Last updated January 29, 2018 at 5:20 pm
The Earth’s formative years may have been even more chaotic than previously thought.
New evidence suggests that when the Earth’s core was extracted from the mantle, the mantle never fully mixed, leaving a number of dense pockets – not unlike chocolate chips in cookie dough.
That is surprising given that the core formation happened in the immediate wake of large impacts from other early Solar System objects that the Earth experienced during its growth, similar to the giant impact event that later formed the Moon.
“Before now, it was widely thought that these very energetic impacts would have completely stirred the mantle, mixing all of its components into a uniform state,” says Colin Jackson from the Smithsonian Institution.
Jackson and colleagues from the Smithsonian and the Carnegie Institution for Science developed their hypothesis while studying the unique and ancient tungsten and xenon isotopic signatures at volcanic hotspots in places such as Hawaii.
Plumes of hot rock surging upward
While it is believed that plumes of hot rock surging upward from the Earth’s mantle at these hotspots originated from the mantle’s deepest regions, the origin of the isotopic signatures has been debated.
The team believes the answer lies in the chemical behaviour of iodine, the parent element of xenon, at very high pressure.
Using diamond anvil cells to recreate the extreme conditions under which Earth’s core separated from its mantle, they determined how iodine was partitioning between metallic core and silicate mantle.
They also demonstrated that if the nascent core separated from the deepest regions of the mantle while it was still growing, then these pockets of the mantle would possess the chemistry needed to explain the unique tungsten and xenon isotopic signatures, provided these pockets remained unmixed with the rest of the mantle all the way up through the present day.
Iodine starts to dissolve into the core
“The key behaviour we identified was that iodine starts to dissolve into the core under very high pressures and temperatures,” said Carnegie’s Neil Bennett.
“At these extreme conditions, iodine and hafnium, which decay radioactively to xenon and tungsten, display opposing preferences for core-forming metal.
“This behaviour would lead to the same unique isotopic signatures now associated with hotspots.”
The researchers also predict that the tungsten and xenon isotopic signatures should be associated with dense pockets of the mantle.
“Like chocolate chips in cookie batter, these dense pockets of the mantle would be very difficult stir back in, and this may be a crucial aspect to the retention of their ancient tungsten and xenon isotopic signatures to the modern day,” Jackson said.
They say their findings support increasing geophysical evidence that there are dense regions of mantle resting just above the core, called ultralow velocity zones and large low shear velocity provinces.
And they believe the methodology they have developed will open new opportunities for directly studying the deep Earth processes.