Carleton University’s Hanika Rizo is the lead author on research that indicates material from the Earth’s core has been leaking into the mantle shell since 2.5 billion years ago. Her findings could indicate cooling and solidification of the inner core or changes in plate tectonics, both of which were vital to establishing life on Earth.

“In our study, we have found the most compelling evidence to date that core material has been leaking into the mantle,” said Rizo, professor in the Department of Earth Sciences. “The evidence comes from measurements of the abundance of the isotope of mass 182 of tungsten (182W) in mantle-derived magmas. While ancient rocks contain excesses in this isotope from 4.3 until 2.5 billion years ago, modern lavas from upwelling mantle plumes, which bring material from the core-mantle boundary to the Earth’s surface, have deficits. Given that the Earth’s core is expected to have deficits in 182W, the isotopic change between the ancient and modern mantle is best explained by core-mantle chemical interaction after the Archean Eon.”

Rizo’s research, which was recently published in Geochemical Perspective Letters, evaluates why material is leaking out of the core. Experiments show that increasing the oxygen concentration of the core-mantle boundary could trigger the exsolution of iron-loving elements like W out of the core and into the mantle. This could happen if deep subduction of oxidized material is introduced into the lower mantle, a geological process resulting from plate tectonics. Alternatively, inner core solidification would also increase the oxygen concentration of the outer core. In this case, Rizo’s results could tell us something about the origin of Earth’s magnetic field. The Earth’s core, which started as entirely liquid metal, has been cooling and partially solidifying over time, which allowed the development of the magnetic field. The time of inner core crystallization is one of the most difficult questions to answer in Earth and planetary sciences.

The Earth’s internal structure is divided into three shells: the core, the mantle and the crust. The Earth’s core, starting at a depth of approximately 2,900 kilometres, is the most inaccessible part of Earth. It is composed mainly of iron metal, unlike the outer silicate mantle and crust shells.

Chemical exchange at the boundary between the core and the mantle has been the subject of debate for decades. Even if thermal, electromagnetic and mechanical interactions across the core-mantle boundary suggest chemical exchange is expected, it has generally been assumed that the core has been chemically isolated since its formation. That’s because geochemical evidence for this exchange has been extremely difficult to find. Until now.

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Tuesday, June 25, 2019 in
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