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

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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The carbon content of Earth and its core

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1919930117 ◽

2020 ◽

Vol 117 (16) ◽

pp. 8743-8749 ◽

Cited By ~ 4

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.

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Modelling Mie scattering in pyrolite in the laser-heated diamond anvil cell: Implications to the core-mantle boundary temperature determination

Physics of The Earth and Planetary Interiors ◽

10.1016/j.pepi.2021.106773 ◽

2021 ◽

pp. 106773

Author(s):

Sergey S. Lobanov ◽

Sergio Speziale ◽

Sascha Brune

Keyword(s):

Diamond Anvil Cell ◽

Mie Scattering ◽

Mantle Boundary ◽

Boundary Temperature ◽

Temperature Determination ◽

Core Mantle Boundary ◽

The Core ◽

Diamond Anvil ◽

Laser Heated

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Partitioning experiments in the laser-heated diamond anvil cell: volatile content in the Earth's core

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

10.1098/rsta.2008.0226 ◽

2008 ◽

Vol 366 (1883) ◽

pp. 4295-4314 ◽

Cited By ~ 12

Author(s):

Andrew P Jephcoat ◽

M. Ali Bouhifd ◽

Don Porcelli

Keyword(s):

Trace Elements ◽

Diamond Anvil Cell ◽

The Core ◽

The Earth ◽

Wide Range ◽

Recent Developments ◽

Mantle Reservoir ◽

History Of ◽

Diamond Anvil ◽

Laser Heated

The present state of the Earth evolved from energetic events that were determined early in the history of the Solar System. A key process in reconciling this state and the observable mantle composition with models of the original formation relies on understanding the planetary processing that has taken place over the past 4.5 Ga. Planetary size plays a key role and ultimately determines the pressure and temperature conditions at which the materials of the early solar nebular segregated. We summarize recent developments with the laser-heated diamond anvil cell that have made possible extension of the conventional pressure limit for partitioning experiments as well as the study of volatile trace elements. In particular, we discuss liquid–liquid, metal–silicate (M–Sil) partitioning results for several elements in a synthetic chondritic mixture, spanning a wide range of atomic number—helium to iodine. We examine the role of the core as a possible host of both siderophile and trace elements and the implications that early segregation processes at deep magma ocean conditions have for current mantle signatures, both compositional and isotopic. The results provide some of the first experimental evidence that the core is the obvious replacement for the long-sought, deep mantle reservoir. If so, they also indicate the need to understand the detailed nature and scale of core–mantle exchange processes, from atomic to macroscopic, throughout the age of the Earth to the present day.

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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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Reconciling metal–silicate partitioning and late accretion in the Earth

Nature Communications ◽

10.1038/s41467-021-23137-5 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Terry-Ann Suer ◽

Julien Siebert ◽

Laurent Remusat ◽

James M. D. Day ◽

Stephan Borensztajn ◽

...

Keyword(s):

High Pressures ◽

Platinum Metal ◽

Core Formation ◽

Enriched Mantle ◽

Metal Silicate ◽

Partitioning Coefficients ◽

Diamond Anvil ◽

Laser Heated ◽

Geochemical Constraints ◽

Island Basalt

AbstractHighly siderophile elements (HSE), including platinum, provide powerful geochemical tools for studying planet formation. Late accretion of chondritic components to Earth after core formation has been invoked as the main source of mantle HSE. However, core formation could also have contributed to the mantle’s HSE content. Here we present measurements of platinum metal-silicate partitioning coefficients, obtained from laser-heated diamond anvil cell experiments, which demonstrate that platinum partitioning into metal is lower at high pressures and temperatures. Consequently, the mantle was likely enriched in platinum immediately following core-mantle differentiation. Core formation models that incorporate these results and simultaneously account for collateral geochemical constraints, lead to excess platinum in the mantle. A subsequent process such as iron exsolution or sulfide segregation is therefore required to remove excess platinum and to explain the mantle’s modern HSE signature. A vestige of this platinum-enriched mantle can potentially account for 186Os-enriched ocean island basalt lavas.

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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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Investigating magma ocean solidification on Earth through laser‐heated diamond anvil cell experiments

Geophysical Research Letters ◽

10.1029/2021gl092446 ◽

2021 ◽

Author(s):

Farhang Nabiei ◽

James Badro ◽

Charles‐Édouard Boukaré ◽

Cécile Hébert ◽

Marco Cantoni ◽

...

Keyword(s):

Diamond Anvil Cell ◽

Magma Ocean ◽

Diamond Anvil ◽

Laser Heated

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Core formation and core composition from coupled geochemical and geophysical constraints

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1505672112 ◽

2015 ◽

Vol 112 (40) ◽

pp. 12310-12314 ◽

Cited By ~ 70

Author(s):

James Badro ◽

John P. Brodholt ◽

Hélène Piet ◽

Julien Siebert ◽

Frederick J. Ryerson

Keyword(s):

Experimental Petrology ◽

Magma Ocean ◽

Core Formation ◽

Cobalt Chromium ◽

Primitive Earth ◽

Left Behind ◽

The Core ◽

Reducing Conditions ◽

Core Composition ◽

Seismic Models

The formation of Earth’s core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal−silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth’s magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7–5% oxygen along with 2–3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.

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