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ABSTRACT: Abstract— Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.
Meteoritics & Planetary Science. 02/2010; 32(6):929 - 944.
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ABSTRACT: A long-standing question in the planetary sciences asks what the Earth is made of. For historical reasons, volatile-depleted primitive materials similar to current chondritic meteorites were long considered to provide the 'building blocks' of the terrestrial planets. But material from the Earth, Mars, comets and various meteorites have Mg/Si and Al/Si ratios, oxygen-isotope ratios, osmium-isotope ratios and D/H, Ar/H2O and Kr/Xe ratios such that no primitive material similar to the Earth's mantle is currently represented in our meteorite collections. The 'building blocks' of the Earth must instead be composed of unsampled 'Earth chondrite' or 'Earth achondrite'.
Nature 04/2002; 416(6876):39-44. · 36.28 Impact Factor
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Nature 03/2002; 415(6873):733-4. · 36.28 Impact Factor
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ABSTRACT: Core formation in asteroid-sized planetesimals occurred early (less than
15 Ma after T(0)) as has been recently demonstrated using W-182/W-184
ratios. Depletion of siderophile elements in the mantle of the HED
parent body has been cited as evidence that metal had segregated into a
core before the eucrites formed. Calculations and experiments indicate
that large degrees of melting are required to efficiently separate metal
from silicate. Given the siderophile element data, it seems necessary to
have large amounts of melting on a Vesta-sized body in order to
facilitate core formation. The high temperatures necessary to melt a
chondritic mantle could be provided early by the decay of Al-26 to Mg-26
and Fe-60 to Ni-60, by a hot T Tauri or FU Orionis stage, or a
superluminous sun. We examined the issue of core formation in a CI
chondritic HED parent body by considering siderophile element
partitioning during metal-silicate equilibrium at high temperatures.
Here we extend this work to investigate the effect of bulk composition
by considering CM, CO, CV, H, L, and LL chondritic starting materials.
02/1997; 28:1177.
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ABSTRACT: We report new measurements of metal/silicate partition coefficients for Ga, Cu, and Sn at 10 to 90 kb and 1250 to 1900°C. We show that all three of these siderophile elements (D(metal/silicate) > 10) become more lithophile at high pressure and temperature conditions. Metal/silicate partition coefficients calculated for the conditions of an early deep magma ocean are all close to or less than 1. Thus, in an early terrestrial magma ocean scenario, the depletions of these elements in the mantle must be caused by volatility rather than by metal/silicate equilibrium. For Mars, moderate mantle depletions of both Sn and Cu (relative to Earth) may be due to separation of an S-rich metallic core. In contrast, the sizable depletions of Ga in the Moon and Vesta must be caused by both volatility and core formation. Volatility controlled depletions of these elements may have been inherited from early solar system materials.
Geochimica et Cosmochimica Acta.
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ABSTRACT: Recent proposals of metal-silicate equilibrium at the base of a deep hydrous magma ocean are based on experimental data obtained under anhydrous conditions. We have undertaken a series of experiments at 10 kbar and 1300°C, designed to isolate the effect of dissolved water on the partitioning of the siderophile elements Ni, Co, Mo, W, and P between metal and hydrous silicate liquid. These experiments show that partition coefficients for Ni, Co, Mo and W remain unchanged under hydrous conditions up to ∼4.0 wt.% dissolved H2O, whereas those for P remain unchanged only up to ∼1.5 wt.% dissolved H2O, above which they increase. Such results indicate that the proposal of a deep hydrous magma ocean for the early Earth is robust across a range of water contents, but the highly charged cation, P, becomes more siderophile at high water contents. Predictive expressions for metal-silicate partitioning from our earlier studies have been augmented with new metal-silicate partition coefficient data. Earlier conclusions that terrestrial upper mantle abundances of Ni, Co, Mo, W, and P are consistent with metal-silicate equilibrium at the base of a deep hydrous magma ocean remain robust with the addition of these new data. These results have two implications for the earliest history of the Earth and its subsequent evolution. First, the high temperature and pressure conditions for both the Earth and the Moon are consistent with the thermal state of the early Earth expected in a giant impactor scenario for the origin of the Moon. Second, wet accretion of the Earth provides an alternative source of Earth's current atmosphere and hydrosphere, and would allow oxidation of originally reduced planetary building blocks.
Earth and Planetary Science Letters.
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ABSTRACT: The accretion and early differentiation of the Earth is the starting point of earth history. The abundances of metal-seeking (siderophile) elements in the mantle are a powerful probe of those events. It has long been known that siderophile element abundances in the Earth's mantle are too high to have resulted from metal-silicate equilibrium at near surface conditions. This mismatch provided support for the idea that the Earth accreted heterogeneously. We report new experimental results for the highly siderophile element Re and show that metal-silicate partition coefficients decrease with increasing temperature (at fixed pressure and relative oxygen fugacity). Calculations using these and previously published experimental results indicate that the abundances of the moderately siderophile elements Fe, Ni, Co, Mo, W, P and, most importantly, the highly siderophile element Re, in Earth's upper mantle are consistent with early equilibration between metal and silicate liquid at the base of a deep (800–1000 km) magma ocean. Reconciliation of mantle abundances of moderately and highly siderophile elements with high temperature and pressure metal-silicate equilibrium would obviate the need for heterogeneous accretion. These new results indicate that the Earth accreted homogeneously, rather than heterogeneously, or that evidence for heterogeneous accretion was erased by early high temperature and pressure melting events.
Earth and Planetary Science Letters.
