Greenlandic rocks shift geological historical perspectives
Rocks dated at 3.4 billion years old, found in Greenland's south-west Isua mountain range, have given up valuable information regarding the structure of Earth during its earliest stages of development. This discovery was made by a French-Danish team led by researchers from the 'Magmas and Volcanoes' Laboratory, a joint research unit of Blaise Pascal University, the Centre National de la Recherche Scientifique (CNRS) and the Institute of Research and Development (IRD). Their research was mainly funded by a European Research Council (ERC) starting grant. What has intrigued the research team are the rocks' lack of neodymium-142, which is an essential chemical element for the study of the Earth's formation.
This missing chemical element supports the hypothesis that the Earth was made up of an ocean of molten magma, which gradually cooled, approximately 100 and 200 million years after its formation. According to the team's research paper, published in the journal Nature, 'the first indisputable evidence for very early differentiation of the silicate Earth came from the extinct 146Sm-142Nd chronometer'.
The Earth is believed to have formed 4.58 billion years ago, by accretion of material in the Solar System. The heat produced by the accretion process, as well as by the decay of radioactive elements, caused this material to melt. As a result, 100 to 200 million years after its formation, Earth must have been made up of an ocean of molten magma, in the centre of which a metallic core formed. Over the years, the ocean gradually cooled, and like liquid chocolate on a cold day, the Earth's crust then appeared, and the process of continental drift soon followed. The crystallisation of the molten magma is likely to have been accompanied by the chemical layering of Earth: concentric layers with distinct chemical compositions became differentiated. It is the signature of these primordial inhomogeneities that the researchers found in the Isua rocks.
What intrigued the scientists and grabbed their attention was the lack of a key chemical element, the isotope neodymium-142, formed by the decay of a now vanished radioactive isotope called samarium-146. The abundance of neodymium-142 is almost identical in all terrestrial rocks. Only two exceptions have been discovered to date, in Canada and Greenland, in certain rocks dating back 3.7 billion years. The composition of these rocks shows evidence of the primordial inhomogeneities that formed as the magma ocean crystallised.
In 2003, for the first time, two groups of French researchers observed an excess of neodymium-142 in certain rocks in the same region. If such excess were to be found in some layers of the primordial Earth, it would mean that other layers must also be depleted in this isotope. However, until these new findings by the French-Danish team, such neodymium-142 deficits remained hypothetical for nine years. Using a sophisticated method, thermal ionisation mass spectrometry, the researchers carried out a very detailed analysis of the concentration of neodymium-142 in Isua rock samples. They discovered a neodymium-142 deficit of 10.6 parts per million, which lends weight to the 'magma ocean' theory.
According to their article, 'results testify to the existence of an enriched component in the Hadean, and may suggest that the southwest Greenland mantle preserved early-formed heterogeneities until at least 3.4?Gyr [billion years] ago'.
These findings should help to improve models of the internal dynamics of Earth during its early stages of development. By discovering a neodymium-142 deficit in relatively young rocks, formed around a billion years after the crystallisation of the magma ocean, the researchers have shown that the primordial inhomogeneities persisted longer than predicted before being eliminated by convective motion in Earth's mantle. In order to obtain more comprehensive data, the scientists now intend to study the composition of other rocks of similar age outcropping, for example in Canada, South Africa and China.
Category: Project results
Data Source Provider: CNRS; Nature
Document Reference: Hanika Rizo et al., 'The elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archean rocks', Nature, 1 November 2012. doi:10.1038/nature11565
Subject Index: Coordination, Cooperation; Earth Sciences; Scientific Research