Australien: Rätsel um rosafarbene Diamanten aus Argyle-Formation gelöst
Seit November 2020 ist die Argyle-Mine geschlossen. Der Bergbaukonzern Rio Tinto befand, der Tagebau im Nordwesten Australiens sei nach 37 Jahren und dem Fund von 865 Millionen Karat Rohdiamanten wirtschaftlich erschöpft. Dabei ist Argyle der einzige bekannte Fundort für seltene und daher begehrte rosafarbene und rote Diamanten. Lange war unklar, wie es zu den außergewöhnlichen Kleinoden in Argyle kommen konnte. Ein Forschungsteam aus Australien in Zusammenarbeit mit Geologen des Bergbaukonzerns hat nun nach eigenen Angaben die »fehlende Zutat« gefunden. Die Ergebnisse hat die Gruppe in der Fachzeitschrift »Nature Communications« veröffentlicht.
Diamond mines are typically found in the middle of continents, in the tectonically stable parts known as cratons. The volcanic mineral Kimberlite, which is named after the South African city Kimberley and not the Australian region Kimberly that includes Argyle, is known for its high diamond content. However, Argyle is located near the coast, at a junction between different land masses away from a craton. In theory, there should be no diamonds there. However, the mine is geologically unique: it is situated in a region where approximately 1.8 billion years ago, the currently connected land masses of the Kimberly region and northern Australia collided, which is believed to make a significant difference.
Einzigartig: Blick auf die Diamantmine Argyle in der westaustralischen Kimberly-Region
Foto: Murray Rayner / AFP
Two out of three factors contributing to the formation of pink diamonds were already known, according to mineralogist Hugo Olierook from Curtin University in Perth, the main author of the study, as stated in an accompanying statement. Firstly, carbon was stored several hundred kilometers deep in the Earth’s mantle, instead of assuming the form of graphite at shallower depths. Secondly, a continent collision distorted the crystal lattice of the diamonds and gave them their color – pink under relatively lighter pressure and brown under stronger pressure. The light brown hues of Champagne and Cognac are even more common in Argyle than others, but they are not lucrative enough for mining purposes.
However, a third factor was still missing as a trigger to explain how the crystals could reach the surface. For this, extreme acceleration was necessary to prevent the carbon from transforming into graphite on its way up. Stretching of the Earth’s crust is believed to have made this possible.
Suche nach dem »nächsten Argyle«
The authors of the Australian study used fine laser beams to determine the age of the crystals, which is around 1.3 billion years, approximately 100 million years more than previously believed. According to the group led by Oliebrook, this aligns with the time when the supercontinent Nuna broke apart, which had united the land of Earth during the Paleoproterozoic era.
The regions of present-day Australia were united at that time, but according to Oliebrook, they were „stretched“. As a result, the „scars“ of the previous collision of land masses opened up and magma shot through them at nearly the speed of sound. Included in this process were the pink diamonds. The force of the eruption is said to have been stronger than any volcanic eruption in human history.
Oliebrook suggests that similar mountainous regions, originating from the end of the supercontinent Nuna, could also contain pink diamonds. A „next Argyle“ could be discovered by specifically searching for locations where the three ingredients – deep carbon, continental collision, and subsequent stretching – come together, explained the researcher. Areas in Canada, Russia, southern Africa, and Australia are potential candidates. However, some of these potential sites may still be covered by sand and other layers of soil.