Rare volcanic rocks that are the source of the world’s diamonds have been discovered in Antarctica.
In 1989, University of Tasmania PhD student Geoff Nichols discovered some unusual boulders in a glacial moraine in the Prince Charles Mountains of East Antarctica, where he was sampling and mapping the rock types of the region.
Twenty four years later, modern technology and expertise united to conclusively identify his samples as rare volcanic rocks known as ‘kimberlite’ — the source of most of the world’s diamonds.
The discovery was reported in the journal Nature Communications in December 2013, by an Australian team that included Dr Nichols and Chief Investigator, Dr Greg Yaxley, a petrologist at the Australian National University.
‘These rocks represent the first reported occurrence of genuine kimberlite in Antarctica,’ Dr Yaxley said.
‘They are of great scientific and commercial importance, as they are the most deeply derived, direct samples of the Earth’s interior, and they are the major hosts of diamond, sometimes in economic abundance.’
Kimberlites are formed by the melting of the earth’s mantle under extreme pressure, at about 200km depth, and in the presence of gaseous ‘volatiles’ such as water, carbon dioxide and methane. The gas-filled rocks ascend rapidly and violently to the surface during volcanic activity, sometimes incorporating diamonds into their structure along the way. Their name comes from the South African town of Kimberley, famous for a late 19th century diamond rush.
Like the diamonds they sometimes carry, kimberlites are rare, often occurring in small outcrops less than 100m across, and mostly restricted to ‘cratonic regions’ — ancient parts of the continents that are geologically stable. But they occur on every continent. In Australia they’ve been found in parts of South Australia, northwest Western Australia and the Northern Territory.
‘It’s not surprising that Antarctica has kimberlites; they’re just very difficult to find,’ Dr Yaxley said.
When Dr Nichols first collected the rocks, it was not immediately apparent that they were kimberlites, but he knew that they were likely generated within the deep mantle. Preliminary work on the rocks suggested they were geochemically related ‘carbonatites’. Later work by University of Tasmania Earth Science Professor, Vadim Kamenetsky, suggested they were ‘carbonate-bearing picrites’.
‘Only the latest more detailed textural, mineralogical and geochemical analysis, and the specialised expertise of each co-author, enabled us to conclusively categorise the rocks as bona fide kimberlites,’ Dr Nichols said.
‘Kimberlite has a very specific mineralogy and chemistry, and we were able to use optical and electron microscopy and whole rock geochemistry to confirm its identity.’
Radiometric dating showed the rocks are some 120 million years old.
‘The Antarctic kimberlites were located at the margin of a major Antarctic transcontinental rift, the Lambert Graben, which may have reactivated when the Indian and Australia-Antarctica plates separated during the Cretaceous period, 145 million to 66 million years ago,’ Dr Yaxley said.
‘This movement along the rift would have caused a little bit of deep melting, resulting in the eruption of these kimberlites.’
The discovery is scientifically interesting as it provides another connection between continents that were once joined together in the supercontinent, Gondwana.
‘The age of the Antarctic kimberlites overlaps with many kimberlites from other world-wide localities, extending a vast Cretaceous, Gondwanan kimberlite province, for the first time, into Antarctica,’ Dr Yaxley said.
The study is an example of how geological research can play an important role in understanding Antarctica’s place in the world, including its geological, geomorphological and climate history. Mining and other non-scientific activities relating to Antarctica’s mineral resources are strictly prohibited under the Protocol on Environmental Protection to the Antarctic Treaty (Madrid Protocol).
Yaxley GM, Kamenetsky VS, Nichols GT, et al (2013). The discovery of kimberlites in Antarctica extends the vast Gondwanan Cretaceous province. Nature Communications 4: 2921, doi: 10.1038/ncomms3921
Corporate Communications, Australian Antarctic Division