Pressure-dependent Iron Isotope Fractionation and Light Elements in Earth's Core

Earth formed from accreted matter surrounding the young Sun. Over time, the iron in this early planetary material moved inward, separating from the surrounding silicate. This process created the planet’s iron core and silicate upper mantle. But, due to the technological impossibility of taking samples from Earth’s core to see which compounds exist there, understanding how this differentiation process occurred is challenging.

DAC and Earth

Seismic data show that in addition to iron, there are “lighter” elements present in the core, but which elements and in what concentrations they exist is a matter of great debate. This is because as the iron moved inward toward the core it interacted with various lighter elements to form different alloyed compounds, which were then carried along with iron into the planet’s depths.

Which elements iron bonded with during this time would have been determined by the surrounding conditions, including pressure and temperature. As a result, working backwards and determining which iron alloy compounds were created during differentiation could tell scientists about the conditions on early Earth and about the planet’s geochemical evolution.

New work from a team led by DCO’s Anat Shahar (Carnegie Institution for Science, USA) and including DCO Extreme Physics and Chemistry Community Co-Chair Wendy Mao (Stanford University, USA) uses this iron record to look at how Earth's core formed, and what light element(s) might be trapped there [1]. The work is published in the journal Science.

The team investigated how pressures mimicking Earth’s core would affect the composition of iron isotopes in various alloys of iron and light elements. Because of differences in isotope masses, there are small but detectable variations in how different isotopes of the same element partition in, or are “picked up” by, either silicate or iron metal. Some isotopes are preferred by certain reactions, which results in an imbalance in the proportion of each isotope incorporated into the end products of these reactions—a process that can leave behind trace isotopic signatures in rocks known as isotope fractionation.

Before now, pressure was not considered a critical variable affecting isotope fractionation. But Shahar and her team demonstrate that for iron, extreme pressure conditions do affect isotope fractionation.

Due to this high-pressure fractionation, reactions between iron and two of the light elements often considered likely to be present in the core—hydrogen and carbon—would have left behind an isotopic signature in the mantle silicate as they reacted with iron and sunk to the core. But evidence of this isotopic signature is lacking in samples of mantle rock, excluding them from the list of potential light elements in the core.

Oxygen, on the other hand, would not have left an isotopic signature behind in the mantle. Other potential core light elements include silicon and sulfur.

“What does this mean? It means we are gaining a better understanding of our planet’s chemical and physical history,” Shahar explained. “Although Earth is our home, there is still so much about its interior that we don’t understand. But evidence that extreme pressures affect how isotopes partition, in ways that we can see traces of in rock samples, is a huge step forward in learning about our planet’s geochemical evolution.”

 

Article adapted from Carnegie Institution for Science, USA.

 

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