Feb. 28 (UPI) -- Modern volcanic hotspots may be linked to molten rock formed billions of years ago, just after Earth formed. According to new research, the study of hotspot lava could offer new insights into the geologic evolution of early Earth.
During Earth's formation, the planet divided into two material layers. Denser, iron metal sank, forming the core, while less-dense, silicate-rich rock rose to form the mantle. During Earth's early history, deep pockets of the mantle rose and separated, solidifying to form Earth's crust. Some portions sank back to the bottom as they solidified and gained density.
The convection-like cycle -- processes of rising and falling, melting and solidifying -- continues today. Over the course of Earth's geologic history, the cycle has created a mostly uniform chemical composition throughout the mantle.
Despite millions of years of churning, however, not all of the mantle has become thoroughly mixed.
In a new paper, published this week in the journal Nature, geologists argue that some parts of the mantle remain unblended, with a chemical composition and texture different from most of the molten rock found in the mantle.
New analysis of volcanic rocks collected from Réunion Island in the Indian Ocean suggest islands formed by volcanic hotspots are linked with these ancient pockets of unmixed mantle.
As part of their analysis, scientists developed a new method for identifying these unique portions of the mantle using radioactive isotopes.
Elemental isotopes with an unstable number of neutrons release energy during their radioactive decay. Over time, this process can change the number of protons and neutrons in the nucleus, causing the element to transform into a entirely new element.
Samarium-146, for example, boasts a half-life of 103 million years, after which it decays into neodymium-142. Samarium-146 was present when Earth first formed, but quickly went extinct, disappearing just 500 million years after Earth formed. As such, an abundance of neodymium-142 serves as a signature of ancient rocks.
Differences in the ratios between neodymium-142 and other isotopes can reveal whether a rock has been significantly altered during its time in the mantle, or remains mostly unchanged since its formation a few billion years ago.
The latest analysis of neodymium isotope ratios suggests plume magma on Réunion is sourced from a pocket of mantle that remains chemically unaltered.
"The mantle differentiation event preserved in these hotspot plumes can both teach us about early Earth geochemical processes and explain the mysterious seismic signatures created by these dense deep-mantle zones," Bradley Peters, a geologist with the Carnegie Institution for Science, said in a news release.