Fast accretion of the Earth with a late Moon-forming giant impact

Abstract

Constraints on the formation history of the Earth are critical for understanding of planet formation processes. 182 Hf-182 W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and 182 Hf-182 W and 87 Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to >100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the 182 W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of mainstage accretion at approximately 30 Myr. A late MGI (>52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in <10.7 AE 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.Hf-W chronometry | planet formation C urrent theories and numerical simulations argue that the Earth grew by numerous collisions between small objects to form larger ones and the last collision with a Mars-sized impactor probably gave rise to the Moon (1-3). Because the core formation processes are thought to have been occurring continuously during the accretion, the time scale of core formation in the growing Earth provides a basis for determining the time scale of formation of the Earth-Moon system (4-7). So far, the best constraint on the time scale of formation of the Earth-Moon system is derived from a combination of 182 Hf-182 W chronology and core formation models (cf. 4, 5, 8). A two-stage model with a single core formation event gives the time of Earth's formation of 28-35 Myr after the onset of the solar system (9-11). A model with a continuous accretion and core formation process and an exponentially decreasing accretion rate leads to a mean time of the Earth's formation of 11 AE 1 Myr (9). This is the time needed to accumulate approximately 63% of the present Earth's mass (4). A model with an early continuous accretion and core formation immediately followed by a Moon-forming giant impact (5) yielded a mean time of Earth's formation of approximately 11.5 Myr and a time of the Moon-forming giant impact of approximately 32 Myr (5). Overall, all such models consistently show that the major mass of the Earth has a mean time of formation of approximately 11 Myr with the complete formation time of Earth, including the Moon-formi...

Early life on land and the first terrestrial ecosystems

“…By contrast, the oldest materials on Earth (zircon crystals) go back~4,400 Ma (Wilde et al 2001), leaving a hiatus of~170 Ma in Earth's geological history. Regardless, it is assumed that the Moon was already formed before 4,400 Ma (Canup and Righter 2000;Yu and Jacobsen 2011) and that the Earth's nucleus, mantle, and lithosphere were already differentiated (Nelson 2004;Boyet and Carlson 2005). At least by~4,200 Ma, but perhaps 200 Ma earlier, large water bodies were in place (Mojzsis et al 2001;Nutman 2006;Cavosie et al 2007, but see alternative views by Deming 2002), while granitic (continental) and basaltic (oceanic) crusts were constantly growing, resurfacing, and remelting, interacting with water in non-uniform regimes that evolved drastically from the Hadean to the Neoarchean (Komiya et al 1999;Nutman et al 2002;Myers 2004;Rino et al 2004;Van Kranendonk 2004 and references therein;Furnes et al 2007a;Adam et al 2012), changing from plume-dominated to platedominated tectonics toward the late Paleoarchean .…”

Is the mantle chemically stratified? Insights from sound velocity modeling and isotope evolution of an early magma ocean

“…Although the timing of such a large-scale differentiation event(s) is unclear, the last magma ocean forming event is thought to have been the Moon-forming giant impact, with the estimated timing of this event of about 30 (Jacobsen, 2005) but not exceeding 100 Ma (Yu and Jacobsen, 2011) after the formation of the first Solar System condensates at 4.567 Ga. Here we assume the onset of upper and lower mantle differentiation to occur at 4.467 Ga.…”

Thermal evolution of the early Moon

“…As discussed earlier, the formation of the Moon could have occurred during the initial 30 to >100 million years (Ma) of the formation of the solar system (e.g., Touboul et al. , ; Yu and Jacobsen ; Kruijer et al. ).…”