For most jewelers, cloudy, yellowish “fibrous diamonds” are too ugly. But for scientists, their crystal structure holds precious secrets that can be traced back one billion years or more.
Geoscientist Yaakov Weiss of the Hebrew University of Jerusalem and his colleagues crushed a part of the South African fibrous diamond to extract the tiny fluid bags trapped in it. This fluid that once formed diamonds has a unique record of conditions deep in the earth long ago. It also contains uranium and thorium, which decay into the isotope helium 4 and gradually leak out of the diamond’s crystal lattice. But no one knows the exact leak rate, which will require determining the age of the diamond and unlocking its history.
By modeling this decay and how much helium 4 might leak over time, Weiss and his colleagues determined the broad age range of these stones. Then, they ruled out the impossible age based on the known structural and thermal conditions in the mantle and crust where the diamonds were formed. Combining these data to arrive at the upper limit of leakage, the researchers can apply it to all the fibrous diamonds they study.They recently described their results exist Nature Communications.
The team traced the fluid back to three different periods, each of which coincided with the dramatic changes on the surface. The oldest diamonds were discovered between 750 million and 2.6 billion years old; scientists reduced their creation to about 1 billion years ago, when tectonic forces were building rugged mountains in what is now South Africa. The next oldest was formed between 300 million and 540 million years ago, which coincides with the formation of the Naukluft Mountains in Namibia. The youngest were formed between 85 million and 118 million years ago, just before an underground eruption blasted them up the crust.
In addition, the fluid is rich in carbon in the oldest diamond, silicon dioxide in the second-old diamond, and salt in the youngest diamond. This may also correspond to major geological changes: for example, when oceanic plates slide under the continental crust, the youngest fluid may come from the oceanic crust pushed into the depths of the earth.
Suzette Timmerman, a geoscientist at the University of Alberta who was not involved in the study, said that no other deep-earth rock or mineral can reach the surface when it changes internally like a diamond. Therefore, the fluid provides a rare direct window into the lithosphere (crust) and upper mantle. “The contents are basically a time capsule,” Timmerman said.
Next, the researchers plan to examine similar correlations between the formation of diamonds from other regions and surface events, Weiss said: “We need to consider whether this explains the evolution of the mantle and the lithosphere.”