Minerals yield signs of early plate tectonics

Minerals preserved within ancient grains of rock, like flies within amber, have provided the earliest hint of plate tectonics on Earth. The minerals suggest that subduction — the process by which one tectonic plate slides beneath another — may have been active more than 4 billion years ago.

The result adds to the growing evidence that Earth was 'modern-looking'— with plate tectonics, mountains, oceans and blue skies — earlier than previously thought.

Little is known about the so-called Hadean or the 'hell-like' period of Earth's history, which spans the time from Earth's formation 4.6 billion years ago until about 3.8 billion years ago. The best geological clues of this era come from hard minerals called zircons, which have survived weathering and can be found within younger rocks.

Mark Harrison of the University of California, Los Angeles, and his colleagues previously found a so-called 'titanium thermometer' that can be used to determine the temperature at which a zircon crystal formed — the higher the temperature, the more titanium the mineral contains. They found that almost all 4-billion-year old zircons formed at about 700 ºC — much lower than the usual temperatures at which these minerals form. That suggested the zircons were saturated in water when they formed. "That was wild," says Harrison, as it indicated that Earth contained water on its surface at a time when it was once thought to be a hellish pit of molten lava.

Cool under pressure

Harrison's team has now taken a closer look¹ at some 4.0–4.2 billion-year-old zircons from the Jack Hills of Australia, investigating the tiny inclusions of other minerals trapped inside them. The titanium thermometer results confirmed that these zircons, too, formed at 700 ºC. The researchers surveyed hundreds of zircons, and found that the most common mineral they contained was muscovite. Muscovite contains water, and today forms when two plates slam together and melt shale, such as where the Pacific plate sinks beneath the west coast of North America.

The team next checked the amount of aluminium present in the mineral inclusions — an indicator of the pressure at which the zircons formed. Only six of the mineral inclusions were big enough to analyze; from these, they found that the zircons must have formed at a minimum of 7,000 atmospheres of pressure — equivalent to being some 25 kilometres beneath the surface.

Given what is known about Earth's temperature at that time, it should have been about three times hotter than 700 ºC at that depth — so something must have been cooling the rocks down. "There's only one place on the planet today where you make magma and the heat is three times below average: subduction zones," says Harrison. Rock in subduction zones is cooled by ocean water.

If there was subduction, Harrison says, there may also have been a global system of plate tectonics. And that has implications for what the planet was like at that time. "The simplest view right now is that plate tectonics produces a planet that looks like it does today," he adds.