2023-10-12 04:56:36
Lost in the mountains of the far north of Western Australia, the partly open-cast Argyle mine has not been active since 2020. While it was one of the world’s main sources of natural diamonds, its reserves were exhausted and its operator Rio Tinto decided to close it. This site was especially renowned for its pink diamonds, of which it accounted for 90% of production worldwide. From a geological point of view, this deposit was an enigma. Hugo Olierook, from Curtin University, Australia, and his colleagues shed light on it by reconstructing the history of the site and its precious stones.
The value of natural diamonds is mainly due to their rarity, which is explained by the extreme conditions necessary for their formation. For carbon to crystallize into diamond, temperatures between 1,100°C and 1,400°C and pressures between 4.5 and 6 gigapascals are required. These conditions are reached at depths of several hundred kilometers. Then the diamonds are quickly brought back to the surface in convection movements of magmatic rock. Diamonds are found mainly in cratons (stable and ancient continental rocks) more than 2.5 billion years old (i.e. dating from the Archean era). The Argyle site is an exception, as the surrounding rock is much younger and dates from the Paleoproterozoic (2.5 to 1.6 billion years old). It is also located on a suture zone, that is to say at the meeting of two cratons and therefore presenting warmer conditions than in the heart of a craton.
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The diamond, witness to the depths
Another particularity of the Argyle site: its pink diamonds. To obtain a diamond of this shade (but also red or brown), greater pressure is required which deforms the network of the initially colorless diamond, as during a collision of continental plates. Since this transformation occurs deep down where colorless diamonds formed, how did pink diamonds get near the surface?
Examples of octahedral diamonds mined from the Argyle mine.
© Murray Rayner
Aerial view of the Argyle diamond mine site, Australia.
© Murray Rayner
To establish a chronology of the events which led to the formation of the Argyle site, Hugo Olierook and his colleagues carried out the dating of rocks from the Australian mine. They notably used uranium-lead dating. Uranium decays into stable lead with a half-life of 700 million years for uranium 235 and 4,500 million years for uranium 238. By measuring the ratio of concentrations of these elements in titanite and zircons (two types of silicates), researchers have identified the major stages in the region’s history.
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Diamond deposits
A significant portion of zircons were formed between 1.87 and 1.85 billion years ago. This corresponds to the meeting of two continental crusts (of which the cratons of Kimberley and northern Australia remain). There was first a phase of subduction, with the Kimberley craton passing under the other plate. Then, under the pressure, the first ended up breaking and the area was sutured forming a shallow and fragile rift. During this episode, pink diamonds were probably formed.
This meeting of the plates is part of the larger context of the formation of the supercontinent Columbia (also called Nuna), which began 2.2 billion years ago and ended 1.8 billion years ago. years when all the continental plates came together. This supercontinent lasted for 500 million years before breaking up 1.3 billion years ago.
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The color of diamonds
Researchers have dated some zircons and titanite to the time of Nuna’s breakup. They would thus be 100 million years older than previously thought. With this new estimate, for Hugo Olierook and his colleagues, a new scenario is emerging. When the supercontinent broke apart, mechanical forces reopened some pathways in the weakened Argyle Rift. This caused melting of rock at depth which came up carrying the diamonds near the surface. The Argyle mine has thus revealed its last secret!
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