In his spare time last summer, Ming Tang's geoscientist from Rice University used to compare niobium content in various rocks in a global mineral database. What he found was worth skipping some nights with friends.
In an article published this month by Communications of nature, Tang, Rice, petrologist Cin-Ty Lee and his colleagues offered an answer to one of the fundamental questions of Earth science: Where do the continents form?
"If our conclusions are correct, every piece of land we are in now began somewhere like the Andes or Tibet with very mountainous surfaces," said Tang, the study's lead author and postdoctoral researcher in the Rice Department . Earth Sciences, Environmental and Planetary Sciences (EEPS). "Today, most places are flat because this is the stable stage of the continental crust. But what we discovered was that when the crust formed, it had to start with mountain-building processes."
The connection between niobium, one of Earth's rarest elements, and the formation of the continent is a story that unfolds over billions of years on scales as small as molecules and as large as mountain ranges. The protagonists are niobium and tantalum, rare metals so similar that geologists often think of them as twins.
"They have very similar chemical properties and behave almost identically in most geological processes," Tang said. "If you measure tantalum and niobium, you will see that their proportion is almost constant in the mantle of the Earth. This means that when you find more niobium in a rock, you will find more tantalum, and when you find less niobium you will find less tantalum."
The mantle is the thickest layer of Earth, covering about 1,800 miles between the planet's core and its thin outer crust. Earth scientists believe that little or nothing moves between the mantle and the core, but the mantle and all that is above – the bottom of the sea, oceans, continents and atmosphere – are connected and many of the Earth's surface atoms today, including atoms in humans and other living things, we run the mantle one or more times in Earth's 4.6 billion years.
The rocks on the continents are an exception. Geologists have found some that are up to 4 billion years old, meaning that they were formed near the surface and remained on the surface without being recycled in the mantle. This is due in part to the nature of the continental crust, which is much less dense than the basaltic rocks beneath the Earth's oceans. Lee, a professor and chair of the EEPS department, said it is no coincidence that Earth is the only rocky planet known to have both continents and life.
"Every day we live on continents and we take most of our resources from the continents," Lee said. "We have oxygen in the air to breathe and just the right temperature to support complex life." These things are so commonplace that we take it for granted but the Earth did not begin with these conditions, they developed later in Earth's history, and the emergence of the continents is one of the things that shaped our planet and made it more habitable. "
Scientists still do not have details about how the continents began and how they grew to cover 30% of the Earth's surface, but a great clue concerns niobium and tantalum, the geochemical twins.
"On average, continental crust rocks have about 20% less niobium than they should in comparison to the rock we see everywhere," Tang said. "We believe that this missing niobium is linked to the mystery of the continents. In solving or finding niobium, we can obtain important information about how the continents are formed."
Geologists have known the imbalance for decades. And it certainly suggests that geochemical processes that produce continental crust also remove niobium. But where was the missing niobium?
This troubling question led Tang to spend his spare time reading the records in the GEOROC database of the Max Planck Institute, a comprehensive global collection of published analyzes of volcanic rocks.
Based on these surveys and months of follow-up testing, Tang, Lee and colleagues provide the first physical evidence that "arclogites" are responsible for missing niobium. Arclogites are accumulated, the remaining slag that accumulates near the base of the continental arches. On rare occasions, pieces of these accumulate on the surface of volcanoes.
The Rice group first submitted archeological samples that Lee had collected in Arizona for his collaborator, Kang Chen, a researcher at China's University of Geosciences in Wuhan. Chen spent a month getting accurate readings of the relative amounts of niobium and tantalum in the samples. The rocks were created when the High Sierras were an active continental arc, like the Andes today.
Chen's tests confirmed high niobium-tantalum indices, but to better understand the mechanism by which this signature was developed, Tang and Lee used high precision laser ablation and "inductively coupled plasma mass spectrometry" in Lee's laboratory to reveal that the mineral rutin was responsible.
"Rutile is the mineral that houses niobium," he said. "It's a natural form of titanium oxide, and it's what really" sees the difference between niobium and tantalum and captures one more than the other. "
But this happens only under specific conditions. For example, Tang said that at temperatures above 1,000 degrees Celsius, rutile retains the normal rates of tantalum and niobium. It just starts to prefer niobium when temperatures drop below 1,000 degrees Celsius. Tang said that the only known place with this set of conditions is deep under the continental arches, such as the Andes today or the High Sierras, about 80 million years ago.
"The reason you need high pressure is that titanium oxide is relatively rare," he said. "You need a lot of pressure to force it to crystallize and fall off the magma."
In an earlier study published in Science Advances in May, Tang and Lee discovered a subtle chemical signature that may explain why the continental crust is poor in iron. Lee said that the discovery and discovery of rutile and niobium illustrates the central importance of continental arches in Earth's history.
"Continental arches are like a magic system that connects everything from climate and oxygen concentrations in the atmosphere to ore deposits," Lee said. "They are a source of carbon dioxide after they die. They can direct the greenhouse or the greenhouse, and they are the building blocks of the continents. "