Fractional crystallization of a basal lunar magma ocean: A dense melt-bearing garnetite layer above 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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Fractional Crystallization of a Martian Magma Ocean and Formation of a Thermochemical Boundary Layer at the Base of the Mantle

10.1002/essoar.10501822.1 ◽

2020 ◽

Author(s):

Garrett Zeff ◽

Quentin Williams

Keyword(s):

Boundary Layer ◽

Fractional Crystallization ◽

Magma Ocean

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Remnant iron oxide/sulfide mattes from a Hadean magma ocean at the core–mantle boundary: Insights from a small scale post-Archean analog

Geochimica et Cosmochimica Acta ◽

10.1016/j.gca.2006.06.702 ◽

2006 ◽

Vol 70 (18) ◽

pp. A347 ◽

Cited By ~ 1

Author(s):

C.-T.A. Lee ◽

A. Lenardic ◽

N. Thiagarajan ◽

A. Agranier ◽

C.J. O’Neill ◽

...

Keyword(s):

Iron Oxide ◽

Small Scale ◽

Magma Ocean ◽

Mantle Boundary ◽

Core Mantle Boundary ◽

The Core

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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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Magma ocean fractional crystallization and cumulate overturn in terrestrial planets: Implications for Mars

Meteoritics and Planetary Science ◽

10.1111/j.1945-5100.2003.tb00013.x ◽

2003 ◽

Vol 38 (12) ◽

pp. 1753-1771 ◽

Cited By ~ 168

Author(s):

Linda T. ELKINS-TANTON ◽

E. M. PARMENTIER ◽

P. C. HESS

Keyword(s):

Fractional Crystallization ◽

Terrestrial Planets ◽

Magma Ocean

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

Geochimica et Cosmochimica Acta ◽

10.1016/j.gca.2006.06.916 ◽

2006 ◽

Vol 70 (18) ◽

pp. A455

Author(s):

E. Ohtani ◽

T. Sakai ◽

T. Kawazoe ◽

T. Kondo

Keyword(s):

Light Elements ◽

Magma Ocean ◽

The Core ◽

Metal Silicate

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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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Nb–Ta Behaviour during Magma-to-Pegmatite Transformation Process: Record from Zircon Megacrysts in Pegmatite

Minerals ◽

10.3390/min11101139 ◽

2021 ◽

Vol 11 (10) ◽

pp. 1139

Author(s):

Sheng He ◽

Ziying Li ◽

Abdullah Al Jehani ◽

Dongfa Guo ◽

Zaben Harbi ◽

...

Keyword(s):

Isotopic Composition ◽

Fractional Crystallization ◽

Transformation Process ◽

Arabian Shield ◽

Magmatic System ◽

Melt Extraction ◽

The Core ◽

Nb Content ◽

T Values ◽

Oscillatory Zonation

Due to the absence of early magma records in pegmatites, it is difficult to investigate the behavior of Nb and Ta during the transformation from magma to pegmatite melt. Zircon megacrysts in an NYF-type (Nb-Y-HREE-F) pegmatite from the Arabian Shield could be divided into three phases from core to margin. The Phase Ι zircon in the core of the zircon megacrysts had typical magma oscillatory zonation with ∑REE content from 300 to 400 ppm, Th/U ratios of less than 0.1 and Nb/Ta ratios of less than 1.0. Phase ΙΙ zircon had oscillatory zonation and was enriched with LREEs mostly with Th/U ratios of 0.1–0.2 and Nb/Ta ratios of 1.0–3.0. Phase ΙΙΙ unzoned zircon had the highest ∑REE content, from 8000 to 15,000 ppm, with Th/U ratios higher than 3.0 and Nb/Ta ratios higher than 5.0. The Hf-O isotopic composition was similar in the different phases of zircon with initial 176Hf/177Hf ratios of 0.28258–0.28277, εHf(t) values from 8.0 to 12.0 and δ18OVSMOW from +4.0‰ to +5.0‰. Zircon megacrysts in the NYF-type pegmatite from the Arabian Shield record the transformation from magma to pegmatite melt. Similar Hf-O isotopic compositions mean a closed magmatic system without contamination by external melt, rock or fluid. The proposed modeling shows that magma with low Nb and Ta concentrations and Nb/Ta ratios could evolve into residual pegmatite melt with a high Nb content and superchondrite Nb/Ta ratio during several stages of melt extraction and fractional crystallization of Ti-rich minerals, such as rutile and titanite. The Nb/Ta ratio can be used as an effective indicator of the transformation process from magma to pegmatite melt.

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

10.5194/egusphere-egu21-8013 ◽

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

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.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.

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