Thursday, April 22, 2021

Ancient rocks reveal the beginning of plate tectonics on Earth

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Tusch and Münker have developed a powerful new method to extract tiny traces of tungsten from ancient rocks. Then they went to look for the rocks.

They first analyzed Archean rocks collected in the Isua region, west of Greenland. Tusch spent 11 months analyzing the samples, but in the end his tungsten 182 data was flat, without significant variation between samples. The researchers speculated that the rocks of Greenland had been warped and heated over the course of their history, confusing their geochemical information.

They needed better rocks so they headed to Pilbara in Western Australia. “It has some of the best preserved Archean rocks on the whole planet,” Münker said. “They haven’t seen much heating compared to similar rocks of this age.”

“I was really keen to find samples that didn’t display the same value over and over again,” Tusch said.

Guided by the co-author Martin Van Kranendonk from the University of New South Wales, the team crisscrossed the Outback in off-road trucks, visiting rusty-red outcrops where ancient volcanic rocks and vegetation look alike: Spinifex bushes on the Outcrops are partly silica, which makes them prickly and inedible to all but termites. They hammered out a promising half-ton of rock and lava that formed between 2.7 billion and 3.5 billion years ago.

To analyze the isotopic ratios of tungsten in ancient rocks, geologists extracted and purified their tungsten using a process called ion exchange chromatography. A dissolved rock sample is loaded into an acidic solution, leading to the vertical separation of different elements.Courtesy of Jonas Tusch

Back in Germany, Tusch goes to work. He used a rock saw to reach into the fresh rock inside each sample, then polished a few slices down to half the width of a human hair to make them translucent for microscopy. He crushed the rest and concentrated the tungsten, then analyzed the isotope ratios of the tungsten in a mass spectrometer.

In nearly two years, the results have spread. This time the isotope ratios were not flat. “It was really nice to watch,” Tusch remarked.

Concentrations of tungsten 182 started to be high in rocks formed 3.3 billion years ago, showing that the mantle was not yet mixing. Then the values ​​declined over 200 million years to reach modern levels 3.1 billion years ago. This decline reflects the dilution of the old tungsten-182 signal when the mantle under Pilbara began to mix. This mixing shows that plate tectonics had started.

The Earth would quickly transform from a water world dotted with volcanic islands resembling Iceland in a world of continents with mountains, rivers and floodplains, lakes and shallow seas.

A new world made for life

The start date of around 3.2 billion years ago helps clarify the impact of plate tectonics on life on Earth.

Life began in advance, over 3.9 billion years ago, and made small bumpy piles in the sediments at Pilbara called stromatolites by 3.48 billion years ago. This shows that plate tectonics is not a prerequisite for life at its most basic level. Still, it’s probably no coincidence that diverse life just as the plate tectonics began.

Plate tectonics came from shallow, sunlit seas and lakes fertilized with nutrients altered by continental rocks. Bacteria evolved in these environments to harvest sunlight through photosynthesis, generate oxygen.

The fossil record shows an explosion of complex and diverse animal life dating back around 540 million years. One of the most abundant and iconic creatures of the time was the trilobite, an armored animal that thrived for tens of millions of years. Photographed here, a trilobite fossil Elrathia kingii.Courtesy of Micha L. Rieser

For another half a billion years, that oxygen remained barely a puff in the air, in part because it immediately reacted with iron and other chemicals. In addition, every oxygen molecule generated during photosynthesis corresponds to a carbon atom, and these easily recombine into carbon dioxide with no net gain of oxygen in the atmosphere, unless the carbon is buried. .

Gradually, however, plate tectonics provided the earth and sediment in which to bury more and more carbon (while providing a lot of phosphorus to stimulate photosynthetic bacteria). The atmosphere finally became oxygenated 2.4 billion years ago.

Oxygen prepared the planet for the emergence of plants, animals, and almost everything in between with an oxygen-based metabolism. Life larger and more complex than microbes requires more energy, and organisms can make far more of the vital, energy-carrying molecule called ATP with oxygen than they can without it. “Oxygen is really important for what we think of as complex life,” said Athena Eyster from the Massachusetts Institute of Technology.

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