Scientists have found material in rock formations that dates back to shortly after the Earth formed, a discovery that may help understand the processes that shaped our planet over the last 4.5 billion years.
Earth formed from the accretion of matter surrounding the young Sun. The heat of its formation caused extensive melting of the planet, leading Earth to separate into two layers when the denser iron metal sank inward towards the centre, creating the core and leaving the silicate-rich mantle floating above.
Over the subsequent 4.5 billion years of Earth's evolution, convection in Earth's interior, like water boiling on a stove, caused deep portions of the mantle to rise upwards, melt, and then separate once again by density.
The melts, since they were less dense than the unmelted rock, rose to form Earth's crust, while the denser residues of the melting sank back downward, altering the mantle's chemical composition in the process.
The mantle residues of crust formation were previously believed to have mixed back into the mantle so thoroughly that evidence of the planet's oldest geochemical events, such as core formation, was lost completely.
However, researchers including Sujoy Mukhopadhyay of University of California Davis, were able find a geochemical signature of material left over from the early melting events that accompanied Earth's formation.
They found it in relatively young rocks both from Baffin Island in Canada, and from the Ontong-Java Plateau in the Pacific Ocean.
These rock formations are called flood basalts because they were created by massive eruptions of lava. The solidified lava itself is only between 60 and 120 million years old.
However, the team discovered that the molten material from inside Earth that long ago erupted to create these plains of basaltic rock owes its chemical composition to events that occurred over 4.5 billion years in the past.
They measured variations of the abundance of an isotope of tungsten in these rocks.
Tungsten contains one isotope of mass 182 that is created when an isotope of the element hafnium undergoes radioactive decay. The time it takes for half of any quantity of hafnium-182 to decay into tungsten-182 is 9 million years.
The team determined that the basalts from Baffin Island, formed by a 60-million-year-old eruption from the mantle hot-spot currently located beneath Iceland, and the Ontong-Java Plateau, which was formed by an enormous volcanic event about 120 million years ago, contain slightly more tungsten-182 than other young volcanic rocks.
Because all the hafnium-182 decayed to tungsten-182 during the first 50 million years of Solar System history, these findings indicate that the mantle material that melted to form the flood basalt rocks that the team studied originally had more hafnium than the rest of the mantle.
The discovery offers new insight into the chemistry and dynamics that shaped our planet's formative processes.
The study was published in the journal Science.
