They are from the earliest period of our planet’s history.
A recent study has found two “birthmarks” in Earth’s mantle consisting of silicate material that formed when our planet was less than 50 million years old.
The research paper, published May 13 in the journal Science, was authored by Tais. W. Dahl, Assistant Professor in Geobiology at the University of Copenhagen, with assistance from University of Maryland Professor of Geology, Richard Walker. Dahl found clear signatures of a distinctive material in two completely separate locations on the globe — Baffin Bay in the north Atlantic and Ontong Java Plateau in the western Pacific Ocean.
According to Walker, this is the first clear indication that portions of the mantle — the 3,000-kilometer-thick layer of hot silicate rocks between the crust and the core — that formed during Earth’s primary accretion (growth) period still exist today.
“What we’ve found are surviving parts of Earth’s primitive mantle that have been preserved for four and a half billion years, and I think that’s kind of exciting!” said Walker in a University of Maryland news release.
Earth likely grew to its current size through the build-up of material from collisions with celestial bodies of increasing size over several tens of millions of years early in the history of the solar system. The last and most massive of these impacts was a collision between proto-Earth and a planetary embryo about the size of Mars, called Theia, that resulted in the formation of the moon.
However, it is commonly believed that it is unlikely any pieces of rock from the earliest period of Earth’s history survived. The collisions are thought to have homogenized material from the Earth’s early mantle, and that any that had survived were likely recycled through plate tectonics.
But that view began to change in 2012 when Walker and colleagues discovered some material from the mantle that had existed 2.8 billion years ago.
According to the authors, “Four and a half billion years of geologic activity have overprinted much of the evidence for the processes involved in Earth’s formation and initial chemical differentiation.” Luckily, some primitive rocks have managed to stay around.
Using high-precision measurements, the team identified early mantle material based on the detection of an overabundance of an isotope of tungsten. The radioactive element hafnium decays into tungsten, and 182-hafnium is a form of the element that was present when our solar system was created, but is no longer found on Earth today.
The decay of 182-hafnium into 182-tungsten happens so quickly that variations in the abundance of 182-tungsten, relative to other isotopes, can only be due to processes that occurred very early in the history of our solar system.
Dahl notes in his paper that the discovery can “provide a view of events that occurred during the first few tens to hundreds of million years of Earth’s history.”