Understanding the early Earth

The Earth formed 4.6 billion years ago but finding evidence for its early composition and history is difficult, with the Earth’s crust and underlying mantle having continuously evolved through melting, and recycling. Ambre Luguet of Bonn University has used new isotopic dating methods to uncover more about the first 750 million years of the earth’s existence, and in particular how the Earth’s mantle has preserved information about the crust formation.

The Earth’s crust, its outermost layer, formed from partial melting of the inner silicate mantle which acts as a reservoir available to generate the crust. So far our limited knowledge of the timing and extent of early crust formation events comes from isotope analysis of zircon found in crust minerals. They have been recycled into much younger rock but like tiny time capsules, they preserve isotopic information about their age and origin over billions of years. ‘The earliest ones recovered on Earth are as old as 4.36 billions of years,’ says Luguet. But she explains, due to the intense bombardment that the Earth has experienced, a lot of early-formed parts of the Earth’s crust may have disappeared.

But what about the underlying mantle? ‘Considering the mother-daughter link between the Earth’s mantle and the Earth’s crust, it makes sense to investigate the first crust formation from the mantle point of view,’ says Luguet. Rather than investigating isotope ratios in Zircon minerals she has looked at Osmium-rich alloys, Osmium-bearing platinum group minerals and sulphides such as erlichmanite (OsS2 - Osmium sulphide), which she describes as ‘the zircons of the mantle’.

State-of-the-art isotopic dating

Luguet performed state-of-the-art isotopic dating on alloys, platinum group minerals and sulphides from the oldest mantle rocks from Botswana, South Africa and Greenland, known as peridotites and the oldest rocks derived from the mantle from Greenland, known as chromitites. The isotopic dating is based on the radioactive decay of the isotope Rhenium-187 into Osmium-187 and Platinum-190 into Osmium-186. ‘The sulphides and platinum group minerals left in the mantle after partial melting are strongly concentrated in Osmium but depleted in Rhenium so the Osmium-187/Osmium-188 isotopic signature, directly corresponds to a partial melting age,’ explains Luguet.

What is new about her method says Luguet, is the size of mineral grains that she has been able to analyse. ‘It is the first time that we measured the isotopic composition in sulphides whose size is less than 20 micrometres. Until now other researchers used laser ablation, which is fast but limits the size of the sulphide being analysed to 80-100 micrometres. Instead, we extracted the sulphide grain from its host rock using a micro-extraction approach combining drilling a ring around the sulphide and scooping it out.’ The new method avoids some of the biases that occur when only larger grains are being analysed so the real complexity and heterogeneity of isotopic compositions can be sampled, ultimately provides more accurate conclusions.

Preserved signatures of partial melting

‘What we have learnt is that the Earth’s mantle via its Osmium-rich minerals does preserve the signatures of partial melting events, the oldest one being 4.36 billion years ago,’ comments Luguet. This figure is similar to the oldest crust formation recorded within the zircon isotope data. Luguet adds that what she calls the ‘genetic’ mother-daughter link between the partial melting of the mantle and crust formation events was known for samples younger than 3.8 billions of years; ‘Now we have showed that it also exists as early as few hundred millions of years after Earth’s formation.’

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