As you might expect with a seeming paradox like that, bridgmanite exists deep within Earth—it’s the material that is believed to make up the vast majority of the lower mantle, Earth’s widest layer, which runs from roughly 410 to 1,796 miles beneath the Earth’s surface. Overall, it makes up roughly 36 percent of the Earth’s volume.
But, like many minerals from deep within the planet, scientists had never seen it. In a new paper published in Science late last week, Oliver Tschauner of the University of Nevada, Las Vegas, and his team describe bridgmanite for the first time.
“Despite appearing for decades in numerous experimental and theoretical studies, characterizations of possible natural samples have not been sufficient to meet International Mineralogical Association criteria for naming new minerals,” Tschauner wrote. “Consequently, any detailed chemical, structural, and petrographic analysis of natural [bridmanite] has remained impossible.”
A cross section of the Earth. Image: Science
We haven’t suddenly figured out how to drill hundreds of miles beneath the Earth’s surface without destroying bridgmanite samples (which are volatile unless highly pressurized by something), and a bridgmanite sample wasn’t launched out of the lower mantle with a volcanic explosion or some other phenomenon. Instead, Tschauner found bridgmanite within a shocked meteorite—one that had undergone an intense, short period of pressurization.
“Hypervelocity impacts on meteorite parent bodies cause short pulses of very high pressures and locally high temperatures that melt and transform meteoritic materials into the same high-pressure phases that make up the deep Earth,” Thomas Sharp, a researcher at Arizona State University’s School of Earth and Space Exploration wrote in an accompanying article about the discovery.
By looking within shocked meteorites, scientists have been able to observe and categorize a few different deep-Earth minerals. But something from within Earth is obviously preferable. Earlier this year, for instance, scientists found the first naturally occurring sample of ringwoodite, which was trapped inside a diamond that had been expelled in Brazil.
Ringwoodite is a mineral that is prevalent in the “transition zone” between the upper and lower mantles. Within that sample, scientists found trapped water, a finding that suggests there is a subterranean ocean that may hold more water than all of Earth’s oceans combined.
Tschauner’s description of bridgmanite gives us no such insights about the inside of the Earth, other than to confirm what scientists believed to have been true for quite some time: The mineral exists, and it can occur naturally under highly pressurized conditions.