In Diamonds' Flaws, Finding the Secret History of Continents


"We've found a way to use traces of sulfur from ancient volcanoes that made its way into the mantle and eventually into diamonds to provide evidence for one particular process of continent building," explained Karen Smit of the Gemological Institute of America, lead author on the group's paper, which appears this week in Science. "Our technique shows that the geologic activity that formed the West African continent was due to plate tectonic movement of ocean crust sinking into the mantle."

Diamonds may be beloved by jewelry collectors, but they are truly a geologist's best friend. Because they originate deep inside the Earth, tiny mineral grains trapped inside of a diamond, often considered undesirable in the gem trade, can reveal details about the conditions under which it formed.

"In this way, diamonds act as mineralogical emissaries from the Earth's depths," explained Carnegie co-author Steve Shirey.


"Solving this mystery is key to understanding how the continents came to exist in their current incarnations and how they survive on an active planet," Shirey explained. "Since this is the only tectonically active, rocky planet that we know, understanding the geology of how our continents formed is a crucial part of discerning what makes Earth habitable."

Some scientists think mantle keels form by a process called subduction, by which oceanic plates sink from the Earth's surface into its depths when one tectonic plate slides beneath another. Others think keels are created by a vertical process in which plumes of hot magma rise from much deeper in the Earth.

A geochemical tool that can detect whether the source of a mantle keel's makeup originated from surface plates or from upwelling of deeper mantle material was needed to help resolve this debate. Luckily, mantle keels have the ideal conditions for diamond formation. This means scientists can reveal a mantle keel's origin by studying inclusions from diamonds that formed in it.


They were able to make this determination because the chemistry of the sulfide mineral grains is only seen in samples from Earth's surface more than 2.5 billion years ago -- before oxygen became so abundant in our planet's atmosphere. This means that the sulfur in these mineral inclusions must have once existed on the Earth's surface and was then drawn down into the mantle by subduction.



"Our work shows that sulfide inclusions in diamonds are a powerful tool to investigate continent construction processes," Smit concluded.

The above story is based on Materials provided by Carnegie Institution for Science.