Interchange Between Earth’s Core and Mantle through Tungsten Isotopes

By measuring minute changes in the ratio of tungsten isotopes, an international collaborative research team has solved a puzzle that has been debated for decades: Is there an interchange of material between the earth’s core and the mantle? The paper has been published in Geochemical Perspective Letters recently.

Research on earth's core has been a very difficult work all the time, one side is the depth is about 2,900 kilometers, which makes it impossible to sample and investigate directly. One the other side, it is the hottest part of the earth, the temperature of the outer core has exceeded 5000 °C. This inevitably affects the mantle that covers the core. It is estimated that 50% of the volcanic heat comes from the core. Volcanic activity is the main cooling mechanism of the earth. Some volcanic activity may be related to earth's core, such as the volcanic islands that are still forming in Hawaii and Iceland, because the mantle plume transfers heat from earth's core to ground.

structure of earth crust to earth core image

New findings indicate that some of the nuclear material from earth’s core does transfer to the bottom of the mantle plume, which has been leaking out for the past 2.5 billion years. This was discovered by measuring minute changes in the isotopes ratio of tungsten (W) elements.

To study the earth’s core, researchers searched for chemical tracers from volcanic rocks formed by the deep mantle. Earth’s core has a very unique chemical composition, mainly composed of iron and nickel, as well as tungsten, platinum, and gold dissolved in an iron-nickel alloy. Therefore, these pro-metal alloys elements are an excellent choice.

Tungsten element has 74 protons with several different isotopes, including tungsten-182 (containing 108 neutrons) and tungsten-184 (containing 110 neutrons). These tungsten isotopes can be used as the most reliable tracer, as the ratio of tungsten-182 to tungsten-184 in the mantle is expected to be much higher than in earth’s core.

This is due to the mantle is rich in another element, Hf, which is insoluble in iron-nickel alloys and has an isotope-182 that is no longer present. This isotope decays into tungsten-182. This results in an additional tungsten-182 in the mantle compared to the center of the earth.

However, detecting isotope changes in tungsten is an extremely difficult challenge because the ratio of tungsten-182 to tungsten-184 varies by a few parts per million (ppm), and the concentration of tungsten in rocks is only a few parts per billion. There are no more than five laboratories in the world can perform this analysis.

New research shows that the ratio of tungsten-182 to tungsten-184 in the mantle has changed significantly during the development of the earth. Among the oldest rocks on earth, the ratio of tungsten-182 to tungsten-184 is much higher than most rocks on modern earth. The change in the ratio of tungsten-182 to tungsten-184 in mantle indicates tungsten in the core has leaked into the mantle for a long time.

difference between the ratio of tungsten isotopes in earth core and mantle shows how the material of core leaks into the mantle plume image

Experiments have shown an increase in oxygen concentration at the boundary between the earth’s core and mantle causes tungsten to separate. Besides, the solidification of the earth's inner core will also increase the oxygen concentration of the outer core. In this case, new findings can provide some information about the evolution of the earth's core, such as the origin of the earth's magnetic field.

The core was originally completely liquid metal, which has been cooling and causing partial solidification over time. The magnetic field is produced by the rotation of the earth's solid core. The crystallization time of the earth's core is one of the most difficult questions to answer in the fields of Earth Science and Planetary Science.

Tungsten isotopes provide tracers for studying the interaction between earth’s core and mantle, as well as studying the dynamics of the earth's interior, which will help to improve our understanding of how and when the earth's magnetic field is turned on.