doi:10.1038/nindia.2018.90 Published online 17 July 2018
About 4.24 billion years old – older than the oldest rocks on Earth – and barely the width of a human hair, tiny grains of zircon have been found trapped in a granite rock in the eastern Indian state of Odisha1. Geologists say these indestructible crystals may shed light on the first mysterious half-billion years of Earth’s history.
The newly found zircon crystals in the Kendujhar district of Odisha are the oldest yet unearthed from Indian soil. Like Earth’s oldest zircons – aged about 4.4 billion years found in the Jack Hills in Western Australia2 – they predate the first recorded rocks on Earth, estimated to be 3.9 billion years old.
Zircon is a mix of silicon, oxygen and zirconium and can outlast the magma rocks in which it is formed. Each crystal, therefore, could be a snapshot of geological events from early Earth.
Scientists haven't yet fully understood the nature and composition of the Earth’s earliest crust and how it gradually evolved to form the outermost layer on which humans live. Zircons, the oldest minerals on Earth, preserve robust records of chemical and isotopic characteristics of the rocks in which they form.
“Studying oxygen isotopes in the zircons could give us valuable information about the presence of liquid water in the first few hundred million years of Earth’s history,” said lead researcher of the Indian study Rajat Mazumder from Curtin University, Malaysia.
Zircons can also tell when the movement of Earth’s plate-like outermost layers, or plate tectonics, began. Such zircon-containing old rocks are being used to analyse various planetary processes, including the search for life on early Earth and on other planets such as Mars, he said.
In 2011, Mazumder teamed up with Trisrota Chaudhuri, a researcher at the Kolkata-based Indian Statistical Institute (and now with the Geological Survey of India), on a trip to Champua, a small town in Kendujhar that had previously yielded 3.6-billion-year-old zircon-containing rocks3. They combed the region and collected samples from many rocks. Back in the lab, they examined the rock samples under microscope and detected the zircon crystals.
With the help of Yusheng Wan, a geologist from the Chinese Academy of Geological Sciences in Beijing, China, they were able to date two zircon crystals back to 4.24 and 4.03 billion years. These crystals were embedded in a 3.4-billion-year-old granite rock.
Shortly after the Earth came into existence, it was filled with a global magma ocean. Study of the zircons reveals that the Earth’s primitive crust was made of rocks solidifying from this magma ocean, Mazumder told Nature India. “Analysis of zircons also indicates that this primordial crust later re-melted to produce silica-rich rocks containing the detected zircon grains,” he said.
Elizabeth Bell from the University of California, Los Angeles, in the United States, who has studied zircons from the Jack Hills, says analysing this new repository of zircon crystals from early Earth would be exciting. Given the dearth of information about that period, this previously unexamined source of zircon is significant, she told Nature India.
Many of the Jack Hills zircons appear to have been derived from sedimentary rocks, and their oxygen isotopic ratios point to the presence of liquid water weathering rocks on Earth's surface since at least 4.3 billion years ago, Bell revealed.
“So, there is nothing that precludes similar lines of evidence from being preserved in these magma-derived zircons found in India,” she said.
1. Chaudhuri, T. et al. Evidence of enriched, Hadean mantle reservoir from 4.2-4.0 Ga zircon xenocrysts from paleoarchean TTGs of the Singhbhum Craton, Eastern India. Sci. Rep. 8, 7069 (2018)