Abstract
Abstract Hf-W chronometry provides constraints on the timing of planetary accretion and differentiation, as the segregation of a metal core from silicates should induce strong fractionation of Hf from W. In most previous studies, it was assumed that a giant impact would perfectly reset the Hf-W chronometer. Here, we show the difficulty of achieving perfect equilibration of the Hf-W system. Perfect equilibration requires iron to split into small droplets. However, since the sedimentation velocities of small droplets are low, the Rayleigh-Taylor instability between the upper metal-containing and lower metal-free layers results in quick overturning of the layers, unless iron droplets were uniformly distributed in the entire mantle. Therefore, the lower metal-free layers cannot be equilibrated. We calculated the isotopic evolution of the Hf-W system, taking into account the partial resetting of this chronometer. Our study led to three conclusions: (1) collision conditions and the number of giant impact events affect the age estimation of core formation, (2) the Earth’sWisotope ratio indicates that more than two-tenths of the volume of the protoearth’s mantle must have been equilibrated at each giant impact, and (3) Mars should have experienced a late, extensive equilibration event; it could have been a single giant impact.
Highlights
Hafnium and tungsten are both highly refractory elements; hafnium is a lithophile element, whereas tungsten is a moderately siderophile element that should partition strongly into the metal phase during metal/silicate segregation
Our study led to three conclusions: (1) collision conditions and the number of giant impact events affect the age estimation of core formation, (2) the Earth’s W isotope ratio indicates that more than two-tenths of the volume of the protoearth’s mantle must have been equilibrated at each giant impact, and (3) Mars should have experienced a late, extensive equilibration event; it could have been a single giant impact
The magma ocean model assumes that growth of the Earth occurred at an exponentially decreasing accretion rate and does not consider the effects of giant impacts
Summary
Hafnium and tungsten are both highly refractory elements; hafnium is a lithophile element, whereas tungsten is a moderately siderophile element that should partition strongly into the metal phase during metal/silicate segregation. Based on new measurements of the W isotope compositions and Hf/W ratios of several meteorites, the age of terrestrial core formation and Earth’s accretion has recently been re-estimated by some groups These estimations were based on either a magma ocean model or a two-stage model (Harper and Jacobsen, 1996; Jacobsen, 1988). The other hand, the two-stage model assumes that Earth’s core formed at a well-defined point in time by a single event (e.g. a giant impact) This model age does not provide a realistic age for core formation, but it does provide the earliest time when core formation can have ceased (Halliday et al, 1996; Halliday, 2004; Kleine et al, 2004a). We discuss the core formation events on Earth and Mars
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