Southwest Research Institute’s Dr. Simone Marchi collaborated on a new study finding the first geophysically plausible scenario to explain the abundance of certain precious metals — including gold and platinum — in the Earth’s mantle. Based on these simulations, scientists found an impact-driven mixing of mantle materials scenarios that could prevent the metals from completely sinking into the Earth’s core. Credit: Southwest Research Institute
A new study suggests impact-driven mixing of mantle materials could create current mantle composition.
Southwest Research Institute’s Dr. Simone Marchi collaborated on a new study finding the first geophysically plausible scenario to explain the abundance of certain precious metals — including gold and platinum — in the Earth’s mantle. Based on the simulations, or model, scientists found that impact-driven mixing of mantle materials scenario that could prevent the metals from completely sinking into the Earth’s core.
The Earth’s Early History
Early in its evolution, about 4.5 billion years ago, Earth sustained an impact with a MarsMars is the second smallest planet in our solar system and the fourth planet from the sun. It is a dusty, cold, desert world with a very thin atmosphere. Iron oxide is prevalent in Mars' surface resulting in its reddish color and its nickname "The Red Planet." Mars' name comes from the Roman god of war.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>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 Understanding vs. New Insights
“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.”
This schematic illustrates the most geophysically plausible explanation for the abundance of HSE metals present in the Earth’s mantle. During the long period of bombardment, impactors would strike the Earth and deliver materials. (a) Liquid metals would sink in the locally produced impact-generated magma ocean before percolating through the partially molten zone beneath. (b) Compression causes the metals in the molten zone to solidify and sink. (c) Then thermal convection mixes and redistributes the metal-impregnated mantle components over long geologic time frames. Credit: Southwest Research Institute
How HSEs Remained in the 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 UniversityEstablished in 1701, Yale University is a private Ivy League research university in New Haven, Connecticut. It is the third-oldest institution of higher education in the United States and is organized into fourteen constituent schools: the original undergraduate college, the Yale Graduate School of Arts and Sciences and twelve professional schools. It is named after British East India Company governor Elihu Yale.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>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.”
Understanding Mantle Dynamics
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.”
Reference: “Vestiges of impact-driven three-phase mixing in the chemistry and structure of Earth’s mantle” by Jun Korenaga and Simone Marchi, 9 October 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2309181120
Source: SciTechDaily
