The earth's core is "leaking

For 2.5 billion years, nuclear material is constantly entering the earth's mantle

For 2.5 billion years, nuclear material has passed into the Earth's mantle, as an isotope study suggests. © Johannes Gerhardus Swanepoel / thinkstock
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Creeping diffusion: The Earth's core appears to have been leaking for around 2.5 billion years - since then, nuclear material has been constantly leaking into the Earth's mantle. Researchers have now tracked this "leakage" using altered levels of tungsten isotopes in mantle rock. The analyzes also prove that the Earth's core was still dense in the early days of the planet. According to the scientists, the leak must have been caused by a subsequent process.

The Earth's core is a defining and enigmatic part of our planet. Only through its combination of solid and liquid iron, the earth gets its magnetic field, because the currents in the liquid outer core drive the geodynamo. At the same time, the heat of the Earth's core provides the energy that enables phenomena such as plate tectonics, mantle plumes and volcanism. On the other hand, one of the mysteries of the earth's core is when the solid inner core froze.

How "dense" is the Earth's core?

But there is another question open: Is there also a material exchange between the Earth's mantle and the Earth's core? It seems clear that the core-mantle boundary is characterized by a large temperature jump and a seismic zone detectable transition zone of 200 to 300 kilometers in thickness. However, the nature of this transition, and the extent to which material from the earth's core penetrates into the lower mantle, is still in dispute.

"We know that the Earth's core has a special chemistry dominated by iron and nickel, as well as dissolved elements such as tungsten, platinum and gold, " explain Hanika Rizo of Carlton University in Ottawa and her colleagues. "Therefore, such metal-affine elements are good indicators of traces of core material in the mantle." However, one must be able to distinguish whether these elements really come from the core or at least from higher layers of the earth.

Tungsten isotopes as an indicator

That could have succeeded Rizo and her team now. For their study, they had studied rocks that came from the lower mantle and leaked at different times on hotspot volcanoes as lava. The researchers focused their attention specifically on the ratio of two tungsten isotopes: 182W and 184W. "These tungsten isotopes could be the most informative tracers for core material because the cladding should have a much higher 182W / 184W ratio, " they explain. display

The reason: 90 percent of the earth's tungsten is concentrated in the earth's core, because this metal in the layer formation of the Earth's interior with other heavy elements in the core dropped down. The Earth's mantle, on the other hand, initially contained much Haffnium-182, an isotope that has completely decayed to tungsten-182 over time. "As a result, the mantle now has an excess of tungsten-182 compared to the Earth's core, " explain the researchers. However, with around 200 parts per million (ppm) it is tiny and difficult to detect.

But if this excess is missing or weaker in mantle rock, this could indicate a "contamination" with nuclear material. Exactly afterwards Rizo and her team searched in their samples.

Leaking for 2.5 billion years

In fact, the researchers got busy. In part of the rock samples from the lower mantle, they registered remarkably reduced levels of tungsten-182. "This suggests that material from the Earth's core has leaked to the base of these mantle plumes, " Rizo and her team report. Thus, the core-cladding boundary, at least for the element tungsten, is not an absolute barrier.

Surprisingly, however, this "leak" in the Earth's core was apparently not always there, as the analysis showed. Because with volcanic mantle rock from the time 2.7 to 4.3 billion years ago, the scientists found no reduced proportion of tungsten-182. "This suggests that no or very little nuclear material was transported into the mantle during this time, " say Rizo and her colleagues. The earth's core has therefore been "leaking" only since the last 2.5 billion years.

Possible processes that could promote the leakage of nuclear material. Rizo et al. / Geochemical Perspective Letters, CC-by-nd 4.0

Plate tectonics or nuclear crystallization as a trigger?

But what causes this "leak" since then? According to the researchers, plate tectonics could play a crucial role in this. Not only does it ensure the movement of the earth plates, it also transports oxygen-rich surface rocks into the deep mantle via subduction. "Experiments show that increasing the oxygen levels at the core-mental boundary can promote the transition of tungsten into the mantle, " explain Rizo and her team.

Alternatively, however, a crystallization of the inner core that began at that time could have increased the oxygen levels at the core boundary. "In that case, our findings would also reveal something about the evolution of the Earth's core, " the researchers said. Because then the solid earth core could be older than expected. (Geochemical Perspective Letters, 2019; doi: 10.7185 / geochemlet.1917)

Source: The Conversation, Geochemical Perspectives

- Nadja Podbregar