Within 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 reprocessed by magma into the remaining crystals. Using machine learning, geologists have revealed that a third of these ancient rocks were sedimentary. This means that more than four billion years ago, at a time when minerals are no longer present, the Earth had an extensive crust exposed to the elements above sea level. The first few hundred million years of Earth’s existence were not as strange to us as we might think.
The atoms of the Earth are largely the same as they were here more than four billion years ago, but nothing solid from that time has been preserved; everything has been reprocessed, often many times. It’s one of the reasons we went to the moon and studied asteroids, to find a direct line that almost led to the birth of the solar system.
The lack of rocks that testify to the first 10 percent of Earth’s existence frustrates geologists. Yet researchers have found an unexpected clue to that lost era in the oldest things on Earth, short of those arriving from space, showing just 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 prove something similar, but not quite as impressive.
The Jack Hills zircons are the oldest surviving remains on Earth. They were formed up to 4.4 billion years ago and were subsequently incorporated into sedimentary rocks that have since been eroded away, leaving only the zircons.
The Jack Hills zircons crystallized from magma, but not from the original magma ocean. This magma was older rocks that were pulled into the Earth to melt. Most of the information about those earlier rocks has been lost in the reprocessing of the magma, but one fact that geologists have been hoping to discover is whether there was sedimentary rock there, or whether it was all igneous.
Igneous rocks can form from cooling magma or lava that we know existed on the early Earth, but sedimentary rocks require a water cycle, where rocks are exposed to the atmosphere above the waterline. Rain erodes them and the material is washed into lakes or oceans to settle and be transformed into new forms of rock.
Professor Ross Mitchell of the Chinese Academy of Sciences and colleagues re-examined Jack Hills zircons, as well as some from the recently discovered South African Green Sandstone Bed that may be nearly as old. 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 roughly abundant S-type granite. This is granite that formed from sediments submerged in magma.
The S-type proportion increases with time, as expected — but if the method Mitchell and colleagues used to identify S-type granites is correct, then 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 S-type proportion rises and falls in line with cycles of supercontinent formation and collapse.
Magma melts with sediment (type S granite) from the Himalayas (left) and the Jack Hills zircon locality in Western Australia (right).
Image Credit: Ross Mitchell
To make an S-type granite, you need a prior process in which rocks are formed, eroded into sediments, and then compressed into new rocks before being pushed into magma. Such a multi-stage process is unlikely to be rapid, so the original islands that protrude from the sea must have existed significantly before the zircons formed. S-type granites in such old zircons would also provide evidence that tectonic cycles that pushed the crust into the mantle occurred at least 4.2 billion years ago.
In other words, if an alien had visited Earth at an early stage, it would have found neither a dry orange world, as was believed a few decades ago, nor an all-encompassing ocean, as was suspected more recently.
The findings complement and extend work published in June, when a team examining the ratio of oxygen isotopes in zircons of the same age found that most formed in the ocean. However, some zircons show signs of having formed in freshwater on land protruding from the ocean, suggesting the presence of continental crust around this time.
The presence of S-type granites in Jack Hills zircons may have been a major debate among a small subset of geologists, but it has implications for a question of much broader interest. The two competing hypotheses for the origin of life are the warm little pond proposed by Darwin, and the hydrothermal vents on the ocean floor.
The warm pond idea, however, requires that the planet had a water cycle with land and freshwater when life first emerged. By pushing back the time to when the first ponds existed, Mitchell and co-authors haven’t proven that this was where life began, but they make a strong argument that ponds are still a contender.
The research was published in Proceedings of the National Academy of Sciences.