Why are the most beautiful diamonds in the world found in Australia?
2023-11-27
For nearly 40 years, Western Australia's Argyle diamond mine has been one of nature's most outstanding treasures. At its peak, the Argyle mine produced more colored diamonds than anywhere else on earth and was world-famous for its unique pink diamonds, dubbed the diamond world’s “pink wonder.”
For decades, researchers have been trying to uncover the origins of the Argyle deposit's dazzling diamonds. Most of the Argyll region's diamond deposits and mines are found in volcanic rocks deep within the Earth, and now, by dating these minerals, scientists think they may finally be piecing together how the deposit was formed some 1.3 billion years ago. In the study, published in Nature Communications, the team speculated that the early breakup of the supercontinent brought Argyll's pink rocks to the Earth's surface from extreme depths (and huge pressures).
The Argyle deposit is located 2,200 kilometers northeast of Perth, Australia, in the country's rugged Kimberley region, and once covered an area the size of 94 football fields. From its opening in 1983 until its closure in 2020, when gem mining was no longer economically viable, the Argyle deposit produced more than 865 million carats of rough diamonds. Most of these diamonds have a light yellow or brown color. But there are also a small percentage of diamonds that are pink, purple or red. In fact, more than 90% of the world’s pink diamonds come from the Argyle deposit.
The pink color of Argyll's most luxurious diamonds is related to the damage they suffered deep in the earth. According to geologist Hugo Olierook of Australia's Curtin University and lead author of the study, these diamonds start out colorless, but the intense pressure created after continental collisions can change The crystal structure of diamonds makes these diamonds turn into colorful colors such as pink. "Diamonds get bent and twisted," Olluk said. "If they twist a little bit, some diamonds will turn pink." Further twisting will turn them brown.
Argyll diamonds probably turned pink and brown about 180 million years ago. At that time, a plate in what is now western Australia collided with the northern part of the Australian plate, deforming the rocks in the area. But this only explains part of the origin of the Argyle deposit. When continental plates collided, diamonds from the region were buried in the Earth's mantle, hundreds of kilometers above the Earth's surface. If the crystals were any closer to the surface, they would be compressed into another structure, changing from sparkling diamonds to dark gray chunks of graphite.
Volcanoes are necessary to bring molten diamonds from the Earth's mantle to the surface. As the molten material rises, carbon dioxide and steam expand, and a volcanic eruption occurs. Olluk likened the phenomenon to popping a champagne cork. In the Argyll region, such an eruption is likely to occur on a beach, where sand and seawater interact with a volcanic rock called lamproite.
To determine when the volcano erupted, the team took two thin slices of volcanic rock from the Argyle deposit and polished them to an extremely small width. By analyzing the mineral composition of the samples under a microscope, the researchers determined that they contained sand from ancient Argyll beaches and dated them using the radioactive elements they contained. By dating the "youngest" sand grains, scientists can estimate when the beach was buried by lava. They also used tiny laser beams to determine the age of the mineral titanite, which is formed when magma fuses with quartz in beach sand.
By comparing the ages of the youngest sand grains and the oldest sphene crystals, the researchers speculated that the Argyll eruption occurred between 1.3 billion and 1.26 billion years ago. This age range was earlier than previously estimated, surprising Olluk and colleagues.
In fact, the timing of that eruption coincided with a tumultuous period in the history of Earth's plate tectonics, when the first supercontinent, called Nuna, was breaking apart. The team speculates that this may have reopened a crack in the ground at the boundary of the continent where Argyll is now located. This in turn triggered volcanic activity that brought diamond-bearing melts to the surface, creating the vast diamond deposits of the Argyll region.
Evan Smith, a researcher at the Gemological Institute of America who studies diamond geology and was not involved in the study, said the new time estimate adds important context to understanding the Argyle eruption.
Olluk believes similar events could occur at other continental borders around the world. But most diamond-bearing deposits are found in the middle of rocky exposed continental plates. This makes the Argyle deposit an exception. When the Argyle deposit was first discovered, most geologists thought it would be futile to search for diamonds along continental plate boundaries because these boundaries are typically elevated by ancient mountain belts and buried beneath soil and sand.
