Scientists at Yale University and the Southwest Research Institute (SwRI) say they have obtained some valuable new information about the gold story. New research offers a theory of how gold, platinum and other precious metals found their way into the shallow layers of Earth's mantle. The story begins with violent collisions of large objects in space, continues in the semi-melted region of Earth's mantle, and ends with the precious metal finding an unlikely resting place, much closer to the planet's surface than scientists expected.
Artistic rendering of a large collision on early Earth. Yale and SwRI scientists have proposed a new theory to explain why gold and platinum are closer to Earth's surface, focusing on a unique "transient" mantle region that traps and distributes these metals. Source: SwRI/Marchi
Ezoic Jun Korenaga, professor of Earth and planetary sciences in the Yale School of Arts and Sciences, and Simone Marchi, a researcher at SwRI in Boulder, Colorado, provide details in a study published in the journal Proceedings of the National Academy of Sciences.
Their new theory offers a possible answer to the unanswered question of how gold, platinum and other precious metals found their way into the shallow layers of Earth's mantle rather than deep into its core. More broadly, the new theory sheds light on how planets form across the universe.
"Our study is a great example of how we reexamined conventional wisdom and made unexpected discoveries," Korenaga said.
The latest research from scientists around the world confirms that billions of years ago, the early proto-Earth collided with a large moon-sized celestial body in space, leaving behind material deposits that folded into today's Earth. But this absorption process has always been a mystery.
In addition to being valued for their scarcity, beauty and use in high-tech products, gold and platinum are also so-called highly "ferrophilic" elements. Their adhesion to iron is so strong that they would almost entirely accumulate in the Earth's metallic core - either fusing directly with it upon impact or sinking rapidly from the mantle into the core. By that logic, they shouldn't be clustered on or near the Earth's surface. However, they do gather.
Korenaga and Marchi's theory revolves around a thin "transient" region in the mantle where shallow parts of the mantle melt while deeper parts remain solid. The researchers found that this region has special dynamic properties that effectively trap falling metal components and slowly transport them to other parts of the mantle.
They theorize that this transport is still ongoing, with the remnants of the transient region appearing as "large low-shear velocity zones" - well-known geophysical anomalies deep in the Earth's mantle.
Marchi: "Such transient regions were almost always formed when large impactors struck the early Earth, which makes our theory quite reliable."
The new theory not only explains previously incomprehensible aspects of Earth's geochemical and geophysical evolution, but also highlights the wide range of time scales involved in Earth's formation, the researchers said.
"One of the striking phenomena we found is that the dynamic changes in the transient mantle region occur in a very short time - about a day, but its impact on the subsequent evolution of the Earth lasts for billions of years," Korenaga said.