Geology: Chemical leftovers from Earth’s formation 4.5 billion years ago may sit near planet’s core

Near the Earth’s core may exist chemical remnants from Earth’s formation 4.5 billion years earlier, much like clumps in a bowl full of batter.

This is the conclusion of a study led by researchers led from the University of Utah, who studied some of the ‘ultra-low velocity zones’ on the core–mantle boundary.

Geologists use the passage of seismic waves through the Earth to plumb its interior — extrapolating features based on how the waves are reflected and refracted. 

These zones have an ultra-low velocity because they slow down seismic waves by half while increasing the density of matter by a third.

The team modeled the formation of certain zones and concluded that they are the dense fractions of magma oceans formed following the impact of the moon.

The material settled down to the bottom, creating a layer structure. This material was pushed in small areas by the mantle over time.

Scroll down to see the video

The Earth’s interior may contain chemical remnants from its formation 4.5 billion years ago, much like clumps in a bowl full of batter. Pictured: a map of the Earth’s interior, showing the locations of the ultra-low velocity zones (yellow) in question above the outer core, as well as sinking crustal material (blue) and so-called ‘superplumes’ (red)

Geologists use the passage of seismic waves through the Earth to plumb its interior — extrapolating features based on how the waves are reflected and refracted (as pictured). Because the velocity of seismic waves passing through ultra-low zones slows by as much as half the speed, and the density increases by upto a third

The investigation was undertaken by geologist Michael Thorne of the University of Utah and his colleagues.

Professor Thorne stated that of all features found in the deep mantle’s depths, it is ultra-low velocity zones which are the most extreme.

He said, “Indeed. These are some of the extreme features found anywhere on Earth.”

Geologists had originally speculated that these ultra-low velocity zones might represent areas where the base of the mantle had partially melted, perhaps providing the source of magma for volcanic ‘hot spots’ like that seen in Iceland.

‘But most of the things we call ultra-low velocity zones don’t appear to be located beneath hot spot volcanoes, so that can’t be the whole story,’ said Professor Thorne.

In their study, the researchers set out to put an alternate theory to the test — that of whether ultra-low velocity zones may be comprised of rock with a different and more archaic composition to that of the rest of the mantle.

According to Thorne, it is possible that ultra-low velocity zones are accumulations of iron oxide. Although this compound may be most familiar as rust it could behave just like a deep-metal metal.

He said that if there were pockets of these near the core they might be able to affect the Earth’s magnet field which is generated at the outer core.

‘The physical properties of ultra-low velocity zones are linked to their origin,’ said paper author and seismologist Surya Pachhai of both the Australian National University, and the University of Utah.

Their origins,  he added, in turn provide ‘important information about the thermal and chemical status, evolution and dynamics of Earth’s lowermost mantle — an essential part of mantle convection that drives plate tectonics.’

Researchers led from the University of Utah studied ‘ultra-low velocity zones’ on the core–mantle boundary. Pictured: a model of one hemisphere, showing the location of two ultra-low velocity zones (red) and the pacific superplume (transparent red)

Researchers focused their research on ultra-low velocity zones located below the Coral Sea. This is between Australia and New Zealand. 

This area is ideal for study because it experiences a high level of seismic activity — allowing the team to build up a high-resolution map of the core–mantle boundary that might shine more light on the mysterious ultra-low velocity zones.

It is not easy to create a seismic image from nearly 1,800 miles worth of crust or mantle.

The team used a technique known as Bayesian inversion to overcome the challenge.

Dr. Pachhai said, “We can make a model for the Earth with ultra-low speed reductions. Then we can run a simulation on the computer that shows us how seismic waveforms might look if this is the Earth.”

“Our next step is to evaluate the predictions with what we have.”

Over the course of hundreds of thousands of iterations of this process, the method is able to produce a mathematically robust model of the Earth’s interior — alongside an understanding of any uncertainties and assumptions involved in such.

‘We can create a model of the Earth that includes ultra-low wave speed reductions and then run a computer simulation that tells us what the seismic waveforms would look like if that is what the Earth actually looked like,’ said seismologist Surya Pachhai. Pictured: snapshots of the final model’s temperature field (left) and residual buoyancy field (right)

Furthermore, the researchers’ models were able to reveal that the ultra-low velocity zones studied most likely have internal structures — specifically, layers — which helps to constrain how these zones most likely formed.

“To our knowledge. this is the first study using such a Bayesian approach at this level of detail to investigate ultra-low velocity zones,” said Dr Pachhai.

“It also is the first to show strong layering in an ultra low velocity zone.”

The team believe that the origin of the ultra-low velocity zones stems from a time more than four billion years ago, when Earth was slowly undergoing fractionation as dense iron sank to the planetary core as lighter minerals floating upwards. 

It is believed that a planetary object around the size of Mars — one which scientists have dubbed Theia — slammed into the young Earth, both throwing up debris into Earth’s orbit to form the moon but also significantly raising the planet’s temperature.

The team believe that the origin of the ultra-low velocity zones stems from a time more than four billion years ago, when a planetary object around the size of Mars — one which scientists have dubbed Theia — slammed into the young Earth (as depicted), both throwing up debris into Earth’s orbit to form the moon but also significantly raising the planet’s temperature

“As an outcome, a large body of molten material, known as a magma ocean, formed,’ explained Dr Pachhai.

As it cooled, this ‘ocean’ would have settled out — with the densest materials sinking down to the bottom of the mantle and forming layers.

And over the course of billions of years, the churning of the material within the mantle would have served to push these dense layers into small patches, forming the ultra-low velocity zones detectable via seismic imaging today.

‘As a result [of the moon-forming impact]Dr. Pachhai said that a large mass of molten matter, also known as a Magma Ocean, was formed. Illustration: The early Earth magma Ocean

As it cooled, this ‘ocean’ would have settled out — with the densest materials sinking down to the bottom of the mantle and forming layers (as pictured)

These dense layers would have been pushed into tiny patches by the constant churning in the mantle over billions of year, creating the ultra-low speed zones that are now detectable using seismic imaging.

Dr. Pachhai noted that ‘the primary and most shocking finding was that ultra-low velocity zone are not homogeneous, but have strong heterogeneities within them (structural or compositional variations).

“This discovery changes the way we view the origins and dynamics of ultra low velocity zones. We found that this type of ultra-low velocity zone can be explained by chemical heterogeneities created at the very beginning of the Earth’s history.

“They still are not well mixed, after 4.5 billion year of mantle Convection”

Some ultra-low velocity zones could preserve some evidence of planet history that had been lost, he said.

‘Our discovery provides a tool to understand the initial thermal and chemical statuses of Earth’s mantle and their long-term evolution,’ Dr Pachhai concluded.

Nature Geoscience published the full results of this study.