Published: Monday, 03 January 2022 20:25
Ultra-low velocity zones sit beneath the central Pacific and Africa, atop the outer core of Earth. In these areas, seismic waves slow by as much as half, and density goes up by a third. Geoscientists initially thought that these zones were areas where the mantle was partially melted, and might be the source of magma for hot spot volcanic regions like Iceland. New research suggests that it’s possible some of ultra-low velocity zones are leftovers from the processes that shaped the early Earth.
“Ultra-low velocity zones could be collections of iron oxide, which we see as rust at the surface but which can behave as a metal in the deep mantle,” said Dr. Michael Thorne, a researcher in the Department of Geology and Geophysics at the University of Utah.
“If that’s the case, pockets of iron oxide just outside the core might influence the Earth’s magnetic field which is generated just below.”
“The physical properties of ultra-low velocity zones are linked to their origin, which in turn provides 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,” added Dr. Surya Pachhai, a postdoctoral researcher in the Department of Geology and Geophysics at the University of Utah and the Research School of Earth Sciences at the Australian National University.
To get a clear picture, Dr. Thorne, Dr. Pachhai and their colleagues studied ultra-low velocity zones beneath the Coral Sea, between Australia and New Zealand.
It’s an ideal location because of an abundance of earthquakes in the area, which provide a high-resolution seismic picture of the core-mantle boundary.
The hope was that high-resolution observations could reveal more about how ultra-low velocity zones are put together.
But getting a seismic image of something through nearly 2,900 km (1,800 miles) of crust and mantle isn’t easy. It’s also not always conclusive — a thick layer of low-velocity material might reflect seismic waves the same way as a thin layer of even lower-velocity material. So the researchers used a reverse-engineering approach.
“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,” Dr. Pachhai said.
“Our next step is to compare those predicted recordings with the recordings that we actually have.”
One particular question the scientists wanted to answer is whether there are internal structures, such as layers, within ultra-low velocity zones.
The answer, according to their models, is that layers are highly likely. This is a big deal, because it shows the way to understanding how these zones came to be.
“To our knowledge this is the first study using such a Bayesian approach at this level of detail to investigate ultra-low velocity zones, and it is also the first study to demonstrate strong layering within an ultra-low velocity zone,” Dr. Pachhai said.
“More than 4 billion years ago, while dense iron was sinking to the core of the early Earth and lighter minerals were floating up into the mantle, a planetary object about the size of Mars may have slammed into the infant planet.”
“The collision may have thrown debris into Earth’s orbit that could have later formed the Moon. It also raised the temperature of the Earth significantly — as you might expect from two planets smashing into each other.”
“As a result, a large body of molten material, known as a magma ocean, formed. The ocean would have consisted of rock, gases and crystals suspended in the magma. The ocean would have sorted itself out as it cooled, with dense materials sinking and layering on to the bottom of the mantle.”
“Over the following billions of years, as the mantle churned and convected, the dense layer would have been pushed into small patches, showing up as the layered ultra-low velocity zones we see today.”
“So the primary and most surprising finding is that the ultra-low velocity zones are not homogenous but contain strong heterogeneities (structural and compositional variations) within them,” Dr. Pachhai explained.
“This finding changes our view on the origin 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 and that they are still not well mixed after 4.5 billion years of mantle convection.”
The team’s paper was published in the journal Nature Geoscience.
S. Pachhai et al. Internal structure of ultralow-velocity zones consistent with origin from a basal magma ocean. Nat. Geosci, published online December 30, 2021; doi: 10.1038/s41561-021-00871-5
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