Early in its evolution, about 4.5 billion years ago, Earth sustained an impact with a Mars-sized planet, and the Moon formed from the resulting debris ejected into an Earth-orbiting disk. A long period of bombardment followed, the so-called "late accretion," when planetesimals as large as our Moon impacted the Earth delivering materials including highly "siderophile" elements (HSEs) - metals with a strong affinity for iron - that were integrated into the young Earth.
"Previous simulations of impacts penetrating Earth's mantle showed that only small fractions of a metallic core of planetesimals are available to be assimilated by Earth's mantle, while most of these metals - including HSEs - quickly drain down to the Earth's core," said Marchi, who coauthored a Proceedings of the National Academy of Sciences (PNAS) paper outlining the new findings. "This brings us to the question: How did Earth get some of its precious metals? We developed new simulations to try to explain the metal and rock mix of materials in the present-day mantle."
The relative abundance of HSEs in the mantle points to delivery via impact after Earth's core had formed; however, retaining those elements in the mantle proved difficult to model - until now. The new simulation considered how a partially molten zone under a localized impact-generated magma ocean could have stalled the descent of planetesimal metals into Earth's core.
"To achieve this, we modeled mixing an impacting planetesimal with the mantle materials in three flowing phases: solid silicate minerals, molten silicate magma and liquid metal," said Dr. Jun Korenaga, the paper's lead author from Yale University. "The rapid dynamics of such a three-phase system, combined with the long-term mixing provided by convection in the mantle, allows HSEs from planetesimals to be retained in the mantle."
In this scenario, an impactor would crash into the Earth, creating a localized liquid magma ocean where heavy metals sink to the bottom. When metals reach the partially molten region beneath, the metal would quickly percolate through the melt and, after that, slowly sink toward the bottom of the mantle. During this process the molten mantle solidifies, trapping the metal. That's when convection takes over, as heat from the Earth's core causes a very slow creeping motion of materials in the solid mantle and the ensuing currents carry heat from the interior to the planet's surface.
"Mantle convection refers to the process of rising hot mantle material and sinking colder material," Korenaga said. "The mantle is almost entirely solid although, over long geologic time spans, it behaves as a ductile and highly viscous fluid, mixing and redistributing mantle materials, including HSEs accumulated from large collisions that took place billions of years ago.
The paper, "Vestiges of impact-driven three-phase mixing in the chemistry and structure of Earth's mantle," was published by PNAS on October 9, 2023.
For more information, visit https://www.swri.org/planetary-science.
Research Report:Vestiges of impact-driven three-phase mixing in the chemistry and structure of Earth's mantle
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