About 4.5 billion years ago, the young Earth was largely a molten world. As the planet gradually warmed and differentiated, dense iron sank toward its center, forming the metallic core, while lighter silicate materials rose to form the mantle. This separation shaped the internal structure of the Earth, but may not have marked the end of the interaction between these two vast reservoirs. In a recent study published in Science Advances, scientists investigated whether Earth’s core has remained isolated from the mantle or whether material has continued to move between them for billions of years. Scientists turned to unusual chemical signatures found in volcanic rocks, which could offer clues dating back to Earth’s earliest history. The findings suggest that hydrogen, a key component of water, has been leaking from the planet’s metallic core into the lower layer of the mantle, ultimately influencing the composition of surface volcanic rocks. Core-mantle exchange The Earth’s interior is not necessarily chemically uniform. Some regions of Earth’s deep mantle retained material from the planet’s formation, surviving the enormous heat and mixing of its early history. It is possible that some unusual material originated in the core and then traveled into the overlying mantle rock. The researchers behind the new study used computer simulations to replicate conditions found deep in the Earth. They set out to examine how two forms of hydrogen, standard light hydrogen and a heavier version called deuterium, behaved when molten rock cooled and crystallized into different minerals. The simulations revealed that as Earth’s ancient magma ocean cooled and crystallized, its two forms of hydrogen became relatively evenly distributed throughout the mantle. Although small variations in isotope ratios developed at different depths, they were insufficient to explain the exceptionally low levels of deuterium observed in volcanic rocks from places such as Baffin Island in the Canadian Arctic and Iceland. Leaky core To help explain the unusual chemical signatures found in the volcanic rock, researchers suggest that water, in the form of hydrogen with a low proportion of deuterium, has likely leaked from Earth’s metallic core into the surrounding mantle over billions of years. As the iron sank to form the core, it absorbed lighter hydrogen, leaving the Earth’s core depleted of deuterium. Another clue pointing toward a leaky core is the presence of unusually high ratios of helium-3 to helium-4 found in volcanic rock, a rare form of helium that may have been preserved in Earth’s core from its early history. The same volcanic rocks also showed unusual signatures of tungsten and ruthenium, pointing to a contribution from the core as well. The findings reinforce the possibility that primordial volatile material has been transferred from the core to the mantle. Such an exchange would have important implications for understanding the Earth’s interior. Instead of acting as completely separate chemical reservoirs, the core and mantle may have exchanged volatile elements over geological time, allowing material from Earth’s deep interior to influence the composition of volcanic rocks at the surface. Researchers hope to carry out future studies to try to discover exactly how material from the core reaches the mantle.
