The Earth's magnetic field protects and makes our planet habitable by preventing harmful particles of high energy from space, including the Sun. The source of this magnetic field is the nucleus in the center of our planet.
But the core is very difficult to study, in part because it begins at a depth of approximately 1,900 kilometers (1,800 miles), making it too deep for sampling and direct investigation.
Yet, we are part of a research team that has found a way to get information about the core of the Earth, with details recently published in Geochemical perspective letters.
It's hot down
The core is the hottest part of our planet and the outer core reaches temperatures above 5,000 degrees Celsius (9,000 Fahrenheit). This must affect the overlying mantle and it is estimated that 50% of the volcanic heat comes from the core.
Volcanic activity is the main cooling mechanism of the planet. Some volcanisms, such as the one that still forms the volcanic islands of Hawaii and Iceland, could be linked to the core by mantle plumes that transfer heat from the nucleus to the Earth's surface.
Yet the exchange of physical materials between the core and the mantle has been debated for decades.
Our results suggest that some basic materials are actually transferred to the base of these mantle plumes, and that the heart has been leaking for 2.5 billion years.
We have discovered this by looking at very small variations in the isotope ratio of the tungsten element (the isotopes are essentially versions of the same element that contain only different numbers of neutrons).
To study the Earth's core, we need to look for chemical tracers of the core material in volcanic rocks derived from the deep mantle.
We know that the nucleus has a very distinct chemistry, dominated by iron and nickel, as well as elements such as tungsten, platinum and gold that dissolve in an iron-nickel alloy. As a result, metal alloy magnet elements are a good choice for finding kernel traces.
The search for tungsten isotopes
Tungsten (chemical symbol W) as the basic element has 74 protons. Tungsten has several isotopes, including 182W (with 108 neutrons) and 184W (with 110 neutrons).
These isotopes of tungsten may be the most conclusive tracers of the base material because the mantle should have 182W /184W ratios as the core.
This is due to another element, hafnium (Hf), which does not dissolve in an iron-nickel alloy and is enriched in the mantle and has an isotope now extinguished (182Hf) which decomposed for 182W. This gives the extra coat 182W compared to tungsten in the core.
But the analysis required to detect tungsten isotope variations is extremely difficult because we look at variations in 182W /184The ratio W in parts per million and the concentration of tungsten in the rocks are as low as tens of parts per billion. Fewer than five laboratories in the world can perform this type of analysis.
Evidence of a leak
Our study shows a substantial change in the 182W /184W mantle report on the life of the Earth. The oldest rocks of the Earth have considerably higher 182W /184W than most rocks of the modern Earth.
The change in the 182W /184The mantle ratio W indicates that the core tungsten has been leaking into the mantle for a long time.
Interestingly, in the oldest volcanic rocks of the Earth, over a period of 1.8 billion years, there has been no significant change in mantle tungsten isotopes. This indicates that between 4.3 billion and 2.7 billion years ago, little or no core material was transferred to the upper mantle.
But over the next 2.5 billion years, the isotopic composition of mantle tungsten has changed dramatically. From this we deduce that a change in plate tectonics, occurred about 2.6 billion years ago towards the end of the Archean period, triggered sufficiently large convection currents in the coat to modify tungsten isotopes of all modern rocks.
Why the leak?
If mantle plumes rise from the core-mantle boundary to the surface, it follows that materials from the Earth's surface must also descend into the deep mantle.
Subduction, a term used to describe the rocks of the earth's surface descending into the mantle, transports oxygen-rich material from the surface into the deep mantle as an integral component of plate tectonics.
Experiments have shown that an increase in oxygen concentration at the boundary between the core and mantle could result in the separation of tungsten from the core and into 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 more about the evolution of the nucleus, including the origin of the Earth's magnetic field.
The core of the Earth began as a completely liquid metal and has cooled and partially solidified over time. The magnetic field is generated by the rotation of the internal solid core. The crystallization time of the inner nucleus is one of the most difficult questions to solve in earth and planet sciences.
Our study provides us with a tracer that can be used to study the core-mantle interaction and the change in the internal dynamics of our planet, and that can help us better understand how and when the magnetic field was activated.
Hanika Rizo, Assistant Professor, Carleton University; David Murphy, Lecturer in Geosciences, Queensland University of Technology, and Denis Andrault, Professor at Clermont Auvergne University.
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