Tungsten isotopic evolution during late-stage accretion: Constraints on Earth–Moon equilibration

Abstract

We couple the results of N-body simulations of late-stage accretion (O'Brien et al. 2006) to a hafnium-tungsten (Hf-W) isotopic evolution code to investigate the evolution of planetary bodies in the inner solar system. Simulations can simultaneously produce planets having Earth-and Mars-like masses and Hf-W systematics by assuming that the tungsten partition coefficient decreases with increasing semi-major axis (e.g. due to increasing oxidation). Simulations assuming that Jupiter and Saturn occupy circular orbits are more successful at reproducing the Hf-W systematics than those assuming presentday Jupiter and Saturn orbits. To generate Earth-like tungsten anomalies, 30-80% of each impactor core is required to re-equilibrate with the target mantle. Some model outcomes yield a target and final impactor having similar (Earth-and Moon-like) tungsten anomalies. However, in no case can the inferred lunar Hf/W ratio be simultaneously matched. This result suggests that the Moon isotopically equilibrated with the Earth's mantle in the aftermath of the giant impact (cf. Pahlevan and Stevenson 2007). Alternatively, either the dynamical models which show the Moon being derived primarily from the impactor mantle, or the accretion timescales obtained by the N-body simulations, are incorrect.

Tungsten isotope composition of the Acasta Gneiss Complex

“…A final and pervasive Moon-forming event must eventually have set the ε 182 W value of the pre-late veneer terrestrial mantle (Münker, 2010;Nimmo et al, 2010;Pahlevan and Stevenson, 2007;Touboul et al, 2007) and also led to the formation of a terrestrial magma ocean (Canup, 2004;Walter et al, 2004). It is as yet unclear to what degree the cores of the proto-Earth and the impactor merged during this event (i.e.…”

In search of late-stage planetary building blocks

“…Of all the MSE, the modification of the mantle by giant impact has been most frequently studied using W isotopes, because of the radiogenic tracer capability inherent in the system (Halliday, 2004;Nimmo et al, 2010;Dwyer et al, 2014). Unfortunately, the W isotopic composition of the mantle today can presently provide only limited constraints regarding the nature of the giant impactor.…”

Geochemical arguments for an Earth-like Moon-forming impactor

“…The parameters used are therefore those relevant to such a collision and are summarized in table 1. The parameters that are the most poorly constrained and strongly influence the calculations (also see [132]) are the degree of equilibration between the impactor core and proto-Earth mantle (k), the metal-silicate partition coefficient for W during lunar core formation (D M ) and the fraction of the Moon that came from the impactor versus the proto-Earth mantle (h). For this reason, we present a range of simulations by varying k, D and h over acceptable ranges (0 < k < 0.4 [134,135]; 50 < D < 100 [128,136]; 0.5 < h < 0.9 [9,137]).…”

“…Finally, k is the fraction of the impactor core reequilibrating with the Earth mantle before being absorbed into Earth's core, i.e. k = 1 indicates full equilibration of the impactor core with the target mantle and k = 0 indicates no re-equilibration [131][132][133].…”