Water occurs naturally as far as at least 250 miles below the Earth’s surface, according to a study published in Science last week by researchers from the University of Chicago and others. The discovery, which relies on extremely bright X-ray beams from the Advanced Photon Source at Argonne National Laboratory, could change our understanding of how water circulates deep in the Earth’s mantle and how heat escapes from the lower regions of our planet.
The researchers identified a form of water known as Ice-VII, which was trapped within diamonds that crystallized deep in the Earth’s mantle. This is the first time Ice-VII has been discovered in a natural sample, making the compound a new mineral accepted by the International Mineralogical Association.
The study is the latest in a long line of research projects at the Advanced Photon Source, a massive X-ray facility used by thousands of researchers every year, which have shed light on the composition and makeup of the deep Earth. Humans cannot explore these regions directly, so the Advanced Photon Source lets them use high-powered X-ray beams to analyze inclusions in diamonds formed in the deep Earth.
“We are interested in those inclusions because they tell us about the chemical composition and conditions in the deep Earth when the diamond was formed,” said Antonio Lanzirotti, a UChicago research associate professor and co-author on the study.
In this case, researchers analyzed rough, uncut diamonds mined from regions in China and Africa. Using an optical microscope, mineralogists first identified inclusions, or impurities, which must have formed when the diamond crystallized. But to positively identify the composition of these inclusions, mineralogists needed a stronger instrument: the University of Chicago’s GeoSoilEnviroCARS’s beam lines at the Advanced Photon Source.
Thanks to the very high brightness of the X-rays, which are a billion times more intense than typical X-ray machines, scientists can determine the molecular or atomic makeup of specimens that are only micrometers across. When the beam of X-rays hits the molecules of the specimen, they scatter into unique patterns that reveal their molecular makeup.
What the team identified was surprising: water, in the form of ice.
The composition of the water is the same as the water that we drink and use every day, but in a cubic crystalline form—the result of the extremely high pressure of the diamond.
This form of water, Ice-VII, was created in the lab decades ago, but this study was the first to confirm that it also forms naturally. Because of the pressure required for diamonds to form, the scientists know that these specimens formed between 410 and 660 kilometers (250 to 410 miles) below the Earth’s surface.
The researchers said the significance of the study is profound because it shows that flowing water is present much deeper below the Earth’s surface than originally thought. Going forward, the results raise a number of important questions about how water is recycled in the Earth and how heat is circulated. Oliver Tschauner, the lead author on the study and a mineralogist at University of Nevada in Las Vegas, said the discovery can help scientists create new, more accurate models of what’s going on inside the Earth, specifically how and where heat is generated under the Earth’s crust. This may help scientists better understand one of the driving mechanisms for plate tectonics.
“This wasn’t easy to find,” said Vitali Prakapenka, a UChicago research professor and a co-author of the study. “People have been searching for this kind of inclusion for a long time.”
For now, the team is wondering whether the mineral Ice-VII will be renamed, now that it is officially a mineral. This is not the first mineral to be identified thanks to research done at the Advanced Photon Source GSECARS beamlines: Bridgmanite, the Earth’s most abundant mineral and a high-density form of magnesium iron silicate, was researched extensively there before it was named. Tschauner was a lead author on that study, too.
“In this study, thanks to the amazing technical capabilities of the Advanced Photon Source, this team of researchers was able to pinpoint and study the exact area on the diamonds that trapped the water,” said Stephen Streiffer, Argonne associate laboratory director for photon sciences and director of the Advanced Photon Source. “That area was just a few microns wide. To put that in context, a human hair is about 75 microns wide.
“This research, enabled by partners from the University of Chicago and the University of Nevada, Las Vegas, among other institutions, is just the latest example of how the APS is a vital tool for researchers across scientific disciplines,” he said.
Other GSECARS co-authors are Eran Greenberg, Dongzhou Zhang and Matt Newville.
In addition to the University of Chicago and UNLV, other institutions cited in the study include the California Institute of Technology, China University of Geosciences, the University of Hawaii at Manoa and the Royal Ontario Museum, Toronto. Data also was collected at Carnegie Institute of Washington’s High Pressure Collaborative Access Team at the Advanced Photon Source and the Advanced Light Source at SLAC National Accelerator Laboratory.
Citation: “Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle,” Tschauner et al, Science, March 9, 2018. DOI: 10.1126/science.aao3030
