The Earth's magnetic field protects and makes our planet habitable by blocking harmful high-energy particles from space, including from the Sun. The source of this magnetic field is the central core of our planet.
But the core is very difficult to study, in part because it starts at a depth of about 2,900 kilometers, making it too deep to investigate directly.
Still, we are part of a research team that has found a way to get information about the Earth's core, with details recently published in Geochemical Perspective Letters .
It's hot down there
The core is the hottest part of our planet with the outside reaching temperatures of over 5,000℃ This must affect the overlying mantle and it is estimated that 50% of volcanic heat comes from the core.
The layers of the Earth, from the outer crust to the inner core. Image: Shutterstock / VRVector Volcanic activity is the main cooling mechanism of the planet. Some volcanoes, such as the still forming volcanic islands of Hawaii and Iceland, may be connected to the core by mantle plumes that transfer heat from the core to the Earth's surface.
However, whether there is any exchange of physical material between the core and the mantle has been the subject of debate for decades.
Our findings suggest that some core material transfers to the base of these mantle plumes, and the core has been leaking this material for the past 2.5 billion years.
We discovered this by looking at very small variations in the ratio of isotopes of the element tungsten (isotopes are basically versions of the same element that just contain different numbers of neutrons).
To study the Earth's core, we need to look for chemical tracers of core material in volcanic rocks derived from the deep mantle.
We know that the core has a very distinctive chemistry, dominated by iron and nickel, along with elements such as tungsten, platinum and gold that dissolve into the iron-nickel alloy. Therefore, alloy-loving elements are a good choice for investigating core traces.
The search for tungsten isotopes
Tungsten (chemical symbol W) has 74 protons as its base element. Tungsten has several isotopes, including 182W (with 108 neutrons) and 184W (with 110 neutrons).
These tungsten isotopes have the potential to be the most conclusive tracers of core material, because the mantle is expected to have much higher 182W/184W ratios than the core.
This is due to another element, hafnium (Hf), which does not dissolve in iron-nickel alloy and is enriched in the mantle, and had a now-extinct isotope (182Hf) that decayed to 182W. This gives the mantle 182W extra relative to the tungsten in the core.
But the analysis required to detect variations in tungsten isotopes is incredibly challenging, as we are looking at variations in the 182W/184W ratio in parts per million and the concentration of tungsten in rocks is as low as tens of parts per billion. Fewer than five laboratories in the world can do this kind of analysis.
Evidence of a leak
Our study shows a substantial change in the 182W/184W ratio of the mantle over the lifetime of the Earth. The oldest rocks on Earth have significantly higher 182W/184W than most rocks on contemporary Earth.
The change in the 182W/184W mantle ratio indicates that tungsten from the core was leaking into the mantle for a long time.
Interestingly, in the oldest volcanic rocks on Earth, over a period of 1.8 billion years there is no significant change in the tungsten isotopes of the mantle. This indicates that from 4.3 billion to 2.7 billion years ago, little or no material from the core was transferred to the upper mantle.
But in the subsequent 2.5 billion years, the tungsten isotope composition of the mantle changed significantly. We infer that a change in plate tectonics at the end of the Archean Eon about 2.6 billion years ago triggered large convective currents in the mantle sufficient to change the tungsten isotopes of all modern rocks.
Why the leak?
If mantle plumes are rising from the core-mantle boundary to the surface, it follows that material from the Earth's surface must also descend into the deep mantle.
Subduction, the term used for rocks from the Earth's surface descending into the mantle, carries oxygen-rich material from the surface into the deep mantle as an integral component of plate tectonics.
Experiments show that increasing oxygen concentration at the core-mantle boundary could cause tungsten to separate from the core and from within the mantle.
Alternatively, solidification of the inner core would also increase the oxygen concentration of the outer core. In this case, our new results could tell us something about core evolution, including the origin of the Earth's magnetic field. 
Image showing the differences in tungsten isotope ratios between the Earth's core and mantle, and how the Earth's core may be leaking material into mantle plumes. Credit: Neil Bennett
The Earth's core started out as entirely liquid metal and has been cooling and partially solidifying over time. The magnetic field is generated by the spinning of the inner solid core. The time of crystallization of the inner core is one of the most difficult questions to answer in Earth and planetary science.
Our study gives us a tracker that can be used to investigate the core-mantle interaction and the changing internal dynamics of our planet, and that can increase our understanding of how and when the magnetic field was triggered.
This article was translated from The Conversation under a Creative Commons license. Read the original article.
