Inside the world’s oldest crystals, collected from the Jack Hills in Western Australia, lie the remains of even older rocks – some of which have been reworked through magma into the surviving crystals. Using machine learning, geologists revealed that a third of these prehistoric rocks were sedimentary. This means that more than four billion years ago, when no minerals survived, the Earth had an extensive 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 mostly the same ones that were there more than four billion years ago, but nothing solid survives from that time; everything has been reworked, usually many times. It’s one of the reasons we went to the moon and studied asteroids to find a direct line almost to the birth of the solar system.
The lack of rocks that testify to the first ten percent of Earth’s existence frustrates geologists. Yet in the oldest things on Earth, beyond arrivals from space, researchers have found an unexpected clue to this 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 do something similar, but not quite as impressive.
Jack Hills zircons are the oldest surviving relics on Earth. They formed 4.4 billion years ago and were subsequently incorporated into sedimentary rocks that have since eroded away, leaving behind only zircons.
The Jack Hills zircons crystallized from magma, but not from the original magmatic ocean. This magma was made from older rocks pulled into the Earth to melt. Most of the information about these earlier rocks was lost as the magma was reworked, but one fact geologists hoped to discover is whether some of them were sedimentary or whether they were all igneous.
Igneous rocks can form from cooling magma or lava that we know existed on early Earth, but sedimentary rocks require a water cycle where the rocks are exposed to an atmosphere above the water surface. Rain erodes them and the material is washed into lakes or oceans to settle and transform into new forms of rock.
Professor Ross Mitchell of the Chinese Academy of Sciences and his colleagues took a new look at the Jack Hills zircons, as well as some from a newly discovered South African greensandstone bed that may be close to their age. By training computers to recognize the fingerprints of sedimentary material in zircons, Mitchell and colleagues were able to determine that a sample of very old zircons contained about an abundance of S-type granite. This is granite formed from sediments that were subducted into magma.
The proportion of S-type increases with time, as might be expected—but if the method used by Mitchell and colleagues 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 rather than growing forever, the S-type fraction rises and falls in accordance with cycles of supercontinent formation and collapse.
Magma melt containing sediment (“S-type granite”) from the Himalayas (left) and the Jack Hills zircon deposit in Western Australia (right).
Image credit: Ross Mitchell
To make 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 down into magma. Such a multi-step process is unlikely to be rapid, so the original islands protruding from the sea must have been there well before the zircons formed. S-type granites in such ancient zircons would also demonstrate tectonic cycles that subducted the crust into the mantle at least 4.2 billion years ago.
In other words, if an alien were to visit Earth at the beginning of its existence, they would find neither a dry orange world, as was assumed a few decades ago, nor an all-encompassing ocean, as was recently assumed.
The findings complement and extend work published in June by the team examining the ratio of oxygen isotopes in similarly aged zircons, most of which were found to have formed in the ocean. However, some zircons show signs of having formed in fresh water on land that protruded from the ocean, suggesting the presence of continental crust at 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. Two competing hypotheses for the origin of life are a warm lake proposed by Darwin and hydrothermal vents on the ocean floor.
However, the warm pond idea requires that the planet had a water cycle with land and fresh water at the time life arose. By pushing back the time when the first ponds existed, Mitchell and co-authors don’t prove that life began there, but they do show that ponds remain a contender.
The study is published in the journal Proceedings of the National Academy of Sciences.