The carbon content of Earth and its core

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

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2007982117 ◽

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.

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Systematics of metal–silicate partitioning for many siderophile elements applied to Earth’s core formation

Geochimica et Cosmochimica Acta ◽

10.1016/j.gca.2010.12.013 ◽

2011 ◽

Vol 75 (6) ◽

pp. 1451-1489 ◽

Cited By ~ 108

Author(s):

Julien Siebert ◽

Alexandre Corgne ◽

Frederick J. Ryerson

Keyword(s):

Core Formation ◽

Earth’S Core ◽

Earth's Core ◽

Metal Silicate ◽

Siderophile Elements

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Metal–silicate silicon isotope fractionation in enstatite meteorites and constraints on Earth's core formation

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2010.04.030 ◽

2010 ◽

Vol 295 (3-4) ◽

pp. 487-496 ◽

Cited By ~ 67

Author(s):

Karen Ziegler ◽

Edward D. Young ◽

Edwin A. Schauble ◽

John T. Wasson

Keyword(s):

Isotope Fractionation ◽

Silicon Isotope ◽

Core Formation ◽

Earth’S Core ◽

Earth's Core ◽

Metal Silicate

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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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Experimental evidence for hydrogen incorporation into Earth’s core

Nature Communications ◽

10.1038/s41467-021-22035-0 ◽

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.

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