Massive, Early Cataclysm on Earth and the Moon Questioned

First Posted: Oct 19, 2015 06:42 AM EDT

Scientists may have learned a bit more about the dates for cataclysm on early moon and the Earth. It turns out that the current dating method that researchers have used may be incorrect.

Durable crystals called zircons are often used to date some of the earliest and most dramatic cataclysms in the solar system. One of these is a super-duty collision that ejected material from Earth to form the moon about 50 million years after Earth formed. Another one of these incidents includes the late heavy bombardment, which was a wave of impacts that may have created hellish surface conditions for young Earth about 4 billion years ago.

Both of these events are widely accepted but unproven. That's why researchers conducted a study of zircons from a gigantic meteorite impact in South Africa.

"While zircon is one of the best isotopic clocks for dating many geological processes, our results show that it is very challenging to use ex situ zircon to date a large impact of known age," said Aaron Casovie, one of the researchers, in a news release.

While many zircons show evidence of shock, once separated from host rocks, they lost contextual information.

The "clock" in a zircon occurs as lead isotopes accumulate during radioactive decay of uranium. With precise measurements of isotopes, researchers can calculate how long lead has been accumulating. If all lead was driven off during the asteroid impact, though, this clock was reset. This means the amount of accumulated lead should record exactly how long ago the impact occurred.

In order to evaluate the assumption of clock-setting by impact, the researchers gathered zircons from Earth's largest impact in South Africa, known to have occurred 2 billion years ago.  It's estimated the original crater was about 300 kilometers in diameter.

The researchers concluded that most of the zircon ages record when they were formed in magma. In other words, they don't record the age of the impact crater, but instead the rocks they formed in, which is about 1 billion years older.

The findings reveal a bit more about zircons, which may influence our view of Earth's early history.

The findings are published in the journal Geology.

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