Ancient continent revealed beneath the tropical island Mauritius

Outcrops of volcanic rocks (trachyte) on Mauritius. Samples of trachyte contained zircons pointing to a much greater age than the volcanic rocks (picture courtesy of Susan F. Webb, Univ. oft he Witwatersrand)

Outcrops of volcanic rocks (trachyte) on Mauritius. Samples of trachyte contained zircons pointing to a much greater age than the volcanic rocks (ccby: M. Wiedenbeck, GFZ).

Zircons pointing to a much longer history of the island than previously thought

31.01.2017: It all began in 2012 with grains of sand collected from a beach on Mauritius. Some of the tiny grains of lava contained even tinier crystals of zircon, a mineral pointing to a much, much longer history for the island than previously thought. Laboratory analyses showed that the young volcanic rocks of Mauritius indeed bury remnants of a much older domain which was seemingly connected to India and Madagascar some 90 million years ago. In 2013, a research team led by the Norwegian Trond H. Torsvik proposed the name “Mauritia” for the sunken continent.

The zircons were analyzed in Potsdam

Now, once again, zircons have enabled scientists to discover more about Mauritia. In the latest research zircon grains were not sampled from a beach but were extracted from a volcanic rock called trachyte that was collected on the island of Mauritius. The rock specimen was brought to Potsdam where zircons were recovered that were then analyzed to determine their age using the secondary ion mass spectrometry (SIMS) lab of the GFZ German Research Centre for Geosciences. The newest results have just been published in the prestigious journal Nature Communications. This study was conducted by Lewis D. Ashwal from the University of the Witwatersrand in South Africa, Michael Wiedenbeck from GFZ, and Trond H. Torsvik, currently guest scientist at GFZ.

Young volcanoes

Located between India and Madagascar, the beautiful islands of the Seychelles and Mauritius attract many tourists. But they are of great interest for geoscientists as well. Whereas the geology of the Seychelles shows old granitic rocks, indicating that the Seychelles archipelago was formerly part of Madagascar and India, Mauritius is of young volcanic origin. Beneath modern-day Mauritius a hotspot heated the oceanic crust, thus inducing melting of the rocks and leading to volcanism. The volcanic activity which created Mauritius started nine million years ago. However, the zircons within the beach sand pointed to continental rocks of much older origin, dating back nearly two billion years. This was the result of the earlier study published by Torsvik and colleagues in 2013 in “Nature Geoscience”.

More pieces of ancient continents

The newly published article in Nature Communications corroborates these findings, refuting any suggestion of wind-blown or pumice-rafted zircons for explaining the earlier results. According to Ashwal and colleagues, the Mascarene Islands show even older traces of the ancient continent Mauritia, reaching back to 3 billion years. When Gondwana broke apart Antarctica, Australia, and India separated from Madagascar and Africa. According to the new results this break-up did not involve a simple splitting of the ancient super-continent of Gondwana. Rather, a complex splintering took place with large fragments of continental crust of variable sizes being left adrift within the evolving Indian Ocean basin. “Our study shows that the supposedly uniform oceanic crust is in fact quite heterogeneous,” says GFZ’s Wiedenbeck, who led the SIMS analyses. He adds: “Most probably the oceanic crust holds many more pieces of ancient continental crust that are yet to be discovered.”

Original study: Ashwal, L., Wiedenbeck, M., Torsvik, T., 2107. Archaen zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius, Nature Communications, DOI: 10.1038/NCOMMS14086