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A new report claims existing assumptions about the composition of the Earth are wrong and don't match the evidence.

The paper presented in a perspective in the journal Nature by Dr Ian Campbell and Professor Hugh O'Neill from the Australian National University, may have implications for how scientists interpret the geochemistry of the Earth, its growth, and the dynamic processes within.

Current theories claim Earth's composition should be the same as the composition of chondritic meteoroids which are among the oldest objects in the solar system. These formed 4.6 billion years ago out of the same molecular gas and dust cloud as the Sun.

"Chondritic meteoroids are thought to have retained their primitive composition, providing a snapshot of the environment they formed in," says O'Neill.

"Knowing this composition lets scientists better understand the composition of the Sun, Earth and solar system as a whole."

Other methods such as spectroscopic studies of the Sun's chemical composition are only 90 per cent accurate, while studying chondritic meteoroids allows an accuracy of up to 99 per cent.

"However new studies of isotope ratios of key rare Earth elements show Earth's composition to be different from that of chondritic meteoroids," says O'Neill.

This means either the matter from which the Earth formed was not chondritic, or Earth has lost matter by collisional erosion in the later stages of planet formation.

Another problem according to O'Neill is that not all chondritic meteoroids are the same.

"They've been affected by later impacts and weathering which has changed their chemical makeup," says O'Neill.
Time for a change

"But so ingrained is the chondritic hypothesis in the way we think, that excuses are made for the planet not being chondritic," says Campbell.

"We've tweaked and re-tweaked the existing hypothesis, but it's now time to abandon the whole thing."

"The excuse most used is that there's a layer down near the core mantle boundary where many of these missing elements are hidden."

According to Campbell, these include thorium, uranium and potassium, which are all heat producing elements.

"The only way this heat can get out, is in mantle plumes which appear on the Earth's surface as hot spots like Iceland and Hawaii," he says.

But according to Campbell, the numbers don't match up because they're only producing a fraction of the heat they should.

Campbell and O'Neill say determining the real composition of early Earth means analysing samples of the ancient mantle found on the crust.

"These are found in places like the greenstone basalts of Western Australia, and the huge volcanic magma flood basins of the Deccan and Siberian traps," says Campbell.

"The element ratios in these samples may reveal whether they match those of chondritic meteorites, or those expected from collisions during the Earth's formation."

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