Metal-silicate fractionation in the deep magma ocean and light elements in the core

The bulk silicate Earth (BSE), and all its sampleable reservoirs, have a subchondritic niobium-to-tantalum ratio (Nb/Ta). Because both elements are refractory, and Nb/Ta is fairly constant across chondrite groups, this can only be explained by a preferential sequestration of Nb relative to Ta in a hidden (unsampled) reservoir. Experiments have shown that Nb becomes more siderophile than Ta under very reducing conditions, leading the way for the accepted hypothesis that Earth’s core could have stripped sufficient amounts of Nb during its formation to account for the subchondritic signature of the BSE. Consequently, this suggestion has been used as an argument that Earth accreted and differentiated, for most of its history, under very reducing conditions. Here, we present a series of metal–silicate partitioning experiments of Nb and Ta in a laser-heated diamond anvil cell, at pressure and temperature conditions directly comparable to those of core formation; we find that Nb is more siderophile than Ta under any conditions relevant to a deep magma ocean, confirming that BSE’s missing Nb is in the core. However, multistage core formation modeling only allows for moderately reducing or oxidizing accretionary conditions, ruling out the need for very reducing conditions, which lead to an overdepletion of Nb from the mantle (and a low Nb/Ta ratio) that is incompatible with geochemical observations. Earth’s primordial magma ocean cannot have contained less than 2% or more than 18% FeO since the onset of core formation.

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Experiments on metal–silicate plumes and core formation

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2008.0194 ◽

2008 ◽

Vol 366 (1883) ◽

pp. 4253-4271 ◽

Cited By ~ 16

Author(s):

Peter Olson ◽

Dayanthie Weeraratne

Keyword(s):

Lower Layer ◽

High Viscosity ◽

Primitive Mantle ◽

Liquid Gallium ◽

Magma Ocean ◽

Core Formation ◽

Thermal Plumes ◽

The Core ◽

Metal Silicate ◽

Sucrose Solutions

Short-lived isotope systematics, mantle siderophile abundances and the power requirements of the geodynamo favour an early and high-temperature core-formation process, in which metals concentrate and partially equilibrate with silicates in a deep magma ocean before descending to the core. We report results of laboratory experiments on liquid metal dynamics in a two-layer stratified viscous fluid, using sucrose solutions to represent the magma ocean and the crystalline, more primitive mantle and liquid gallium to represent the core-forming metals. Single gallium drop experiments and experiments on Rayleigh–Taylor instabilities with gallium layers and gallium mixtures produce metal diapirs that entrain the less viscous upper layer fluid and produce trailing plume conduits in the high-viscosity lower layer. Calculations indicate that viscous dissipation in metal–silicate plumes in the early Earth would result in a large initial core superheat. Our experiments suggest that metal–silicate mantle plumes facilitate high-pressure metal–silicate interaction and may later evolve into buoyant thermal plumes, connecting core formation to ancient hotspot activity on the Earth and possibly on other terrestrial planets.

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Accretion and core formation: constraints from metal–silicate partitioning

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2008.0115 ◽

2008 ◽

Vol 366 (1883) ◽

pp. 4339-4355 ◽

Cited By ~ 26

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.

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A synthesis of geochemical constraints on the inventory of light elements in the core of Mars

Icarus ◽

10.1016/j.icarus.2018.06.023 ◽

2018 ◽

Vol 315 ◽

pp. 69-78 ◽

Cited By ~ 6

Author(s):

Edgar S. Steenstra ◽

Wim van Westrenen

Keyword(s):

Light Elements ◽

The Core ◽

Geochemical Constraints

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Experiments on turbulent metal-silicate mixing in a magma ocean

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2011.08.041 ◽

2011 ◽

Vol 310 (3-4) ◽

pp. 303-313 ◽

Cited By ~ 55

Author(s):

Renaud Deguen ◽

Peter Olson ◽

Philippe Cardin

Keyword(s):

Magma Ocean ◽

Metal Silicate

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Metal-silicate-water reaction under high pressure — Formation of metal hydride and implications for composition of the core and mantle

Chemical Geology ◽

10.1016/0009-2541(88)90060-5 ◽

1988 ◽

Vol 71 (4) ◽

pp. 365

Author(s):

S. Akimoto ◽

T. Suzuki

Keyword(s):

High Pressure ◽

Metal Hydride ◽

The Core ◽

Metal Silicate

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Inefficient compaction in small planetary cores -- application to the Moon

10.5194/egusphere-egu21-14959 ◽

2021 ◽

Author(s):

Marine Lasbleis

Keyword(s):

Magnetic Field ◽

Latent Heat ◽

Energy Budget ◽

Inner Core ◽

Light Elements ◽

The Moon ◽

Small Bodies ◽

Typical Size ◽

The Core ◽

Planetary Cores

<div> Growth of the solid inner core is generally considered to power the Earth's present geodynamo. Cristallisation of a solid central inner core has also been proposed to drive the lunar dynamo and to generate a magnetic field in smaller bodies. In a previous work, we estimated the compaction of planetary cores for different scenarios of growth (with or without supercooling) and different sizes of the inner core. Our main results indicated that small inner cores are unlikely to compact efficiently the liquid trapped during the first steps of the growth. This is especially true for small bodies for which the typical size of the core is similar to the compaction length. The light elements are thus trapped during the cristallisation, reducing the release of latent heat and of light elements. We present here a model to include the effect of an inefficient compaction in the energy budget of a planetary core and investigate the implications for the dynamo evolution in small bodies. We apply this model for the evolution of the core of the Moon. </div>

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Solubility of carbon and nitrogen in a sulfur-bearing iron melt: Constraints for siderophile behavior at upper mantle conditions

American Mineralogist ◽

10.2138/am-2019-7103 ◽

2019 ◽

Vol 104 (12) ◽

pp. 1857-1865 ◽

Cited By ~ 1

Author(s):

Alexander G. Sokol ◽

Alexander F. Khokhryakov ◽

Yuri M. Borzdov ◽

Igor N. Kupriyanov ◽

Yuri N. Palyanov

Keyword(s):

Liquid Iron ◽

Upper Mantle ◽

Carbon Budget ◽

Iron Alloy ◽

Stability Field ◽

Magma Ocean ◽

Carbon Solubility ◽

Carbon And Nitrogen ◽

Liquid Iron Alloy ◽

The Core

Abstract Carbon solubility in a liquid iron alloy containing nitrogen and sulfur has been studied experimentally in a carbon-saturated Fe-C-N-S-B system at pressures of 5.5 and 7.8 GPa, temperatures of 1450 to 1800 °C, and oxygen fugacities from the IW buffer to log fO2 ΔIW-6 (ΔIW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of iron and wüstite). Carbon saturation of Fe-rich melts at 5.5 and 7.8 GPa maintains crystallization of flaky graphite and diamond. Diamond containing 2100–2600 ppm N and 130–150 ppm B crystallizes in equilibrium with BN within the diamond stability field at 7.8 GPa and 1600 to 1800 °C, while graphite forms at other conditions. The solubility of carbon in the C-saturated metal melt free from nitrogen and sulfur is 6.2 wt% C at 7.8 GPa and 1600 °C and decreases markedly with increasing nitrogen. A 1450–1600 °C graphite-saturated iron melt with 6.2–8.8 wt% N can dissolve: 3.6–3.9 and 1.4–2.5 wt% C at 5.5 and 7.8 GPa, respectively. However, the melt equilibrated with boron nitride and containing 1–1.7 wt% sulfur and 500–780 ppm boron dissolves twice less nitrogen while the solubility of carbon remains relatively high (3.8–5.2 wt%). According to our estimates, nitrogen partitions between diamond and the iron melt rich in volatiles at DNDm/Met=0.013−0.024. The pressure increase in the Fe-C-N system affects iron affinity of N and C: it increases in nitrogen but decreases in carbon. The reduction of C solubility in a Fe-rich melt containing nitrogen and sulfur may have had important consequences in the case of imperfect equilibration between the core and the mantle during their separation in the early Earth history. The reduction of C solubility allowed C supersaturation of the liquid iron alloy and crystallization of graphite and diamond. The carbon phases could float in the segregated core liquid and contribute to the carbon budget of the overlying silicate magma ocean. Therefore, the process led to the formation of graphite and diamond, which were the oldest carbon phases in silicate mantle.

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Linking the core heat content to Earth's accretion history

10.5194/egusphere-egu2020-10540 ◽

2020 ◽

Author(s):

Renaud Deguen ◽

Vincent Clési

Keyword(s):

Thermal State ◽

Thermal Evolution ◽

Heat Content ◽

Magma Ocean ◽

The Core ◽

Current State ◽

Partitioning Coefficients ◽

The Mean ◽

Mantle Diapirism ◽

Do So

The composition of Earth's mantle, when compared to experimentally determined partitioning coefficients, can be used to constrain the conditions of equilibration - pressure P, temperature T, and oxygen fugacity fO2 - of the metal and silicates during core-mantle differentiation. This places constraints on the thermal state of the planet during its accretion, and it is tempting to try to use these data to estimate the heat content of the core at the end of accretion. To do so, we develop an analytical model of the thermal evolution of the metal phase during its descent through the solid mantle toward the growing core, taking into account compression heating, &#160; viscous dissipation heating, and heat exchange with the surrounding silicates. For each impact, the model takes as initial condition the pressure and temperature at the base of the magma ocean, and gives the temperature of the metal when it reaches the core. The growth of the planet results in additional pressure increase and compression heating of the core. The thermal model is coupled to a Monte-Carlo inversion of the metal/silicates equilibration conditions (P, T, fO2) in the course of accretion from the abundance of Ni, Co, V and Cr in the mantle, and provides an estimate of the core heat content at the end of accretion for each geochemically successful accretion. The core heat content depends on the mean degree of metal-silicates equilibration, on the mode of metal/silicates separation in the mantle (diapirism, percolation, or dyking), but also very significantly on the shape of the equilibration conditions curve (equilibration P and T vs. fraction of Earth accreted). We find that many accretion histories which are successful in reproducing the mantle composition yield a core that is colder than its current state. Imposing that the temperature of the core at the end of accretion is higher than its current values therefore provides strong constraints on the accretion history. In particular, we find that the core heat content depends significantly on the last stages of accretion.&#160;

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