Plate tectonics is a fundamental factor in the sustained habitability of Earth, but its time of onset is unknown, with ages ranging from the Hadean to Proterozoic eon. University of Rochester’s Dr. John Tarduno and colleagues examined plate tectonics from a time 3.9 billion years ago, when they believe the first traces of life appeared on Earth. But they found that mobile plate tectonics was not occurring during this time. Instead, they discovered, Earth was releasing heat through what is known as a stagnant lid regime.
Plate tectonics allows heat from Earth’s interior to escape to the surface, forming continents and other geological features necessary for life to emerge.
Accordingly, there has been the assumption that plate tectonics is necessary for life. But the new research casts doubt on that assumption.
“Ideally, the presence or absence of a mobile lithosphere can be tested using paleomagnetism, but even the best preserved oldest rocks on Earth have experienced metamorphism, and this places severe restrictions on the type of magnetic carriers that might retain primary signals,” Dr. Tarduno and co-authors explained.
“Single-crystal paleointensity, whereby single mineral crystals that contain magnetic inclusions capable of recording the ancient field are studied, provides an approach to see through this metamorphism.”
“The only known detrital crystals that can be accurately dated, and that are able to provide constraints on lithospheric mobility spanning the multi-hundred-million-year timescales that typify plate-tectonic cycles, are detrital zircons bearing primary magnetic inclusions.”
Zircons are tiny crystals containing magnetic particles that can lock in the magnetization of Earth at the time the zircons were formed.
By dating the zircons, geoscientists can construct a timeline tracing the development of Earth’s magnetic field.
The strength and direction of Earth’s magnetic field change depending on latitude. For example, the current magnetic field is strongest at the poles and weakest at the equator.
Armed with information about zircons’ magnetic properties, scientists can infer the relative latitudes at which the zircons formed. That is, if the efficiency of the geodynamo is constant and the intensity of the field is changing over a period, the latitude at which the zircons formed must also be changing.
But Dr. Tarduno’s team discovered the opposite: the zircons they studied from South Africa indicated that during the period from about 3.9 to 3.4 billion years ago, the strength of the magnetic field did not change, which means the latitudes did not change either.
Because plate tectonics includes changes in latitudes of various land masses, plate tectonic motions likely weren’t occurring during this time and there must have been another way Earth was removing heat.
Further reinforcing their findings, the authors found the same patterns in zircons they studied from Western Australia.
“We aren’t saying the zircons formed on the same continent, but it looks like they formed at the same unchanging latitude, which strengthens our argument that there wasn’t plate tectonic motion occurring at this time,” Dr. Tarduno said.
The findings appear this week in the journal Nature.
_____
J.A. Tarduno et al. Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetism. Nature 618, 531-536; doi: 10.1038/s41586-023-06024-5
