted in common lowered freeboard–enhanced sea level conditions at the passive margins of the ‘southern’ cratons. The diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and/or the thermal state of the cratons. From ca 2.4–2.2 Ga, cratons that make up the present-day continents of India, Africa and Australia had relatively high continental freeboard and lowered sea levels. Glacigenic deposits are preserved on the Kaapvaal (Africa), Singhbhum (India) and Pilbara (Australia) cratons. The second broadly defined drowning event, at ca <2.2 and >2.15 Ga, was probably due to post-glacial climatic amelioration. Freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned. In India, carbonates were more prominent than the siliciclastic sediments (including prominent black shales) seen in Africa and Australia.

Two-stage melting and the geochemical evolution of the mantle: a recipe for mantle plum-pudding

We explore a geochemical model for mantle evolution where a sequence of hotspot and ridge upwelling has melted the mantle to make hotspot and mid-ocean ridge basalts and their residues, and plate subduction has re-cycled and stirred all of these differentiation products back into the mantle. After billions of years this process has mixed various `plums' of incompatible-element rich veins within a matrix made from the residues of melting that have been depleted in incompatible elements. We propose that the mantle flows upward and melts in a two-stage process. During the first stage, plume upwelling and melting creates an enriched ocean island basalt by extracting a low degree melt (∼1–4%) from the rising mantle mixture. The plums are easier to melt, so proportionally more of the incompatible elements are extracted from these components. After melt extraction, the mixture of leftovers is depleted in composition, even though it still contains ∼96–99% of the mass of the original plume upwelling. These depleted leftovers are hot and buoyant so they pond beneath the lithosphere as an asthenosphere layer. When they rise and melt a second time beneath a mid-ocean ridge, a depleted mid-ocean ridge basalt is extracted. The now extremely depleted leftovers, ∼85% of the mass of the original plume upwelling, accrete to oceanic lithosphere which eventually subducts to recycle leftovers, eroded continental crust, and basaltic plums back into the mantle. Observed trace element, rare gas, and isotopic contrasts between oceanic island and mid-ocean ridge basalts can be produced by a recipe which assumes that throughout Earth history these two sequential stages of deep plume and shallower ridge melting have both created and reprocessed the plums and residues that make up the present-day mantle. In this recipe the two-stage melting process does not change through time, but the rate of mantle overturn slows over time in proportion to the decrease in radioactive heat production.

On the partitioning of mantle heat loss below oceans and continents over time and its relationship to the Archaean paradox

Summary The Archaean paradox stems from the fact that while certain lines of evidence suggest that Archaean continental geotherms were similar to those at present, other lines of evidence suggest that the average mantle heat flux was considerably greater. The simplest qualitative solution, which holds that a greater proportion of heat loss was carried by the creation and subduction of oceanic lithosphere in the Earth’s past, has lacked a clear physical basis. At a fundamental level, it requires that the ratio of mantle heat loss below oceans to that below continents be an increasing function of overall convective vigour and, to date, no self-consistent model has been used to suggest a means by which this could be so. A simple means is suggested by models that allow accumulations of chemically light near-surface material, analogues to continents, to form within the upper thermal boundary layer of a thermally convecting and chemically dense fluid, an analogue to the mantle. The physical properties of continental crust cause the thermal coupling condition between a model continent and the convectively unstable mantle below to be different from that which exists at the interface between mantle and oceans. For the latter, a spatially constant temperature condition holds due to (1) the participation of oceanic crust in convective mantle overturn and (2) the large effective thermal conductivity of oceans relative to the mantle. For the former, a spatially near-constant equilibrium heat flux condition holds due to (1) the long-term stability of continental crust against remixing into the mantle and (2) the comparable thermal conductivities of distinct rock types. This leads to a laterally variable thermal condition at the surface of the convecting mantle, which, in turn, leads to different local mantle heat flux behaviour in continental versus oceanic regions. As a result, heat flux below continents can increase at a slower rate, with increasing convective vigour, than it does elsewhere. Model heat-flux scalings are used to assess the degree to which continental geotherms can be time-stabilized by this means. Results suggest that the thermal surface heterogeneity imposed on the mantle by the presence of continental crust can prevent wide-scale crustal melting in the Archaean by forcing oceans to carry a greater proportion of the Earth’s heat-loss load in the past.