Mysterious crystals found on an Australian beach reveal a stunning connection to Antarctica

Grenades have fascinated humans for millennia. The ancient Egyptians decorated their jewelry with garnets, and the Greeks called them “lamp stones” because they believed they could help them see in the dark. Recently, crystallized garnet particles have highlighted a fascinating geological link between Australia and Antarctica, uncovering an ancient Antarctic mountain range now under ice. These groundbreaking findings were published in Communications Earth & Environment.

Detrital minerals and sediments in sand can reveal the tectonic history of their locations. For this study, scientists analyzed detrital garnet geochronology to piece together the orogenic events recorded in sediments. Geologists from the University of Adelaide discovered these garnet crystals while exploring the beach at Petrel Cove in South Australia. “This journey started with the question of why there was so much garnet on the beach at Petrel Cove,” says researcher Jacob Mulder.

Classical detrital minerals and sediments in the sand are known to provide useful information about the tectonic history of the place where they were found. Thus, for this study, scientists determined the detrital garnet geochronology to reconstruct the orogenic events from the sedimentary record. These garnet crystals were discovered when some geologists from the University of Adelaide combed the beach at Petrel Cove in South Australia. “This journey started with the question of why there was so much garnet on the beach at Petrel Cove,” said Jacob Mulder, one of the team’s researchers.

While Australia is known for its pink lakes, pink wildflowers and pink desert roses, the pink sand that covered the beach piqued the curiosity of researchers. They wondered what the source of these garnet crystals could be. Most garnets are formed when sedimentary rocks with a high aluminum content, such as shale, are metamorphosed and subjected to heat and pressure. The high heat and pressure break the chemical bonds in the rocks and cause minerals to recrystallize, according to Geoscience Australia. Garnet is typically destroyed by prolonged exposure to the marine environment. So when geologists found garnet in Australia’s pink sands, they concluded that it did not come from a local source of rock. They suggested in the study that it likely traveled from a nearby source.

The research, led by PhD student Sharmaine Verhaert and Associate Professor Stijn Glorie, used a new, advanced method to show that the garnet grains found were approximately 590 million years old. According to the research paper, more than 550 individual garnet grains from modern beaches and Permo-Carboniferous strata in South Australia were analyzed using the recently established, high-throughput laser ablation Lu-Hf dating method. The method, called ‘lutetium-hafnium dating’, uses a laser system attached to a mass spectrometer to determine the age of the particles.

These garnet grains were separated from four modern beach sand samples and two samples from the Permo-Carboniferous glaciogenic Cape Jervis Formation, the latter being part of the Troubridge Basin that once covered large parts of southern Australia. They also determined garnet ages based on local rock units.

From previous studies, it was known that garnet was formed mainly as a result of two events. One of these events was the Delamerian orogeny, the event that created the Adelaide Fold Belt about 490 to 514 million years ago. The second event was the formation of Gawler Craton in western South Australia, about 1.4 to 3.3 billion years ago. Interestingly, the age of the garnet found on the South Australian beach did not match any of these time scales.

“The garnet is too young to come from the Gawler Craton and too old to come from the eroding Adelaide Fold Belt,” Verhaert said, adding: “Garnet requires high temperatures to form and is usually associated with the formation of large mountain belts. , and this was a time when the South Australian crust was relatively cool and not mountainous.”

Further investigation of ice sheets and ice flows revealed that the garnet-rich glacial sand was washed over this beach by a northwest-moving ice sheet when Australia and Antarctica were connected in the supercontinent Gondwana. About 85 million years ago, the supercontinent began to split in two. This was largely due to mantle plumes and other tectonic activity, according to the Australian Antarctic Program.

When the discovered garnet was dated using technical equipment, its age was found to date from the same period as the outcrop of the Transantarctic Mountains in East Antarctica, an area of ​​uplifted rocks completely covered by thick ice sheets for millions of years. Researchers hypothesized that this area harbors evidence of a 590-million-year-old mountain belt hiding beneath the Antarctic ice. “Although it is not currently possible to sample directly beneath this ice sheet, it is conceivable that millions of years of ice transport have eroded the bedrock beneath it and transported this load of garnet northwestward into the Antarctic-Australian conjugate,” says Glorie. .

He added that the garnet deposits were then stored locally in the glacial sediments until they were freed by erosion, and the waves and tides dragged them onto the South Australian beach, turning the white sand a pink blush. “It is fascinating to think that we have been able to trace tiny grains of sand on a beach in Australia to a previously undiscovered mountain range beneath the Antarctic ice,” the researchers said.