Cratons are the oldest, most stable parts of Earth's crust, and as such hold clues to Earth's early evolution. Dewashish Upadhyay, a geochemist now at the Indian Institute of Technology in Kharagpur, analysed the make-up of isotopes in rocks from India's Bastar craton and found that some of the rocks carry the signature of a differentiation event — the separation of materials with different geochemical properties. This event must have taken place during the first 400 million years of Earth's history, possibly when a magma 'ocean' covering the planet solidified.

The area encompassing the Bastar craton and the neighbouring Eastern Ghats Granulite Belt of southeastern India is rich in old rocks. The presence of alkaline igneous rock — a type of rock derived from Earth's mantle — marks the location where the Indian subcontinent split apart from another land mass 1.4 billion years ago and then collided with East Antarctica 1.1 billion years ago.

For his graduate project at the University of Bonn in Germany, Upadhyay made extended visits to the craton to collect samples and map the area. His initial analyses determined that the rocks were enriched in what he calls 'incompatible elements' — elements that prefer to remain in magma rather than become incorporated into crystallizing minerals.

During the course of this work, the geological field was stirred up by the discovery of rocks in southwest Greenland that provided evidence that a terrestrial magma ocean had crystallized to produce Earth's crust and mantle within 400 million years of the Solar System's formation. Isotopic analysis of the Greenland rocks had revealed an excess of neodymium-142 compared with terrestial standards, which indicated a separation of elements during that early, geochemically tumultuous time.

142Nd is produced by the α-decay of samarium-146, a now-extinct nuclide. Major geochemical differentiation events, such as the formation and crystallization of a magma ocean, could have fractionated samarium and neodymium, producing reservoirs with contrasting 142Nd compositions. Because the Greenland reservoirs were rich in 142Nd, researchers started to look for a complementary reservoir with a 142Nd deficit. And Upashyay wondered whether his rocks, rich as they were in incompatible elements, could have come from such a reservoir.

To test his suspicions, he needed to measure the ratio of 142Nd to 144Nd, a naturally occurring radioisotope that serves as a reference point. “If we can measure variation in 142Nd, it implies there was some sort of a differentiation event that fractionated samarium from neodymium,” Upadhyay says.

Upadhyay got the opportunity to do this analysis as a postdoc in Klaus Mezger's laboratory at the University of Münster in Germany. It revealed a deficit in 142Nd in four out of seven rock samples. He and his coauthors concluded that the rocks were derived from a source formed during Hadean time — Earth's earliest geological aeon — and that some such Hadean reservoirs may be preserved in the mantle beneath cratons (see page 1118).

Now Upadhyay is back in India as a faculty member. He is working to set up his own lab, which will be the first in the country able to do the kind of isotopic analysis required to track elements — and so Earth's history.