Scientists have unraveled a mystery regarding Earth's surface-level presence of dense precious metals. These metals remained in the upper crust after immense cosmic collisions, even though they should have sunk into the core. The abundance of metals like gold, platinum, and palladium on the surface has long perplexed researchers.
Their chemical overabundance suggests they arrived during early Earth's impacts with large space rocks. However, a new study offers a plausible explanation for their retention in the planet's shallower regions, shedding light on an enduring geological enigma.
The Mystery of Precious Metals Found Near Earth's Surface
In the study, titled "Vestiges of impact-driven three-phase mixing in the chemistry and structure of Earth's mantle" published in PNAS, researchers have proposed a solution to the long-standing mystery surrounding the presence of dense metals like gold, platinum, and palladium near Earth's surface.
Despite their high density, these metals can seep through the mantle and become trapped within solidifying rock. This phenomenon effectively retains them within the Earth's crust, possibly accounting for the existence of enigmatic features known as low-velocity shear zones deep within the mantle.
According to Simone Marchi, a co-author of the study and a researcher at the Southwest Research Institute in Boulder, Colorado, the impacts of giant space rocks during Earth's early history led to the creation of regions slightly denser than the surrounding materials. These impacts allowed for the metals to percolate and eventually find their way back to the Earth's surface.
Gold, platinum, palladium, as well as other platinum-group metals and the transition metal rhenium, are collectively categorized as "highly siderophile elements" due to their affinity for binding with iron.
If, as scientists hypothesize, these metals arrived on Earth via asteroids and planetoids in the early solar system's chaotic period, they should have penetrated the crust, reached the mantle, and subsequently descended towards the iron-rich core. However, this expected trajectory did not occur as anticipated.
Simulations Reveal Earth's Hidden Metal Mystery
In their investigation, researchers Simone Marchi and Jun Korenaga conducted simulations of ancient impacts on early Earth and made a surprising discovery: the challenge of preventing heavy metals like gold, platinum, and palladium from reaching the core was more complex than previously believed, shedding new light on a long-standing geological enigma.
The simulation shows that a massive space rock, potentially as large as the moon, disintegrated the impactor, giving rise to a molten magma ocean that penetrated deep into the mantle.
Within this molten expanse, a transitional layer formed, comprising rock that was partially molten and partially solid, allowing metals from the impactor to diffuse into this semi-molten zone, distributing them throughout the mantle.
Unlike pure, dense metals, which should have rapidly sunk towards the core, the metal-infused mantle region was only slightly denser than its surroundings. As it slowly descended into areas of higher pressure, it solidified, trapping small metal fragments before they could reach the core.
Over billions of years, mantle convection processes transported these trapped metals towards Earth's crust, where they became accessible for human mining operations, providing essential materials for jewelry and electronics.
The researchers also speculated that the metal-rich regions within the mantle might still be detectable today in seismic reconstructions of the mantle, particularly in areas known as large low-velocity shear provinces (LLSVPs), potentially hinting at remnants of ancient impacts responsible for delivering precious metals like gold and platinum to Earth.
Future research may involve simulating similar impacts on young planets like Mars or Venus to gain insights into this process on other terrestrial planets with distinct geological compositions.
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