Inside the world’s oldest crystals, collected from Western Australia’s Jack Hills, are the remains of even older rocks – some of which were reworked through magma into the surviving crystals. With the help of machine learning, geologists have discovered that a third of these primary rocks were sedimentary. This means that more than four billion years ago, at a time from which no minerals survive, the Earth had a vast crust exposed to the elements above sea level. Earth’s first few hundred million years were not as strange to us as we might think.
Earth’s atoms are largely the same as they were here more than four billion years ago, but nothing solid from that time survives; everything is recycled, usually many times. It’s one of the reasons why we went to the Moon and study asteroids, to find a direct line almost to the origin of the Solar System.
The lack of rocks that testify to the first ten percent of Earth’s existence frustrates geologists. However, in the oldest things on Earth, arrivals from space aside, researchers have found an unexpected clue about that lost era, revealing how quickly the planet evolved into something familiar. It comes just a month after the same tiny crystals were used in a different way to try something similar, but not quite as impressive.
The Jack Hills zircons are the oldest surviving relics on Earth. They formed up to 4.4 billion years ago and were then incorporated into sedimentary rocks that have since eroded away, leaving only zircons behind.
The Jack Hills zircons crystallized from magma, but not from the original magma ocean. This magma was made from older rocks pulled down to Earth to melt. Most of the information about those earlier rocks has been lost in the reworking of the magma, but one fact geologists have been hoping to find is whether any of them were sedimentary, or whether they were all igneous.
Igneous rocks can form from the cooling of magma or lava that we know existed on the early Earth, but sedimentary rocks require a water cycle, where the rocks are exposed to the atmosphere above the water line. Rain erodes them and the material is washed into lakes or oceans to settle and transform into new rock forms.
Professor Ross Mitchell of the Chinese Academy of Sciences and colleagues have taken a new look at the Jack Hills zircons, as well as some from the newly discovered South African Green Sandstone bed that can be roughly matched to their age. By training computers to recognize fingerprints of sedimentary material within zircons, Mitchell and colleagues were able to determine that a sample of very old zircons contains abundant S-type granite. This is granite formed from sediments that were submerged in magma.
The percentage of S-type increases with time, as expected—but if the method Mitchell and colleagues used to identify S-type granites is correct, zircons formed 4.24 billion years ago were made of 35 percent S-type granite. In an interesting tangent, the authors found that instead of increasing forever, the proportion of S-type rises and falls in line with cycles of supercontinent formation and collapse.
Magma melt including sediment (S-type granite) from the Himalayas (left) and the Jack Hills zircon discovery site in Western Australia (right).
Image credit: Ross Mitchell
To make an S-type granite, you need a previous process in which rocks are formed, eroded to become sediments, and then compressed into new rocks before being pushed into magma. Such a multistage process is unlikely to be rapid, so the original islands emerging from the sea must have been there considerably before the formation of the zircons. S-type granites in such ancient zircon would also prove tectonic cycles that subducted the crust into the mantle occurred at least 4.2 billion years ago.
In other words, if an alien had visited the Earth early in its existence, they would have found neither a dry orange world, as was supposed a few decades ago, nor an all-encompassing ocean, as suspected in the times of last.
The findings complement and extend work published in June, when a team investigating the ratio of oxygen isotopes within zircons of similar age found that most formed within the ocean. However, some of the zircons show signs of having formed in fresh water on land that emerged from the ocean, indicating the presence of continental crust around this time.
The presence of S-type granites in the Jack Hills zircons may have been a matter of great debate among a small subset of geologists, but it has implications for a question of much wider interest. The two competing hypotheses for the origin of life are the small warm pool proposed by Darwin and hydrothermal vents at the bottom of the ocean.
However, the warm-pool idea requires that the planet had a water cycle of land and fresh water at the time life appeared. By pushing back the time when the first ponds existed, Mitchell and co-authors haven’t proven that it was where life began, but they make a strong case that ponds remain a contender.
The study is published in the Proceedings of the National Academy of Sciences.