The effect of melt composition on metal-silicate partitioning of siderophile elements and constraints on core formation in the angrite parent body

Abstract The Earth differs from other terrestrial planets in having a substantial silica-rich continental crust with a bulk andesitic composition1. The compositional dichotomy between oceanic and continental crust is likely related to water-rich subduction processes2. Over the past decade, the discovery of meteorites with andesitic bulk compositions have demonstrated that continental-crust like compositions can be attained through partial melting of chondritic protoliths3,4,5. Here we show that a newly identified achondrite meteorite, Erg Chech (EC) 002, is a high-Mg andesite but that, unlike previous andesitic achondrites has strongly fractionated and low abundances of the highly siderophile elements (HSE), reminiscent of Earth’s upper continental crust6. The major and HSE composition of EC 002 can be explained if its asteroid parent body underwent metal-silicate equilibrium prior to silicate partial melting without losing significant volatile components. The chemistry of pyroxene grains in EC 002 suggests it approximates a parental melt composition, which cannot be produced by partial melting of pre-existing basaltic lithologies, but more likely requires a metal-free chondritic source. Erg Chech 002 likely formed by ~ 15% melting of the mantle of an alkali-undepleted differentiated asteroid. The discovery of EC 002 shows that extensive silicate differentiation after metal-silicate equilibration was already occurring in the first two million years of solar system history7, and that andesitic crustal compositions do not always require water-rich subduction processes to be produced.

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

Science ◽

10.1126/science.1214967 ◽

2012 ◽

Vol 336 (6077) ◽

pp. 72-75 ◽

Cited By ~ 60

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.

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

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

10.1098/rsta.2008.0147 ◽

2008 ◽

Vol 366 (1883) ◽

pp. 4315-4337 ◽

Cited By ~ 62

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.

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