October Editorial      

The Earth's oldest rocks

The Earth may have had a solid crust for longer than scientists formerly believed. This is the conclusion which emerged in June this year from a team of scientists led by John Valley, of the University of Wisconsin-Madison. His team reported evidence that the Earth's crust existed 4.3 billion years ago (see August 2008 editorial ). The dating was based on analysis of the Detrital zircons found in the Jack Hills of Western Australia. However, although the team could date the earliest rocks, these rocks no longer exist as they were destroyed during the early, very violent, past of the Earth when it was subjected to meteorite bombardment. Now a team from McGill University in Montreal, Canada has gone one step further by finding rocks which they have dated at 4.28 billion years old. These rocks come from a greenstone belt known as the Nuvvuagittuq greenstone belt, in northern Quebec. Their latest research has been published in the September issue of Science magazine (Science 26 September 2008: Vol. 321. no. 5897, pp. 1828 - 1831). The technique used to analyse the rocks was based on measuring the element samarium. To be more precise. Samarioum 146 which is radioactive and decays into a stable isotope (in this case 142 neodymium). From there it is possible to measure the levels of Nd in the rock samples and estimate the time for the radioactive decay. The results show that some rock types analysed have lower ¹⁴²Nd/¹⁴⁴Nd ratios than the terrestrial standard which means that they had to originate from a reservoir whichwas originally deficient in ¹⁴²Nd. The samarium radioactive system is now extinct. This means that radioactive samarium no longer exists on Earth. Actually, what makes it more useful in this analysis is that the samarium was extinct at around 4.1 to 4.2 billion years ago. As a resultsamarium is not very useful for dating younger rocks. To employ the samarium system of dating, the rocks have to be older than 4.2 billion years. In fact, the authors point out that the rocks from Nuvvuagittuq greenstone belt are the very first terrestrial rocks old enough for this method of dating to be possible.

But is there any other explanation which could account for the existence of these isotopes? When the team started their research about four years ago, they already knew that the rocks were very old. The original age was estimated at about 3.8 billion years. This of course is much younger than the required 4.2 billion years required by the samarium system of dating. So the team hypothesized that either the rocks were much older than 3.8 billion years. Alternatively, the anomaly in the chemical composition of these rocks was because the source of the rocks was very old mantle - older than 4.2 billion years - which gave rise to the rocks. The only problem with this theory is that nobody has found this mantle. So the scientists are more inclined to believe that actually what they are looking at is very old rocks rather than younger rocks formed from a very old mantle reservoir which is deficient in ¹⁴²Nd. Whatever the final answer turns out to be, the findings are very exciting.

But why this obsession with old rocks? Finding a really old rock on Earth is rare but they are essential for figuring out how the crust on the Earth formed, and as a result of this formation how the planet become what it is today. It is estimated that our Solar System is around 4.6 billion years old. So if indeed the rocks from the greenstone belt in Canada are 4.28 billion years old, that would put the formation of these rocks really early in the evolution of our planet, when the earth was just cool enough to have a crust.

But the discovery of these ancient rocks may have yet more significance. These ancient rocks have a banded iron formation - fine ribbon-like bands of alternating magnetite and quartz. The banded iron formation, BIF in short, is typical of rock precipitated in deep sea hydrothermal vents (see also BIF) - which have been put forward as potential habitats for early life on Earth. The presence of BIF as early as 4.28 billion years ago would suggest that even then the Earth had an ocean with hydrothermal circulation. Now extrapolating further, the formation of BIFs is thought to be initiated by a bacterium called Cyanobacteria. Cyanobacteria was the very first life form to use water and the sun for photosynthesis, a process which produced free oxygen as a waste product. The increase in oxygen levels caused the oxidation of free iron in the sea. So if the BIF can be proven to had been formed at 4.28 billion years, some type of bacteria, if not necessarily Cyanobacteria, should have been present to initiate this BIF formation. If this is true we are not only looking at the Earth's oldest rocks but also at the oldest evidence of life anywhere.