A team of geoscientists from Australia, Canada and the UK has detected primordial chemical signatures preserved within young kimberlites, small-volume volcanic rocks that are the source of most diamonds.
“Knowing the chemical signature of Earth’s original building blocks is the holy grail of geochemistry,” said Professor Graham Pearson, a researcher in the Department of Earth and Atmospheric Sciences at the University of Alberta and co-author of a paper published in the journal Nature.
“This knowledge can help us understand the formation of the planets in the Solar System as well as their relationship to each other.”
Professor Pearson and colleagues analyzed samples from three large kimberlite fields in southern Africa, Brazil and western Canada.
“We found that kimberlites’ chemical (helium isotope) signatures are remarkably uniform and very like the chondritic building blocks thought to have formed the Earth 4.55 billion years ago,” Professor Pearson said.
“We think this is the cleanest signal of this primordial signature found in any group of rocks on Earth.”
The Earth’s interior is constantly cycling, as minerals at the planet’s surface are continually returned to great depths through movement of the tectonic plates.
“This mixing makes it very difficult to detect the starting ingredients — a bit like trying to detect the brand of flour that was used by tasting an over-cooked cake,” Professor Pearson said.
“The final twist is that in the last 200 million years, these signatures became more scarce, likely due to events associated with the breakup of the supercontinent, Pangaea.”
“The discovery of these primordial signatures suggest that reflect the melting of deep, isolated regions of pristine material within the Earth that have remained untouched by billions of years of tectonic plate recycling — revealing a glimpse of the holy grail that will shed new light on how Earth, and planets like it, formed.”
Jon Woodhead et al. 2019. Kimberlites reveal 2.5-billion-year evolution of a deep, isolated mantle reservoir. Nature 573: 578-581; doi: 10.1038/s41586-019-1574-8
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