History of Earth's Early Carbon Cycle Recorded in 2-Billion-Year-Old Diamonds

For jewelry makers, imperfections in diamonds are flaws. But for geologists, these flaws hold valuable clues. Certain flaws, called mineral inclusions, reveal details such as how long the diamond took to form and the processes that shaped its growth.

Members of DCO’s Reservoirs and Fluxes Community Gareth Davies, Suzette Timmerman and Janne Koornneef (all from Vrije Universiteit, Amsterdam) investigated mineral inclusions within diamonds from Botswana. They identified one containing silicate material that formed 2.3 billion years ago in its interior and a 250 million-year-old garnet crystal towards its outer rim, the largest age range ever detected in a single specimen. Their analysis, published in a new paper in Earth and Planetary Science Letters, suggests that carbon exchange and deposition between the atmosphere, biosphere, oceans, and geosphere may have changed significantly over the past 2.5 billion years [1].

“Although a jeweler would consider diamonds with lots of inclusions to be flawed, for a geologist these are the most valuable and exciting specimens,” said Davies. “We can use the inclusions to date different parts of an individual diamond, and that allows us to potentially look at how the processes that formed diamonds may have changed over time and how this may be related to the changing carbon cycle on Earth.”

The researchers examined 16 diamonds from two mines in northeastern Botswana: seven specimens from the Orapa mine and nine from the Letlhakane mine. Although the mines are located just 40 kilometers apart, the diamonds from the two sources had significant differences in the age range and chemical composition of inclusions.

The Orapa diamonds contained material dating from between around 400 million to more than 1.4 billion years ago. The Letlhakane diamond inclusions ranged from less than 700 million to 2.5 billion years old. In every case, the team could link the age and composition of material in the inclusions to distinct tectonic events occurring locally in the Earth’s crust, such as a collision between plates, continental rifting, or magmatism. This suggests that diamonds form when triggered by heat fluctuations and magma fluid movement associated with these events.

The Letlhakane diamonds also provided a rare opportunity to look back in time to the early Earth. The oldest inclusions date back to before the Great Oxidation Event (GOE) around 2.3 billion years ago, when oxygen produced by multicellular cyanobacteria started to fill the atmosphere, radically changing the weathering and sediment formation processes and thus altering the chemistry of rocks.

“The oldest inclusions in the diamonds contain a higher proportion of the lighter carbon isotope. As photosynthesis favors the lighter isotope, carbon 12, over the heavier carbon 13, this ‘light’ ratio finding suggests that organic material from biological sources may have been more abundant in diamond-forming zones early in the Earth’s history than we find today,” explained Timmerman. “Higher temperatures in the Earth’s interior before the GOE may have affected the way that carbon was released into the diamond-forming regions beneath the Earth’s continental plates and may be evidence of a fundamental change in tectonic processes. However, we are currently working with a very small dataset and need further studies to establish if this is a global phenomenon.”

Article adapted from materials provided by Europlanet Media Centre.

Image: A selection of unprocessed inclusion-bearing gem quality diamonds from Letlhakane in Botswana. The shiny metal-like inclusions are sulfide, while the dark surrounding areas are graphite that formed in cracks created by the differential expansion of the sulfide and diamond when brought to the surface from a depth of over 150 km. The bottom left diamond contains an orange garnet and a green clinopyroxene. Credit: M. Gress, VU Amsterdam

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