Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact

Core formation should have stripped the terrestrial, lunar, and martian mantles of highly siderophile elements (HSEs). Instead, each world has disparate, yet elevated HSE abundances. Late accretion may offer a solution, provided that ≥0.5% Earth masses of broadly chondritic planetesimals reach Earth’s mantle and that ~10 and ~1200 times less mass goes to Mars and the Moon, respectively. We show that leftover planetesimal populations dominated by massive projectiles can explain these additions, with our inferred size distribution matching those derived from the inner asteroid belt, ancient martian impact basins, and planetary accretion models. The largest late terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially modified Earth’s obliquity by ~10°, whereas those for the Moon, at ~250 to 300 kilometers, may have delivered water to its mantle.

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Highly Siderophile Element Constraints on Accretion and Differentiation of the Earth-Moon System

Science ◽

10.1126/science.1133355 ◽

2007 ◽

Vol 315 (5809) ◽

pp. 217-219 ◽

Cited By ~ 127

Author(s):

James M. D. Day ◽

D. Graham Pearson ◽

Lawrence A. Taylor

Keyword(s):

Platinum Group Element ◽

Group Element ◽

The Moon ◽

Core Formation ◽

Data Set ◽

Highly Siderophile Elements ◽

Earth’S Mantle ◽

The Earth ◽

Siderophile Elements

A new combined rhenium-osmium– and platinum-group element data set for basalts from the Moon establishes that the basalts have uniformly low abundances of highly siderophile elements. The data set indicates a lunar mantle with long-term, chondritic, highly siderophile element ratios, but with absolute abundances that are over 20 times lower than those in Earth's mantle. The results are consistent with silicate-metal equilibrium during a giant impact and core formation in both bodies, followed by post–core-formation late accretion that replenished their mantles with highly siderophile elements. The lunar mantle experienced late accretion that was similar in composition to that of Earth but volumetrically less than (∼0.02% lunar mass) and terminated earlier than for Earth.

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The Earth-Moon Connection

Zeitschrift für Naturforschung A ◽

10.1515/zna-1989-1004 ◽

1989 ◽

Vol 44 (10) ◽

pp. 891-923 ◽

Cited By ~ 2

Author(s):

A. E. Ringwood

Keyword(s):

Genetic Relationship ◽

The Moon ◽

Core Formation ◽

Earth’S Mantle ◽

Terrestrial Atmosphere ◽

The Earth ◽

Wide Range ◽

Siderophile Elements ◽

The Impact ◽

Close Genetic Relationship

Abstract The early thermal state of the Earth provides important constraints on hypotheses relating to its origin and its connection with the Moon. The currently popular giant impact hypothesis of lunar origin requires the Earth’s mantle to have been completely melted during the impact. Differentiation of a molten mantle would have produced strong chemical and mineralogical stratification, causing the mantle to become gravitationally stable and resistant to convective rehomogenization. The resulting composition and mineralogy of the upper mantle and primitive crust would have been dramatically different from those which have existed during the past 3.8 b. y. It is concluded that the Earth’s mantle was not extensively melted at the conclusion of accretion of the planet and therefore the hypothesis that the Moon was formed by the impact of a martian-sized planetesimal on the proto-Earth is probably incorrect. Nevertheless, a wide range of geochemical evidence demonstrates the existence of a close genetic relationship between the Moon and the Earth’s mantle. The key evidence relates to the processes of core formation in planetary bodies and resultant abundance patterns of siderophile elements which remain in their silicate mantles. Because of the complexity of the core formation process within a given body and the multiplicity of chemical and physical processes involved, the mantle siderophile signature is expected to be a unique characteristic. Thus, the siderophile signatures of Mars and of the eucrite parent body are quite distinct from that of the Earth’s mantle. Lunar siderophile geochemistry is reviewed in detail. It is demonstrated that a large group of siderophile elements display similar abundances in the terrestrial and lunar mantles. The similarity implies that a major proportion of the material now in the Moon was derived from the Earth’s mantle after core formation. This implication, however, does not require that the bulk compositions of the lunar and terrestrial mantles should be essentially identical, as is often assumed. Factors which may contribute to significant compositional differences between the two bodies within the context of a close genetic relationship are reviewed. The most promising mechanism for removing terrestrial material from the Earth’s mantle arises from the impacts of a number of large (0.001 to 0.01 ME) but not giant (≥ 0.1 ME) planetesimals after core formation and at the terminal stage of the Earth’s accretion. These impacts evaporated several times their own masses of mantle material and shock-melted considerably more. However, they did not lead to complete or extensive (e.g. > 50%) melting of the entire mantle. Impact-generated clouds of shock-melted spray and vapours were accelerated to high velocities in the presence of a primitive terrestrial atmosphere that co-rotated with the Earth. This provided an effective means of transferring angular momentum from the Earth to the ejected material which condensed to form a ring of Earth-orbiting planetesimals and moonlets. The Moon was formed by coagulation from material derived from the outer regions of this ring. Accretion of the Earth in the presence of the gases of the solar nebula and the co-rotating primitive terrestrial atmosphere may also have provided a mechanism for generating the rapid prograde spin of the proto-Earth.

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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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