Geochemists trace down the fate of Earth’s missing niobium
Scientists from the Universities at Cologne and Bonn, Germany, have shown that Earth’s missing niobium had already been lost on the asteroids from which the Earth was formed.
Cologne/Germany (upm) – The earliest stages of Earth’s formation, which took place over 4.5 billion years ago, are still not well understood. This is because the oldest rock record on Earth only dates back as far as ca. 4 billion years, i.e., half a billion years after the Earth formed. Geologists are therefore dependent on indirect information to decipher Earth’s early history. However, geological events that took place when the Earth formed have left their chemical traces in much younger rocks, e.g. in the form of abnormal contents of rare trace elements or in the decay products of radioactive isotopes. The abundances of the rare element niobium, which occurs only at very small abundances in rocks (1 gram per ton of rock or even less), are one such tracer. It has been known for about a decade that approximately 20% of niobium is missing from the silicate Earth that comprises crust and mantle. The reason for this niobium depletion was unknown, but it has long been speculated that the missing niobium must have been lost during Earth’s infancy.
The Cologne-Bonn research team carried out extremely accurate measurements of niobium concentrations in various types of meteorites from the asteroid belt. "These asteroids are the only remnants of the early solar system from over 4.5 billion years ago," explains co-author Toni Schulz, now at the University of Vienna, who has previously examined and selected many of the precious meteorite samples for the study. All larger planets in the solar system formed by the collision of such asteroidal bodies. As the researchers reported in the recently published edition of the international science magazine "Nature Geoscience", their measurements led to the completely surprising discovery that the niobium is often found in the metallic core of such asteroids in significant amounts. "This is not what we expected," says geochemist Carsten Münker from Cologne "We always assumed that the niobium is concentrated in the outer, silicate-rich mantles of asteroids, but not in their metallic cores, because it is traditionally assumed to be a “rock-loving” element". Because this discovery was so unusual, the research team decided to conduct experiments in the laboratory to understand why niobium behaves in such an unexpected way in asteroids. "Surprisingly, it was found from the results of these experiments that, when these asteroids heated up and melted in the early solar system, considerable amounts of niobium fractionated into their metallic cores," says petrologist Raúl Fonseca from Bonn who carried out the laboratory experiments.
The results of the Cologne-Bonn research team now provide new insights into the growth of the early Earth some 4.5 billion years ago. It has been known for some time that the Earth has grown by collision of asteroids until it reached its present size. The new study suggests that in many cases the metal nuclei of the incoming asteroids appear to have directly merged with the metal core of the young Earth. "Earth’s missing niobium is in its metallic core, at a depth of more than 2900 kilometers," says Carsten Münker from Cologne. "But we now know that Earth’s missing niobium had already been lost on the asteroids in the young solar system, the very building blocks of planet Earth," adds his co-author Raúl Fonseca from Bonn.
Literatur: Carsten Münker, Raúl O.C. Fonseca, Toni Schulz: Silicate Earth’s missing Niobium may be sequestered into Asteroidal Cores. Nature Geoscience: http://dx.doi.org/10.1038/ngeo3048, available from Oct 23, 2017.
Website Prof. Carsten Münker:
Website Dr. Raul Fonseca: