New Delhi — Geologists have identified a cluster of rocks in southern India that they say is a signature of volcanic eruptions that triggered the separation of Madagascar from India and may have been the source of the iconic Kohinoor diamond.

Geochronological studies on the outcrop called the Timmasamudram cluster, 40km southwest of Anantpur in Andhra Pradesh, suggest it is about 90 million years old and belongs to a family of rocks called kimberlites that are known to be rich in diamonds.

While more than 100 kimberlites had earlier been catalogued in southern peninsular India, they are all about one billion years old. Timmasamudram is the first 90 million-year-old kimberlite cluster from southern India.

“The Timmasamudram kimberlite cluster is significant,” said N.V. Chalapathi Rao, professor of geology at the Banaras Hindu University, who led the studies.

“Its age and its chemistry tell us it is connected to the magmatic event underlying the India-Madagascar separation, which also happened about 90 million years ago,” he told The Telegraph .

The Geological Survey of India has over the past several decades catalogued dozens of kimberlite rocks in southern India, broadly clustered into three fields: southern Wajrakarur kimberlites, northern Narayanpet kimberlites, and central Raichur kimberlites.

Timmasamudram is located within Wajrakarur, the largest of these three fields. Some of the other sites in this field too have been recognised as sources of diamonds.

The researchers say it is possible the Timmasamudram kimberlite cluster was the original source of the Kohinoor, which was found about 200km north of the location of the rock.

“But over millions of years, rivers and their tributaries may have shifted with changes in surface terrain,” Rao said. “During this process, the Kohinoor and other diamonds may have been transported from their original sites to the Krishna riverbed.”

A website on famous diamonds says the Kohinoor passed from one ruler to another in India before it became the property of the East India Company and was presented to Queen Victoria in 1850. The diamond is now part of the British crown jewels, and is on display in London.

Geological studies from India and Madagascar suggest that massive plumes of magma led to their separation after both had broken away from the ancient super-continent called Gondwanaland.

“To break a continent, we need deep-seated thermal activity,” said Ashish Dongre, a research scholar at the University of Johannesburg in South Africa who is studying geological mechanisms to explain the origins of kimberlites in India.

Kimberlites represent deep rocks that originate some 150km beneath the earth’s surface. The temperatures and pressures at those depths are conducive to the formation of diamonds, Telegraph reported

“Magma plumes transport diamonds to the surface or close to the surface,” Rao said.

Geologists caution that while some kimberlites are rich in diamonds, they may not be economical for commercial diamond mining.

The abundance of diamonds in kimberlites depends on the rate of the magma eruptions and the oxygen levels in the magma, Suresh Patel, professor of geology at the department of earth sciences at the Indian Institute of Technology, Mumbai, told this newspaper.

“In oxygen-rich magma, the diamond is destroyed: it turns into carbon dioxide -and if the rate of the magmatic eruption is slow, the diamond can turn into graphite,” said Patel, who is independently studying the Timmasamudram kimberlites.

The Geological Survey of India had earlier delineated the Timmasamudram kimberlites into four sections, or pipes, so-called because they resemble carrot-shaped structures extending into the ground.

Patel and his colleagues have found that a pipe named TK1 is poor in diamonds, possibly because of an oxygen-rich magma environment, while the TK4 pipe is relatively rich in diamonds.

The TK4 pipe is 14 metres by six metres and has been shown to reach depths of up to 30 metres, Patel said. “Such a small kimberlite section is not economical for diamond mining because the total volume of the kimberlite material is low.”

Patel had two years ago studied a 65-million-year old kimberlite near Behradih village in Jharkhand’s Koderma district. The Behradih kimberlite is 300 metres by 160 metres and extends to a depth of at least 100 metres.

“The volume of the kimberlite material in Behradih certainly makes it economical to look for diamonds there,” Patel said.
telegraphindia