The 2 Billion Years of Earth's Core: A Comprehensive Guide
The blazing core of Earth has been observed interacting with other, otherworldly layers, thus it is not an isolated entity. This is supported by a recent study that discovered some of the planet's interior seeps into mantle plumes, some of which eventually reach the surface of Earth.
The researchers noted that this finding contributes to the resolution of a long-running controversy regarding the amount of material exchanged between the core and mantle.
The researchers published their results in The Conversation, a website where academics share their research with the general public. "Our findings suggest some core material does transfer into the base of these mantle plumes, and the core has been leaking this material for the past 2.5 billion years," they said.
The metal tungsten, which is element 74 on the periodic chart, allowed for the discovery. A dating profile for tungsten might state that it is a siderophile, or "iron lover." Given that iron and nickel make up the majority of Earth's core, it should come as no surprise that tungsten makes up a large portion of its composition.
Tungsten's profile would also indicate that it has a few isotopes, such as W-182 and W-184. The researchers discovered that these isotopes could aid them in resolving the core-leaking mystery as they were planning their investigation.
Hafnium, another element, is a lithophile, which means it is found in the Earth's silicate-rich mantle and loves rocks. The radioactive isotope of hafnium, Hf-182, decays into W-182 after 8.9 million years. The scientists deduced that this meant the mantle should have more W-182 than the core.
The 182W/184W ratio of ocean island basalts, which originate from mantle plumes, "may therefore indicate chemical exchange between the core and the source of mantle plumes," the researchers said in the report.
However, the tungsten difference would be negligibly tiny: It was anticipated that the mantle and core would have tungsten-182 compositions that differed by only 200 parts per million. The researchers stated in The Conversation that "fewer than five laboratories in the world can do this type of analysis."
Furthermore, because the core starts at a depth of almost 1,800 miles below the surface, studying it is difficult. To put that into perspective, the Kola Superdeep Borehole in Russia, which is around 7.6 miles deep, is the deepest hole that has ever been excavated by mankind.
The next best thing, therefore, was to study the rocks that seeped to the surface of the Earth from the deep mantle at the Pilbara Craton in Western Australia, as well as the hotspots of Reunion Island and the Kerguelen Archipelago in the Indian Ocean...
Leak detected
The presence of tungsten in these rocks indicated a core leak. The W-182-to-W-184 ratio in Earth's mantle underwent a significant alteration during Earth's existence, the researchers discovered. They found that, strangely, the W-182-to-W-184 ratio of Earth's earliest rocks is higher than that of the majority of rocks today.
The researchers stated in The Conversation that "the change in the 182W/184W ratio of the mantle indicates that tungsten from the core has been leaking into the mantle for a long time."
The age of Earth is roughly 4.5 billion years. Tungsten isotopes, however, showed no appreciable alteration in the planet's oldest mantle rocks. According to the researchers, this indicates that there was either minimal or no material exchange from the core to the upper mantle between 4.3 billion and 2.7 billion years ago.
However, there has been a significant shift in the mantle's tungsten isotope makeup over the last 2.5 billion years. What caused this to occur? The researchers hypothesized that material from Earth's surface may be sliding down into the deep mantle like a see-saw if mantle plumes are erupting from the core-mantle barrier. The researchers noted that the presence of oxygen in this surface material can have an impact on tungsten.
"Subduction, the term used for rocks from Earth's surface descending into the mantle, takes oxygen-rich material from the surface into the deep mantle as an integral component of plate tectonics," the investigators wrote in The Conversation. "Experiments show that an increase in oxygen concentration at the core-mantle boundary could cause tungsten to separate out of the core and into the mantle."
Alternatively, the oxygen content in the outer core may have grown when the inner core solidified after Earth formed, according to the researchers. "In this case, our new results could tell us something about the evolution of the core, including the origin of Earth's magnetic field," they wrote in the Conversation.
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