Hadean zircons (4.2 Ga) found in India

[Doc Socks Junior]

Northeastern India is the latest region in which material from Earth's formative years has been discovered. This geological time is known as the Hadean, since the Earth was a destructive, hellish place. Although new research has led to doubts as to how "hellish" the Hadean actually was, the fact remains that no rocks are preserved from this time (4.56 - 4.05 billion years ago). However, there are some remnants of rocks from this time period left.

Zircons!

Zircons are a simple zirconium silicate mineral. Zirconium, silicon, and oxygen. The helpful nature of zircon comes from radioactive uranium substituting for zirconium in the crystal lattice of the mineral. Uranium decays in a number of ways, resulting in different forms of lead. We extract the tiny (under 100 micrometer!) zircon crystals from rocks, and measure the lead isotopes in them. From that, we can get a U-Pb age for the zircon, and figure out when the rock formed.

http://www.thehindu.com/sci-tech/science/worlds-second-oldest-rock-is-from-odisha/article23829527.ece

Quote

A rock sample recovered nearly eight years ago from Champua in Odisha’s Kendujhar district has put India at the forefront of geological research in the world. Scientists have found in the rock a grain of magmatic zircon (a mineral that contains traces of radioactive isotopes) that is an estimated 4,240 million years old — a discovery of great promise to study the earth’s early years.

The news article is actually "fake news" to a certain degree. This is not the world's second oldest rock, the rock just contains the world's second oldest zircon. The robust nature of zircon allows it to survive being recycled into different rocks. What occurred in this case, it seems, is that as the Older Metamorphic Tonalitic Gneiss (the rock unit in question) formed ~3.4-3.5 billion years ago, it came up through a different rock, containing much older (4.24-4.0 billion year old) zircons. The OMTG melted some of the rock it came through, and the zircons came along for the ride.

The actual scientific report is open access! Go science.

https://www.nature.com/articles/s41598-018-25494-6

Quote

Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb analyses of zircons from Paleoarchean (~3.4 Ga) tonalite-gneiss called the Older Metamorphic Tonalitic Gneiss (OMTG) from the Champua area of the Singhbhum Craton, India, reveal 4.24-4.03 Ga xenocrystic zircons, suggesting that the OMTG records the hitherto unknown oldest precursor of Hadean age reported in India. 

Xenocrystic means what was explained earlier - a crystal from another rock incorporated into a younger one.

I've actually been to the area in question, but mainly doing a different type of research. Beautiful area.