Xenon May Hide in Core With Iron and Nickel

Researchers at the Carnegie Institution for Science mimicked conditions in Earth’s core to show that the normally aloof element xenon will combine with iron and nickel under intense pressure and at high temperature.

Meteorites are time capsules from the earliest days of the Solar System. One type, called carbonaceous chondrites, contain some of the most-primitive known samples of Solar System material, including a lot more xenon than occurs in our own planet’s atmosphere. This missing xenon has stumped geophysicists for decades.

An international team including DCO members Alexander Goncharov, Hanyu Liu, Sergey Lobanov, (all at Carnegie Institution for Science, USA) and colleagues, is chasing down the solution to this longstanding puzzle. The researchers suspect that the missing xenon may be in Earth’s core. They recreated the core’s temperature and pressure in the lab to show that under these conditions, xenon will interact with iron and nickel. They published their findings in a new paper in Physical Review Letters [1].

“Xenon is one of a family of seven elements called the noble gases, some of which, such as helium and neon, are household names,” said lead author Elissaios Stavrou, now at Lawrence Livermore National Laboratory. “Their name comes from a kind of chemical aloofness; they normally do not combine, or react, with other elements.”

Because xenon doesn’t play well with others, it’s deficiency in Earth’s atmosphere, even in comparison to other, lighter noble gases, like krypton and argon, which theoretical predictions tell us should be even more depleted than xenon, is difficult to explain.

The research team focused their attention on the idea that the missing xenon might be found deep inside Earth, specifically hidden in compounds with nickel and, especially, iron, which forms most of the planet’s core.

Extreme pressure changes chemical properties of xenon
The application of extreme pressure dramatically affects the chemical properties of xenon, so that it stops acting aloof and interacts with iron and nickel. This illustration shows how the changes in the electromagnetic properties of xenon, iron, and nickel under these intense pressures allow for the formation of XeFe₃ and XeNi₃. Credit: Stavrou et al

Scientists have known for a while that although xenon doesn’t form compounds under ambient conditions, under the extreme temperatures and pressures of planetary interiors it isn’t quite so aloof. “When xenon is squashed by extreme pressures, its chemical properties are altered, allowing it to form compounds with other elements,” explained Lobanov, now at Stony Brook University.

Using a laser-heated diamond anvil cell, the researchers squeezed and heated the elements to mimic the conditions found in Earth’s core and employed advanced spectroscopic tools to observe how xenon interacted with both nickel and iron.

The researchers found that xenon and nickel formed XeNi3 under nearly 1.5 million times normal atmospheric pressure (150 GPa) and at temperatures of above about 1,200 degrees Celsius. Furthermore, at nearly 2 million times normal atmospheric pressure (200 Gpa) and at temperatures above about 1,700 degrees Celsius, they synthesized complex XeFe3 compounds.

“Our study provides the first experimental evidence of previously theorized compounds of iron and xenon existing under the conditions found in Earth’s core,” said Goncharov. “However, it is unlikely that such compounds could have been made early in Earth’s history, while the core was still forming, and the pressures of the planet’s interior were not as great as they are now.”

The researchers are investigating whether a two-stage formation process could have trapped xenon in Earth’s early mantle and then later incorporated it into XeFe3 when the core separated and the pressure increased. 

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