Water may exist in Earth’s lower mantle, shows diamonds study

Washington: Water may be more common than expected at extreme depths approaching 640 kilometres and possibly beyond — within Earth’s lower mantle, says a study that explored microscopic pockets of a trapped form of crystallised water molecules in a sampling of diamonds from around the world.

For the study, published in the journal Science, diamond samples from locations in Africa and China were studied through a variety of techniques, including the use of infrared light at the US Department of Energy’s Lawrence Berkeley National Laboratory.

The tiny traces of crystallized water, trapped in spaces called inclusions that measure just a few microns (millionths of a meter) in length, contain the molecular signature of ice VII.

This crytallised water likely formed from liquid water existing at very high pressures, according to the study.

The structure and chemical studies helped the scientists to determine the pressures and temperatures at which the diamonds formed.

This allowed the researchers to estimate the depths of their formation.

“It seems many diamonds come from greater depths,” said the study’s lead author
Oliver Tschauner, Professor at the University of Nevada, Las Vegas.

The researchers found it surprising that so many of the studied diamonds from a random sampling seemed to originate from deep inside the Earth, within and even beyond the so-called transition zone sandwiched between Earth’s upper and lower mantles.

While only about 60 diamonds had previously been confirmed to originate at depths greater than about 306 kms, the latest study added several more to this tally.

Researchers concluded that some of the inclusions likely were formed from fluid existing at depths of 402 kms to 547 kms beneath Earth’s surface. Others may have formed at depths ranging from 610 kms to 800 kms — possibly within Earth’s “shallow” lower mantle.

“It’s not just a curiosity to have a diamond residing deep in Earth’s mantle – this is direct evidence for aqueous fluid in the deep Earth,” Tschauner said.

The pressures that formed these deeper diamonds are estimated at approximately 24 to 25 gigapascals, which is about 224 times more pressure than exists at the bottom the ocean’s deepest point in its Mariana Trench.

The composition of the fluid that was trapped in the inclusions appears to be complex, with traces of carbonates, oxides, and salt, Tschauner said.