Geological Science

Zircons (and the secrets they hold) are forever

4-billion-year-old crystals offer clues to when plate tectonics began, setting stage for life on Earth

May 18, 2022
The Scitech

The three zircon crystals used in the study: the hafnium isotope, oxygen isotope, and trace element compositions. Zircons are like time capsules that retain chemical clues of the Earth’s first 500 million years.

Scientists have long understood that plate tectonics, the drift of separate, rigid plates that make up the Earth’s crust, formed continents and mountains and was crucial to the evolution of the planet’s surface from one of molten lava and rock to an environment hospitable to life.

What’s been less clear is when it began.

A team of Harvard-led researchers have analyzed some very rare, ancient, and nearly indestructible crystals the size of small grains of sand called zircons for chemical clues about the onset of plate tectonics. The study, published in AGU Advances, suggests that 3.8 billion years ago there was a major transition in the geochemistry of these zircons that make them look much more like the zircons that are formed today in the red-hot environments where plate tectonics happen.

“Prior to 3.8 billion years ago, the planet doesn’t seem to be as dynamic,” said Nadja Drabon, a Harvard assistant professor of Earth and Planetary Sciences and the paper’s first author. “Today, there’s lots of crust that gets constantly destroyed in what are called subduction zones and new crust is created. Many [previous] zircons showed that back then once the early crust formed, it lived for a really long time — about 600 million years in this case. While there was some internal reworking, we never created new granitic crust. … Then 3.8 billion years ago, everything changes.”

Think of zircons as tiny time capsules that retain chemical clues of the Earth’s first 500 million years. Some were formed in the magma of the planet more than 4 billion years ago when the Earth, geologically speaking, was still in its infancy. It makes them the oldest known materials on Earth. Their secrets can be understood by zapping them with lasers, which is what the researchers did for their analysis.

Source: Harvard University news release. Author: Juan Siliezar Harvard Staff Writer