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 The niobium and tantalum concentration in the mantle constrains the composition of Earth’s primordial magma ocean

2020 ◽  
Vol 117 (45) ◽  
pp. 27893-27898
Author(s):  
Dongyang Huang ◽  
James Badro ◽  
Julien Siebert
Keyword(s):  
Diamond Anvil Cell ◽  
Magma Ocean ◽  
Core Formation ◽  
The Core ◽  
Bulk Silicate Earth ◽  
Reducing Conditions ◽  
Metal Silicate ◽  
Diamond Anvil ◽  
Laser Heated ◽  
Tantalum Concentration

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.

The redox state of the mantle during and just after core formation

2008 ◽  
Vol 366 (1883) ◽  
pp. 4315-4337 ◽  
Author(s):  
D.J Frost ◽  
U Mann ◽  
Y Asahara ◽  
D.C Rubie
Keyword(s):  
Oxygen Fugacity ◽  
Oxidation State ◽  
Upper Mantle ◽  
Partition Coefficients ◽  
Metal Loss ◽  
Core Formation ◽  
The Core ◽  
Mantle Mineral ◽  
Metal Silicate ◽  
Siderophile Elements

Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal–silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe 2 O 3 content of the mantle that probably occurred in the first 1 Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe 2 O 3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal–silicate partition coefficients for O and Si, we have modelled core–mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O 3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe 2 O 3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe 2 O 3 during core formation, FeO would have disproportionated to produce Fe 2 O 3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe 2 O 3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

Experiments on metal–silicate plumes and core formation

2008 ◽  
Vol 366 (1883) ◽  
pp. 4253-4271 ◽  
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.

scholarly journals Experimental evidence for hydrogen incorporation into Earth’s core

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Shoh Tagawa ◽  
Naoya Sakamoto ◽  
Kei Hirose ◽  
Shunpei Yokoo ◽  
John Hernlund ◽  
...  
Keyword(s):  
Terrestrial Planet ◽  
Outer Core ◽  
Core Formation ◽  
Earth’S Core ◽  
Hydrogen Incorporation ◽  
Earth's Core ◽  
The Core ◽  
The Earth ◽  
Metal Silicate ◽  
Important Constituent

AbstractHydrogen is one of the possible alloying elements in the Earth’s core, but its siderophile (iron-loving) nature is debated. Here we experimentally examined the partitioning of hydrogen between molten iron and silicate melt at 30–60 gigapascals and 3100–4600 kelvin. We find that hydrogen has a metal/silicate partition coefficient DH ≥ 29 and is therefore strongly siderophile at conditions of core formation. Unless water was delivered only in the final stage of accretion, core formation scenarios suggest that 0.3–0.6 wt% H was incorporated into the core, leaving a relatively small residual H2O concentration in silicates. This amount of H explains 30–60% of the density deficit and sound velocity excess of the outer core relative to pure iron. Our results also suggest that hydrogen may be an important constituent in the metallic cores of any terrestrial planet or moon having a mass in excess of ~10% of the Earth.

Effect of H2O on metal–silicate partitioning of Ni, Co, V, Cr, Mn and Fe: Implications for the oxidation state of the Earth and Mars

2016 ◽  
Vol 192 ◽  
pp. 97-121 ◽  
Author(s):  
V. Clesi ◽  
M.A. Bouhifd ◽  
N. Bolfan-Casanova ◽  
G. Manthilake ◽  
A. Fabbrizio ◽  
...  
Keyword(s):  
Oxidation State ◽  
The Earth ◽  
Metal Silicate

scholarly journals Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements

Science ◽  
2012 ◽  
Vol 336 (6077) ◽  
pp. 72-75 ◽  
Author(s):  
Christopher W. Dale ◽  
Kevin W. Burton ◽  
Richard C. Greenwood ◽  
Abdelmouhcine Gannoun ◽  
Jonathan Wade ◽  
...  
Keyword(s):  
Body Size ◽  
Solar System ◽  
Oxidation State ◽  
Parent Body ◽  
The Body ◽  
The Moon ◽  
Core Formation ◽  
Highly Siderophile Elements ◽  
The Core ◽  
Siderophile Elements

Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.

scholarly journals The carbon content of Earth and its core

2020 ◽  
Vol 117 (16) ◽  
pp. 8743-8749 ◽  
Author(s):  
Rebecca A. Fischer ◽  
Elizabeth Cottrell ◽  
Erik Hauri ◽  
Kanani K. M. Lee ◽  
Marion Le Voyer
Keyword(s):  
Carbonaceous Chondrite ◽  
Magma Ocean ◽  
Core Formation ◽  
Earth’S Core ◽  
Multistage Model ◽  
Earth's Core ◽  
Metal Silicate ◽  
Diamond Anvil ◽  
Laser Heated ◽  
Carbon Inventory

Earth’s core is likely the largest reservoir of carbon (C) in the planet, but its C abundance has been poorly constrained because measurements of carbon’s preference for core versus mantle materials at the pressures and temperatures of core formation are lacking. Using metal–silicate partitioning experiments in a laser-heated diamond anvil cell, we show that carbon becomes significantly less siderophile as pressures and temperatures increase to those expected in a deep magma ocean during formation of Earth’s core. Based on a multistage model of core formation, the core likely contains a maximum of 0.09(4) to 0.20(10) wt% C, making carbon a negligible contributor to the core’s composition and density. However, this accounts for ∼80 to 90% of Earth’s overall carbon inventory, which totals 370(150) to 740(370) ppm. The bulk Earth’s carbon/sulfur ratio is best explained by the delivery of most of Earth’s volatiles from carbonaceous chondrite-like precursors.

Isotopes as clues to the origin and earliest differentiation history of the Earth

2008 ◽  
Vol 366 (1883) ◽  
pp. 4129-4162 ◽  
Author(s):  
Stein B Jacobsen ◽  
Michael C Ranen ◽  
Michael I Petaev ◽  
John L Remo ◽  
Richard J O'Connell ◽  
...  
Keyword(s):  
Time Scale ◽  
Magma Ocean ◽  
Crust Formation ◽  
Early Atmosphere ◽  
The Earth ◽  
Metal Silicate ◽  
Deep Earth ◽  
History Of ◽  
Mean Time ◽  
Late Veneer

Measurable variations in 182 W/ 183 W, 142 Nd/ 144 Nd, 129 Xe/ 130 Xe and 136 Xe Pu / 130 Xe in the Earth and meteorites provide a record of accretion and formation of the core, early crust and atmosphere. These variations are due to the decay of the now extinct nuclides 182 Hf, 146 Sm, 129 I and 244 Pu. The l82 Hf– 182 W system is the best accretion and core-formation chronometer, which yields a mean time of Earth's formation of 10 Myr, and a total time scale of 30 Myr. New laser shock data at conditions comparable with those in the Earth's deep mantle subsequent to the giant Moon-forming impact suggest that metal–silicate equilibration was rapid enough for the Hf–W chronometer to reliably record this time scale. The coupled 146 Sm– 147 Sm chronometer is the best system for determining the initial silicate differentiation (magma ocean crystallization and proto-crust formation), which took place at ca 4.47 Ga or perhaps even earlier. The presence of a large 129 Xe excess in the deep Earth is consistent with a very early atmosphere formation (as early as 30 Myr); however, the interpretation is complicated by the fact that most of the atmospheric Xe may be from a volatile-rich late veneer.

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