The long-term evolution of the Earth mantle with a basal magma ocean

2021 ◽  
Author(s):  
Stephane Labrosse ◽  
Adrien Morison ◽  
Daniela Bolrão ◽  
Antoine Rozel ◽  
Maxim Ballmer ◽  
...  
Keyword(s):  
Phase Equilibrium ◽  
Mantle Convection ◽  
Large Scale ◽  
Seismic Velocity ◽  
Fractional Crystallisation ◽  
Magma Ocean ◽  
Compositional Evolution ◽  
The Core ◽  
The Earth ◽  
Flow Through

<p>The early evolution of the Earth was likely affected by a large scale magma ocean, in particular in the aftermath of the giant impact that formed the Moon. The exact structure and dynamics of the Earth following that event is unknown but several possible scenarios feature the existence of a basal magma ocean (BMO), whose last remaining drops may explain the current seismically detected ultra low velocity zones. The presence of a BMO covering the core carries many implications for the dynamics and evolution of the overlying solid mantle. The phase equilibrium between the magma and the solid mantle allows matter to flow through the boundary by melting and freezing. In practice, convective stresses in the solid create a topography of the interface which displaces the equilibrium. Heat and solute transfer in the liquid acts to erase this topography and, if this process is faster than that the producing topography, the boundary appears effectively permeable to flow. This leads to convective motions much faster than in usual mantle convection. We developed a mantle convection model coupled to a model for the thermal and compositional evolution of the BMO and the core that takes into account the phase equilibrium at the bottom of the solid mantle. It also includes the fractional crystallisation at the interface and net freezing of the magma ocean. Early in the history, convection in the mantle is very fast and dominated by down-welling currents. As fractional crystallisation proceeds, the magma ocean gets enriched in FeO which makes the cumulate to also get richer. Eventually, it becomes too dense to get entrained by mantle convection and starts to pile up at the bottom of the mantle, which inhibits direct mass flow through the phase change boundary. This allows a thermal boundary layer and hot plumes to develop.</p><p>This model therefore allows to explain the present existence of both residual partial melt and large scale compositional variations in the lower mantle, as evidenced by seismic velocity anomalies. It also predicts a regime change between early mantle convection dominated by down-welling flow to the onset of hot plumes in the more recent past.</p>

Ups and Downs

2018 ◽  
Author(s):  
Roy Livermore
Keyword(s):  
Mantle Convection ◽  
Laboratory Experiments ◽  
Inner Core ◽  
Computer Modelling ◽  
Great Depth ◽  
New Horizons ◽  
The Core ◽  
The Earth ◽  
The World ◽  
The 1960S

Despite the dumbing-down of education in recent years, it would be unusual to find a ten-year-old who could not name the major continents on a map of the world. Yet how many adults have the faintest idea of the structures that exist within the Earth? Understandably, knowledge is limited by the fact that the Earth’s interior is less accessible than the surface of Pluto, mapped in 2016 by the NASA New Horizons spacecraft. Indeed, Pluto, 7.5 billion kilometres from Earth, was discovered six years earlier than the similar-sized inner core of our planet. Fortunately, modern seismic techniques enable us to image the mantle right down to the core, while laboratory experiments simulating the pressures and temperatures at great depth, combined with computer modelling of mantle convection, help identify its mineral and chemical composition. The results are providing the most rapid advances in our understanding of how this planet works since the great revolution of the 1960s.

scholarly journals Plate-tectonic evolution of the Earth: bottom-up and top-down mantle circulation

2016 ◽  
Vol 53 (11) ◽  
pp. 1103-1120 ◽  
Author(s):  
W.G. Ernst ◽  
Norman H. Sleep ◽  
Tatsuki Tsujimori
Keyword(s):  
Mantle Convection ◽  
Ultrahigh Pressure ◽  
Return Flow ◽  
Plate Tectonic ◽  
Magma Ocean ◽  
Top Down ◽  
Bottom Up ◽  
Near Surface ◽  
The Earth ◽  
Over Time

Intense devolatilization and chemical-density differentiation attended accretion of planetesimals on the primordial Earth. These processes gradually abated after cooling and solidification of an early magma ocean. By 4.3 or 4.2 Ga, water oceans were present, so surface temperatures had fallen far below low-pressure solidi of dry peridotite, basalt, and granite, ∼1300, ∼1120, and ∼950 °C, respectively. At less than half their T solidi, rocky materials existed as thin lithospheric slabs in the near-surface Hadean Earth. Stagnant-lid convection may have occurred initially but was at least episodically overwhelmed by subduction because effective, massive heat transfer necessitated vigorous mantle overturn in the early, hot planet. Bottom-up mantle convection, including voluminous plume ascent, efficiently rid the Earth of deep-seated heat. It declined over time as cooling and top-down lithospheric sinking increased. Thickening and both lateral extensional + contractional deformation typified the post-Hadean lithosphere. Stages of geologic evolution included (i) 4.5–4.4 Ga, magma ocean overturn involved ephemeral, surficial rocky platelets; (ii) 4.4–2.7 Ga, formation of oceanic and small continental plates were obliterated by return mantle flow prior to ∼4.0 Ga; continental material gradually accumulated as largely sub-sea, sialic crust-capped lithospheric collages; (iii) 2.7–1.0 Ga, progressive suturing of old shields + younger orogenic belts led to cratonal plates typified by emerging continental freeboard, increasing sedimentary differentiation, and episodic glaciation during transpolar drift; onset of temporally limited stagnant-lid mantle convection occurred beneath enlarging supercontinents; (iv) 1.0 Ga–present, laminar-flowing asthenospheric cells are now capped by giant, stately moving plates. Near-restriction of komatiitic lavas to the Archean, and appearance of multicycle sediments, ophiolite complexes ± alkaline igneous rocks, and high-pressure–ultrahigh-pressure (HP–UHP) metamorphic belts in progressively younger Proterozoic and Phanerozoic orogens reflect increasing negative buoyancy of cool oceanic lithosphere, but decreasing subductability of enlarging, more buoyant continental plates. Attending supercontinental assembly, density instabilities of thickening oceanic plates began to control overturn of suboceanic mantle as cold, top-down convection. Over time, the scales and dynamics of hot asthenospheric upwelling versus lithospheric foundering + mantle return flow (bottom-up plume-driven ascent versus top-down plate subduction) evolved gradually, reflecting planetary cooling. These evolving plate-tectonic processes have accompanied the Earth’s thermal history since ∼4.4 Ga.

scholarly journals Properties of the Core-Mantle Boundary

1988 ◽  
Vol 129 ◽  
pp. 377-377
Author(s):  
Bradford H. Hager
Keyword(s):  
Mechanical Properties ◽  
Thermal Boundary Layer ◽  
Seismic Velocity ◽  
Velocity Versus ◽  
Compositional Gradient ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Earth ◽  
Thick Region

The core-mantle boundary (CMB), separating the molten metallic core from the overlying solid silicate mantle, marks the largest discontinuity in mechanical properties within the Earth. The ∼ 200 km thick region just above the CMB, named D″ by Bullen (1950), is characterized by an anomalous gradient in seismic velocity versus depth. D″ was originally interpreted as a region with a strong compositional gradient due to the accumulation of dense material at the base of the mantle. Subsequently, the anomalous gradient was interpreted as the result of a strong temperature gradient in a hot thermal boundary layer at the base of the mantle, an interpretation motivated by the requiremnet that heat involved in generating the geodynamo must be transported out of the core and through the mantle by convection.

scholarly journals Timescales of chemical equilibrium between the convecting solid mantle and over- and underlying magma oceans

Solid Earth ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 421-437
Author(s):  
Daniela Paz Bolrão ◽  
Maxim D. Ballmer ◽  
Adrien Morison ◽  
Antoine B. Rozel ◽  
Patrick Sanan ◽  
...  
Keyword(s):  
Chemical Equilibrium ◽  
Large Scale ◽  
Numerical Models ◽  
Solid Material ◽  
Fractional Crystallisation ◽  
Chemical Fractionation ◽  
Magma Ocean ◽  
Material Transfer ◽  
Magma Oceans

Abstract. After accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core–mantle boundary and grows upwards or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle as well as its present-day dynamics and composition. In this work, we use numerical models to study convection in a solid mantle bounded at one or both boundaries by magma ocean(s) and, in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re)melting of solid material at the interface between these reservoirs. When these crystallisation and remelting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle–magma ocean boundaries promotes chemical equilibrium and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies (Lebrun et al., 2013), which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints.

Accretion and core formation: constraints from metal–silicate partitioning

2008 ◽  
Vol 366 (1883) ◽  
pp. 4339-4355 ◽  
Author(s):  
Bernard J Wood
Keyword(s):  
Oxidation State ◽  
Maximum Pressure ◽  
Magma Ocean ◽  
Core Formation ◽  
The Core ◽  
Refractory Elements ◽  
The Earth ◽  
Metal Silicate ◽  
Restricted Volume ◽  
Si Content

Experimental metal–silicate partitioning data for Ni, Co, V, Cr, Nb, Mn, Si and W were used to investigate the geochemical consequences of a range of models for accretion and core formation on Earth. The starting assumptions were chondritic ratios of refractory elements in the Earth and the segregation of metal at the bottom of a magma ocean, which deepened as the planet grew and which had, at its base, a temperature close to the liquidus of the silicate. The models examined were as follows. (i) Continuous segregation from a mantle which is chemically homogeneous and which has a fixed oxidation state, corresponding to 6.26 per cent oxidized Fe. Although Ni, Co and W partitioning is consistent with chondritic ratios, the current V content of the silicate Earth cannot be reconciled with core segregation under these conditions of fixed oxidation state. (ii) Continuous segregation from a mantle which is chemically homogeneous but in which the Earth became more oxidized as it grew. In this case, the Ni, Co, W, V, Cr and Nb contents of core and mantle are easily matched to those calculated from the chondritic ratios of refractory elements. The magma ocean is calculated to maintain a thickness approximately 35 per cent of the depth to the core–mantle boundary in the accreting Earth, yielding a maximum pressure of 44 GPa. This model yields a Si content of the core of 5.7 per cent, in good agreement with cosmochemical estimates and with recent isotopic data. (iii) Continuous segregation from a mantle which is not homogeneous and in which the core equilibrates with a restricted volume of mantle at the base of the magma ocean. This is found to increase depth of the magma ocean by approximately 50 per cent. All of the other elements (except Mn) have partitioning consistent with chondritic abundances in the Earth, provided the Earth became, as before, progressively oxidized during accretion. (iv) Continuous segregation of metal from a crystal-melt mush. In this case, pressures decrease to a maximum of 31 GPa and it is extremely difficult to match the calculated mantle contents of the highly incompatible elements Nb and W to those observed. Progressive oxidation is required to fit the observed mantle contents of vanadium. All of the scenarios discussed above point to progressive oxidation having occurred as the Earth grew. The Earth appears to be depleted in Mn relative to the chondritic reference.

scholarly journals Mantle plumes and mantle dynamics in the Wilson cycle

2019 ◽  
Vol 470 (1) ◽  
pp. 87-103 ◽  
Author(s):  
Philip J. Heron
Keyword(s):  
Mantle Convection ◽  
Large Scale ◽  
Thermal Evolution ◽  
Mantle Flow ◽  
Mantle Plumes ◽  
Mantle Dynamics ◽  
The Core ◽  
Deep Mantle ◽  
Igneous Provinces ◽  
Wilson Cycle

AbstractThis review discusses the thermal evolution of the mantle following large-scale tectonic activities such as continental collision and continental rifting. About 300 myr ago, continental material amalgamated through the large-scale subduction of oceanic seafloor, marking the termination of one or more oceanic basins (e.g. Wilson cycles) and the formation of the supercontinent Pangaea. The present day location of the continents is due to the rifting apart of Pangaea, with the dispersal of the supercontinent being characterized by increased volcanic activity linked to the generation of deep mantle plumes. The discussion presented here investigates theories regarding the thermal evolution of the mantle (e.g. mantle temperatures and sub-continental plumes) following the formation of a supercontinent. Rifting, orogenesis and mass eruptions from large igneous provinces change the landscape of the lithosphere, whereas processes related to the initiation and termination of oceanic subduction have a profound impact on deep mantle reservoirs and thermal upwelling through the modification of mantle flow. Upwelling and downwelling in mantle convection are dynamically linked and can influence processes from the crust to the core, placing the Wilson cycle and the evolution of oceans at the forefront of our dynamic Earth.

The initial condition for the long-term evolution of terrestrial planets as determined by Reactive Freezing of the Basal Magma Ocean

2020 ◽  
Author(s):  
Maxim Ballmer ◽  
Rob Spaargaren ◽  
Ananya Mallik ◽  
Daniela Bolrão ◽  
Adrien Morison ◽  
...  
Keyword(s):  
Fractional Crystallization ◽  
Mantle Convection ◽  
Terrestrial Planets ◽  
Initial Condition ◽  
Magma Ocean ◽  
Molten Layer ◽  
Compositional Evolution ◽  
Mantle Boundary ◽  
Reactive Crystallization

<p>Terrestrial planets evolve through various stages of large-scale melting, or magma oceans, due to the energy release during accretion and differentiation. Any magma ocean is thought to become progressively enriched in FeO and incompatible elements upon freezing due to fractional crystallization. The resulting upwards enrichment of the related cumulate (=crystal) packages drives gravitational overturn(s) of the incipient mantle, and ultimately stabilizes a FeO-enriched molten layer at the core-mantle-boundary (CMB)<sup>1</sup>. Such a molten layer, previously termed basal magma ocean (BMO)<sup>2</sup>, is thought to also fractionally crystallize, but downwards instead of upwards, and over much longer timescales than the surficial magma ocean. This BMO fractional crystallization due to slow planetary cooling analogously implies the stabilization of a thick FeO-enriched layer at the CMB. Such a layer would essentially remain stable forever, as being too dense to be entrained by convection of the overlying mantle. However, at least for Earth, geophysical observations rule out the preservation of such a deep dense global layer. Here, we investigate the consequences of an alternative mechanism for BMO freezing, reactive crystallization, on the initial condition of solid-state mantle convection and long-term planetary evolution.</p><p>Based on scaling relationships, we show that any cumulates, which crystallize from the BMO (e.g., due to initial cooling or reaction) are readily entrained by mantle convection. Once the BMO-mantle boundary is exposed, the BMO reacts with the mantle to form reactive cumulates. Reaction is driven by disequilibrium between mantle rocks and the BMO, a situation that is inevitable independent of BMO initial composition. As reactive cumulates are continuously entrained by mantle convection, the BMO continues to freeze by reactive crystallization. Based on lower-mantle mineral-melt phase equilibria<sup>3</sup>, we calculate the compositional evolution of the BMO, and the chemistry of the BMO cumulate package. We demonstrate that for a wide range of BMO initial compositions, the cumulate package consists of two discrete layers: the first is pure bridgmanite close to the MgSiO<sub>3</sub> end-member; the second is mostly bridgmanite+ferropericlase that is moderately enriched in FeO and incompatibles, i.e. similar in composition to FeO-enriched pyrolite. The mass or thickness of the cumulate package depends on reaction kinetics, but is significantly larger than that of the BMO. The bridgmanitic layer is expected to be entrained by mantle convection due to its intrinsic buoyancy, but resist efficient mixing due to its intrinsic strength, thereby potentially providing an explanation for seismic scatterers/reflectors and ancient geochemical reservoirs<sup>4</sup>. The moderately FeO-enriched layer is expected to stabilize thermochemical piles, providing a candidate origin for the seismically-observed large low shear velocity provinces (LLSVPs)<sup>5</sup>.</p><p>These results have implications for the long-term (thermal) evolution of planets in general. Earth-sized terrestrial (exo-)planets and super-Earths should also initially host a MgSiO<sub>3</sub>-rich layer as well as a moderately FeO-enriched layer. In contrast, small terrestrial planets such as Mars may host a more strongly Fe-rich deep dense global layer as long as no BMO is stabilized in their histories.</p><p>[1] Ballmer+, G-cubed 2017; [2] Labrosse+, Nature 2007; [3] Boukaré+, JGR Solid-Earth 2015; [4] Ballmer+, Nat.Geosci. 2017; [5] Ballmer+, G-cubed 2016.</p><p> </p>

scholarly journals Timescales of chemical equilibrium between the convecting solid mantle and over-/underlying magma oceans

2020 ◽  
Author(s):  
Daniela Paz Bolrão ◽  
Maxim Dionys Ballmer ◽  
Adrien Morison ◽  
Antoine Billy Rozel ◽  
Patrick Sanan ◽  
...  
Keyword(s):  
Chemical Equilibrium ◽  
Large Scale ◽  
Numerical Models ◽  
Solid Material ◽  
Fractional Crystallisation ◽  
Chemical Fractionation ◽  
Magma Ocean ◽  
Material Transfer ◽  
Magma Oceans

Abstract. After accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core-mantle boundary and grows upwards, or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle, as well as its present-day dynamics and composition. In this work we use numerical models to study convection in a solid mantle bounded at either or both boundaries by magma ocean(s), and in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re-)melting of solid material at the interface between these reservoirs. When these crystallisation/re-melting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle magma-ocean boundaries promotes chemical equilibrium, and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies (Lebrun et al., 2013), which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints.

STUDY OF ENVIRONMENTAL AWARENESS AMONG SENIOR SECONDARY SCHOOL STUDENTS IN RELATION TO THEIR GENDER AND ACADEMIC STREAM

2015 ◽  
Vol 3 (2) ◽  
pp. 262-265
Author(s):  
Dr.Navdeep Kaur
Keyword(s):  
Environmental Education ◽  
Natural Resources ◽  
Large Scale ◽  
Human Life ◽  
Environmental Awareness ◽  
Human Beings ◽  
School Students ◽  
The Earth ◽  
Stages Of Learning ◽  
The Masses

Since its evolution environment has remained both a matter of awe and concern to man. The frontier attitude of the industrialized society towards nature has not only endangered the survival of all other life forms but also threatened the very existence of human life. The realization of such potential danger has necessitated the dissemination of knowledge and skill vis-a-vis environment protection at all stages of learning. Therefore, learners of all stages of learning need to be sensitized with a missionary zeal. This may ensure transformation of students into committed citizens for averting global environment crisis. The advancement of science and technology made the life more and more relaxed and man also became more and more ambitious. With such development, human dependence on environment increased. He consumed more resources and the effect of his activities on the environment became more and more detectable. Environment covers all the things present around the living beings and above the land, on the surface of the earth and under the earth. Environment indicates, in total, all of peripheral forces, pressures and circumstances, which affect the life, nature, behaviour, growth, development and maturation of living beings. Irrational exploitation (not utilization) of natural resources for our greed (not need) has endangered our survival, and incurred incalculable harm. Environmental Education is a science, a well-thought, permanent, lasting and integrated process of equipping learning experiences for getting awareness, knowledge, understanding, skills, values, technical expertise and involvement of learners with desirable attitudinal changes about their relationship with their natural and biophysical environment. Environmental Education is an organized effort to educate the masses about environment, its functions, need, importance, and especially how human beings can manage their behaviour in order to live in a sustainable manner.  The term 'environmental awareness' refers to creating general awareness of environmental issues, their causes by bringing about changes in perception, attitude, values and necessary skills to solve environment related problems. Moreover, it is the first step leading to the formation of responsible environmental behaviour (Stern, 2000). With the ever increasing development by modern man, large scale degradation of natural resources have been occurred, the public has to be educated about the fact that if we are degrading our environment we are actually harming ourselves. To encourage meaningful public participation and environment, it is necessary to create awareness about environment pollution and related adverse effects. This is the crucial time that environmental awareness and environmental sensitivity should be cultivated among the masses particularly among youths. For the awareness of society it is essential to work at a gross root level. So the whole society can work to save the environment.

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