The nature of Earth’s earliest crust and crustal processes remains enigmatic due to the almost complete lack of a rock record older than 4 billion years (Hadean Eon) and the scarcity of rocks between 4 and 3.6 billion years (Eoarchean Era). One unresolved question is when mobile-lid tectonics commenced. To study the Hadean, the only direct record is from the mineral zircon, but Hadean zircon is rarely found. In new research, scientists from Harvard University and elsewhere analyzed Hadean to Eoarchean zircon crystals found in the Barberton Greenstone Belt, South Africa. They found that zircons older than 3.8 billion years were not formed in a subduction zone setting, a hallmark of modern plate tectonics. Instead, the early crust originated from remelting of crust derived from a relatively undepleted mantle and, in comparison to modern mantle-derived crust, was distinctive in its longevity. After 3.8 billion years ago, geochemical signatures of the zircons start showing similarities to those of zircons derived from modern subduction zones. This shift can also be seen in other zircon suites in the 3.8-3.6 billion year time window, possibly indicating major transition in crustal geochemistry and global onset of mobile-lid tectonics in that time period.
Hadean Earth. Image credit: Alec Brenner.
Earth’s crust and the top layer of mantle just under it are broken up into rigid plates that move slowly on top of viscous but mobile lower layers of mantle rock.
Heat from Earth’s core drives this slow but inexorable motion, responsible for volcanoes, earthquakes, and the uplift of mountain ranges.
Estimates for when this process revved up and modern crust formed range from over 4 billion years ago to just 800 million years ago.
Uncertainty arises because the geologic record from Earth’s youth is sparse, due to the surface recycling effect of plate tectonics itself.
Almost nothing remains from the Hadean Eon, Earth’s first 500 million years.
“Prior to 3.8 billion years ago, Earth doesn’t seem to be as dynamic,” said Dr. Nadja Drabon, a researcher in the Department of Earth and Planetary Sciences at Harvard University and the Department of Geological Sciences at Stanford University.
“Today, there’s lots of crust that gets constantly destroyed in what are called subduction zones and new crust is created.”
“Many previous zircons showed that back then once the early crust formed, it lived for a really long time — about 600 million years in this case.”
“While there was some internal reworking, we never created new granitic crust…. Then 3.8 billion years ago, everything changes.”
Zircons retain chemical clues of the Earth’s first 500 million years. Some were formed in the magma of the planet more than 4 billion years ago when the Earth, geologically speaking, was still in its infancy. It makes them the oldest known materials on Earth.
Their secrets can be understood by zapping them with lasers, which is what the researchers did for their analysis.
The authors saw that 3.8 billion years ago as the planet was cooling a lot of new crust was suddenly being formed and that the geochemical signatures of zircons began to look like those generated in subduction zones, the places where two colliding tectonic plates meet and one slides under the other and into the mantle where it is recycled (code word for burned to a crisp).
“It’s not clear whether there were subduction zones 3.8 billion years ago, but what is known is that the new crust being formed was likely a result of some type of plate tectonics,” they said.
The scientists gathered 3,936 new zircons from a 2017 expedition in South Africa. The crystals ranged from 4.1 billion to 3.3 billion years old.
They looked at three different geochemical features of the zircons: the hafnium isotope, oxygen isotope, and trace element compositions. Each gave them a different piece of the puzzle.
For instance, the hafnium isotope offered hints about the formation and evolution of the Earth’s crust; the oxygen isotopes about whether there were oceans; and the trace elements about the composition of the crust.
The data suggested that the rate of crust formation started picking up almost 4 billion years ago.
The authors also looked at data from other studies on ancient zircons that have been found around the world to see whether they saw evidence of a similar shift. They did when it came to data on hafnium isotopes.
“All of them show this shift between 3.8 and 3.6 billion years ago,” Dr. Drabon said.
“There wasn’t much data on the other two geochemical features and hopes to focus on those next, including looking at when oceans started forming.”
“I don’t even know where to start. There’s so much to do.”
The findings were published in the journal AGU Advances.
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Nadja Drabon et al. Destabilization of Long-Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga. AGU Advances, published online April 21, 2022; doi: 10.1029/2021AV000520
