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Barberton Mountains keep rocking the geological world


Barberton Mountains keep rocking the geological world

First it revealed stunning clues about life on Earth, now the site is disclosing some of the planet's earliest secrets

Cape Town bureau chief

Two months after evidence from SA’s latest world heritage site pushed back the start of terrestrial life by 500 million years, it has provided a new window into Earth’s origins.
Zircon crystals from the Barberton Greenstone Belt, east of Pretoria, are expected to reveal some of the planet’s early secrets, including clues to the chemistry of its crust during the Archean Eon, between 4 billion and 2.5 billion years ago.
Ancient crystals of zircon, a durable mineral found in rock that has been squeezed and heated, first emerged in Western Australia. The new examples, from Weltevreden Farm west of Barberton, were subjected to less heating and squeezing, raising hopes that they may be more revealing about the chemistry of Earth’s early years.
Earth’s first 1.5 billion years are shrouded in mystery because there is no significant rock record older than 4 billion years, and very little until about 3 billion years ago.
Now the Barberton Makhonjwa Mountains world heritage site will help to fill in the gaps following the discovery of zircon crystals believed to be up to 4.1 billion years old.
Scientists from Louisiana State University in the US have found evidence at the site that komatiites, 3 billion-year-old volcanic rocks found within the Earth’s mantle, have a different composition than modern equivalents.
Within rocks from 10 lava flows thought to be the hottest ever to erupt on Earth’s surface, they discovered a well-preserved mineral called olivine, the major constituent of Earth’s upper mantle.
“Discovering fresh unaltered olivine in these ancient lavas was a remarkable find. The fieldwork was wonderfully productive and we were eager to return to the lab to use the chemistry of these preserved olivine crystals to reveal clues of the Archean Mantle,” said Keena Kareem, a member of the Louisiana team.
Results of the team’s work over more than three decades in the Barberton Makhonjwa Mountains have been published in the October edition of Nature Geoscience.
The researchers suspect parts of early Earth’s magma ocean are preserved in the approximately 3.2 billion-year-old minerals.
“Oxygen isotopes in these fresh olivines support the existence of ancient chunks of the frozen magma ocean. Rocks like this are very rare and scientifically valuable,” said Louisiana geology professor Gary Byerly.
Researcher Benjamin Byerly said: “We attempted to reconcile the findings with some of the conventional explanations for lavas with oxygen isotope compositions like these, but nothing could fully explain all of the observations.
“It became apparent that these rocks preserve signatures of processes that occurred over 4 billion years ago and that are still not completely understood.”
In August, Times Select reported that fossilised remains of microbial life found on the Barberton Greenstone Belt date back 3.22 billion years, forcing scientists to change their view that the earliest primitive organisms crawled out of the ocean 2.7 billion years ago.

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