Crystals Reveal Earth’s Violent Ancient Past

Researchers from Trinity College Dublin have confirmed that the oldest dated minerals on Earth were formed by asteroid impacts. This is pretty exciting because it makes our geological history a little more coherent (which is more than we can say about our future…)

So let’s go back a bit. When trying to find out about Earth’s origins, geologists look to Hadean detrital zircon – literally, bits of rock formed during the hellish early period of nascent Earth. These tiny crystals are like tiny looking glasses into our ancient past, revealing the geological conditions of the time, and possibly even when water first appeared on our planet.

The earliest known zircon crystals are from around 4 billion years ago – and this has long been a problem for researchers. Zircon forms when molten rock solidifies. Okay, so where did the molten rock come from? Plate tectonics! But wait, tectonic plates weren’t around 4 billion years ago…

One answer has been that plate tectonics started earlier than we think, but that just doesn’t seem to fit the data. And researchers haven’t been able to prove that zircon could be formed by asteroid impacts. Until now.

This week, Trinity College PhD researcher Gavin Kenny and his colleagues published a paper which proves that zircon can be formed by asteroid impacts. The project has taken them from Dublin to Ontario to Stockholm, and they presented the results recently at the Lunar and Planetary Science Conference (LPSC) in Houston, Texas.

By studying samples from the Sudbury crater in Canada, the team found that zircon crystals formed by that impact were identical to those formed 2 billion years earlier. So what does this mean? Essentially, scientists don’t have to figure out how plate tectonics could have started earlier than expected. We already know that the poor old earth was being pummelled by asteroids in the past. Finally, the geology and the chemistry align!

“Given what we know about the early Earth, the simplest explanation is that the oldest zircons likely formed in impact craters,” Kenny says.

I asked Kenny what this means for future research. “Very little research has been done on the effects of an impact on pre-existing zircons that were already on Earth and then struck. How does this affect their chemistry? This is a new field and perhaps in the coming years it will tell us even more about the early Earth.”

Kenny G. G., Whitehouse M.J., Kamber B. S., ‘Differentiated impact melt sheets may be a potential source of Hadean detrital zircon’. Geology, April 2016; DOI: 10.1130/G37898.1

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