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Planet Earth
International  
29 November 2017

New study questions timing on formation of Earth’s crust and metals distribution

An Early Earth study on ancient magmatic rocks has raised questions about the processes that shaped continents and distributed metals between 4 and 2.5 billion years ago.

The study reveals that Earth’s continental crust started evolving an unexpected 300 million years earlier than previously understood, in a process identified for the first time.

Most of the continental crust on Earth is believed to have formed between 4 to 2.5 billion years ago, known as the Archaean Eon among geologists. Intense and long-lasting volcanic action at that time formed a proto-crust that would have broadly resembled today’s oceanic crust.

In a study published in GeoScience Frontiers, Geology researchers from the University of Johannesburg show that Earth’s continental crust started evolving, or differentiating, 300 million years earlier than previously understood.

The Earth developed from a seething mass of magma, which is molten rock, to the habitable planet we know now. The traditional view saw crust formation broadly as a process in three phases. However, the new results indicate that several cycles of thickening and melting of the proto-crust produced further magmas, each with a different composition. So the Earth’s crust differentiation was far earlier and more complex than what was traditionally believed,” says co-author Marlina Elburg.

“This process of crust differentiation is a fundamental step in the history of the Earth, which eventually led to formation of continents, the distribution of base and precious metals, and setting the stage for the development of life on land,” adds Elburg.

The traditional approach

To estimate when the Earth’s crust started forming, previous researchers relied on analysing rocks that formed deep in the Earth’s continental crust from magma slowly cooling and crystallising, called plutons.

It was possible to do that because plutons are now exposed on the surface of the Earth. After plutons were formed, the rocks that formed above them in shallower portions of the crust, were removed by billions of years of destructive geological processes.

Previous linear understanding of crust formation

Using this traditional approach, previous researchers described the process of Earth’s crust differentiation broadly in three phases.

The first phase was understood as a time of volcanic activity that produced magnesium (Mg)-rich rocks, known as komatiites and basalts.

In the second phase 3.5-3.2 billion years ago, the magnesium (Mg)-rich rocks melted to yield silica (Si) - and sodium (Na)-rich plutons, a series of rocks collectively known as TTG in geology terms.

In a third phase starting at 3.2 billion years ago, partial melting of the TTG rocks yielded silica(Si)- and potassium(K)-rich rocks, called GMS.

This scenario envisages a linear geochemical development of the Earth’s crust over time, with the concentrations of silica (Si) and potassium (K) increasing in younger rocks.

Ancient eruption fragments analysed

The new research from the University of Johannesburg shows that the traditional approach has given us a biased view of the geochemical development of the earth – and an inaccurate estimate for when this happened, says co-author Axel Hofmann.

“We took existing data about volcanic rocks and analysed fragments of rocks within 2 km of the surface which formed when magma crystallised and solidified (shallow intrusive rocks) and combined that with data from better-known plutons. We also sampled fragments of magmatic rocks high in feldspar and quartz (felsic rock) formed during ancient erosion events and preserved as cobbles in a conglomerate,” adds Hofmann.

In this way, the team could assess how the chemistry of magmatic rocks on the Kaapvaal craton of southern Africa changed in the period 3.6 to 3.2 billion years ago.

Unexpected results

“What we found is that potassium (K)-rich granitic rocks, which were thought to only become an important part of the earth’s crust around 3.2 billion years ago, were already forming 300 million years earlier, from 3.5 billion years ago, says Elburg.

“Also, at 3.5 billion years ago, the shallow crust already contained copious amounts of high-potassium(K) rocks, in contrast with the traditional understanding of crust formation,” she adds .

The results give a more complete account of the composition and internal structure of the continental crust in the Archaean and throws a new light on the Earth’s geochemical development.

Crust formation not so simple after all

The new understanding is that the Palaeoarchaean continental crust was vertically zoned, with most of potassium (K)-rich magmatic rocks concentrated in its shallow portions and the sodic (Na) rocks concentrated at deeper levels.

After formation of the continent was complete, this shallow, potassium (K)-rich level of the crust was subsequently largely lost to erosion, but pebbles of these high-potassium (K) rocks can still be found in younger sedimentary rocks.

The new findings have important implications for the processes that played a role in the development and stabilisation of continents, as low-density high-silica (Si) rocks such as those of the GMS suite are more likely to form stable cratons.

At the same time, this transfer of elements towards the shallow crust may have also affected the distribution of base and precious metals, with strong implications on Archaean mineralisation processes.

The full implications of the research results, particularly for understanding the processes that formed continents and distributed base and precious metals between 4 billion and 2.5 billion years ago, need to be considered carefully.

“Metals such as gold, silver and copper are expected to have been affected by the same processes that concentrated sodium and potassium in the upper crust. However, further research will be needed to investigate this important implication of our study and to understand whether these metals could have been concentrated to the extent of forming an ore deposit”, says co-author Andrea Agangi.

Where did this research appear?

The research study “A review of Palaeoarchaean felsic volcanism in the eastern Kaapvaal craton: linking plutonic and volcanic records” appeared in the journal Geoscience Frontiers at  http://www.sciencedirect.com/science/article/pii/S1674987117301366

DOI = https://doi.org/10.1016/j.gsf.2017.08.003

Feature image credit: www.123rf.com

 

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