Compared to other types of volcanic rocks that erupt at Earth’s surface, kimberlites are unique. They come from the deeper parts of the mantle, contain much higher amounts of carbon dioxide and other volatile compounds than most volcanic rocks, erupt rapidly and violently, and often carry diamonds. Kimberlites are also surprisingly consistent – so consistent that a new study proposes that all kimberlite eruptions came from the same place, deep within Earth.
DCO Reservoirs and Fluxes Community members Andrea Giuliani (ETH Zurich, Switzerland) and Graham Pearson (University of Alberta, Canada), with colleagues at the University of Melbourne in Australia and Durham University in the United Kingdom, looked at hundreds of kimberlite samples of different ages and geographic locations. In a new paper, published in Nature, they show this primitive, isolated reservoir remained unchanged until about 200 million years ago, around the same time that the supercontinent Pangaea was breaking up. The study is the first to use isotopes, which are atoms of the same element with different numbers of neutrons, to see how kimberlites have changed over time on a global scale.
“Kimberlites can give us a glimpse – if not the best glimpse – of the deepest part of the carbon cycle,” said Giuliani. Based on seismic data from sound waves sent into the subsurface, these deep reservoirs likely still exist today and “kimberlites give us a temporal perspective on these domains.”
Giuliani and Pearson teamed up with lead authors Jon Woodhead and Janet Hergt (both at University of Melbourne, Australia) to pool their data from kimberlite samples collected worldwide, with ages ranging from about 2 billion years to 40 million years old. Most of the samples came from diamond mining and exploration, donated by De Beers and other diamond companies. “To put together such a collection is a unique effort,” said Giuliani. “It has included the help of a lot of people over 20 years.”
To make their analyses, the researchers crushed up the rocks and separated out parts of the rock containing the elements neodymium and hafnium, which are robust tracers of processes occurring in the deep Earth. Then they measured the concentrations of different isotopes of these elements and compared the ratios over time to see how the composition of kimberlites, and the mantle reservoir where they originated, changed in the last 2 billion years.
“We started to analyze the data and realized that we were sitting on something pretty cool,” said Giuliani. They were surprised to see that for much of Earth’s history, the kimberlites appeared practically unchanged. Furthermore, the isotope ratios appear to be highly similar to our best guess of the composition of very early Earth, just after it formed from solar system materials. This finding suggests that the deep reservoir perhaps stayed isolated from the very beginning.
Around the time of Pangaea’s break up, however, the isotope ratios started to shift. The supercontinent’s demise likely sent unprecedent amounts of crust into the deep mantle through subduction, which the researchers suspect may have reached the deep, kimberlite reservoir.
The findings may have implications for the timing and extent of mantle convection, the slow, lava-lamp-like churning of the mantle. There is evidence that in recent times, subducting plates travel down to the core-mantle boundary and hot plumes erupt from the same place, suggesting that we have full convection today. But this study speculates it’s possible that in the past, mantle convection was separated into layers, leaving the deep reservoir of kimberlite material untouched until about 200 or so million years ago.
One caveat the authors point out is that these findings are based on isotopes from only two systems. Additional experiments will give a fuller picture of how kimberlites evolved and when their deep source stopped being isolated. “Perhaps other isotopic systems, such as strontium and lead, might give us a better resolution of the reservoir’s interaction with subducted recycled material,” said Giuliani. Currently he is examining the carbon isotopes from the kimberlite samples to see how the carbon content changed over time, and to see if or when the kimberlite source material became mixed with surface carbon. “If we can answer this question even more robustly, we can probably understand much better how Earth worked in the past.”
Main image: A thin section of the carbonate-rich Peuyuk kimberlite from Somerset Island, Canada. Credit: Andrea Giuliani