A Tenham Meteorite Sheds Light on Bridgmanite, a Mysterious Mineral

A thin section of a Tenham meteorite reveals a vein of bridgmanite.

Credit: Tschauner et al., 2014, Science/AAAS

The Tenham meteorite, a rock from space, is giving scientists the first glimpse of a mineral long thought to be the most abundant mineral on Earth, but which researchers lacked a natural sample of until now.

This discovery could shed light on the structure and dynamics of the inner Earth, as well as the early history of the solar system, according to the new paper.

"The search for this mineral in meteorites has been going on for decades, it was just a matter of finding the right method for detecting it," said lead study author Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas.

The mineral is a high-density version of magnesium iron silicate. It is the most abundant mineral on Earth, and makes up about 38 percent of the planet's volume, but it's only stable at very high pressures and temperatures, so for decades, researchers had only seen lab-generated versions of it.

Under the heat and pressure found in Earth's lower mantle, which extends from about 410 to 1,615 miles (660 to 2,600 kilometers) below the planet's surface, magnesium silicate can form what is called a perovskite structure, which can be imagined as an array of double pyramids that are joined at their corners.

The centers of each pyramid are made of silicon, the apexes and corners are made of oxygen, and magnesium and iron reside in the spaces between each double pyramid.

But scientists had not discovered a naturally occurring version of this mineral until now, the mineral would not survive the long journey from the lower mantle to Earth's surface because it would readily transform into lower-density minerals.

The fact that scientists had not found any specimens of magnesium iron silicate perovskite in nature also meant it could not get an official mineral name from the International Mineralogical Association (IMA).

This presented geologists with the odd situation of a nameless mineral being the most abundant one on Earth.

Since researchers could not find a naturally occurring version of magnesium iron silicate perovskite from Earth, they instead looked to space.

They hypothesized that high-speed cosmic impacts could generate the pressures and temperatures needed to create this mineral, and samples of it could then come to Earth as meteorites knocked off their parent asteroids or planets.

Recently, Tschauner and his colleagues carefully isolated magnesium iron silicate perovskite in a Tenham meteorite.

The mineral was given has the official name of "bridgmanite," after the father of high-pressure experiments, Nobel laureate Percy Bridgman, according to the report, published in the Nov. 28 issue of the journal Science.

The researchers analyzed a Tenham meteorite, a rock that was part of a meteor shower that rained down on Australia on a spring night in 1879.

This meteorite bore signs that it was part of an asteroid that experienced a great impact.

The stone also possessed minerals called akimotoite and ringwoodite, which are similar in composition and origin to bridgmanite.

Tschauner and his colleagues used high-energy X-rays from a synchrotron, a particular type of cyclic particle accelerator, descended from the cyclotron, in which the guiding magnetic field (bending the particles into a closed path) is time-dependent, being synchronized to a particle beam of increasing kinetic energy.

These intense X-rays do little damage to bridgmanite, thus helping the scientists prove its composition and crystal structure.

The researchers found that bridgmanite was higher in iron and sodium than they had expected based on synthetic samples.

"This gives interesting insights for what might be going on in the lower mantle," Tschauner said.

Tschauner added that detecting bridgmanite in other meteorites could shed light on the strength of the impacts their parent bodies experienced.

The pressures and durations of these impacts in turn "allow us to estimate the size of the parent bodies of these meteorites, and with enough data, we can, for given points in time in the solar system's history, figure out how large bodies in the solar system were," Tschauner said.

Evidence of 'oceans worth' of water in Earth's mantle detected

Schematic cross section of the Earth’s interior highlighting the transition zone layer (light blue, 410-660 km depth), which has an anomalously high water storage capacity. 

The study by Schmandt and Jacobsen used seismic waves to detect magma generated near the top of the lower mantle at about 700 km depth.

Dehydration melting at those conditions, also observed in the study’s high-pressure experiments, suggests the transition zone may be nearly saturated with H2O dissolved in high-pressure rock. 

Credit: Steve Jacobsen/Northwestern University

Researchers have found evidence of a potential "ocean's worth" of water deep beneath the United States.

Although not present in a familiar form, the building blocks of water are bound up in rock located deep in the Earth's mantle, and in quantities large enough to represent the largest water reservoir on the planet, according to the research.

For many years, scientists have attempted to establish exactly how much water may be cycling between the Earth's surface and interior reservoirs through the action of plate tectonics.

Northwestern University geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt have found deep pockets of magma around 400 miles beneath North America, a strong indicator of the presence of H₂O stored in the crystal structure of high-pressure minerals at these depths.

"The total H₂O content of the planet has long been among the most poorly constrained 'geochemical parameters' in Earth science. Our study has found evidence for widespread hydration of the mantle transition zone," says Jacobsen.

For at least 20 years geologists have known from laboratory experiments that the Earth's transition zone, a rocky layer of the Earth's mantle located between the lower mantle and upper mantle, at depths between 250 and 410 miles, can, in theory, hold about 1 percent of its total weight as H₂O, bound up in minerals called wadsleyite and ringwoodite.

However, as Schmandt explains, up until now it has been difficult to figure out whether that potential water reservoir is empty, as many have suggested, or not.

If there does turn out to be a substantial amount of H₂O in the transition zone, then recent laboratory experiments conducted by Jacobsen indicate there should be large quantities of what he calls "partial melt" in areas where mantle flows downward out of the zone.

This water-rich silicate melt is molten rock that occurs at grain boundaries between solid mineral crystals and may account for about 1 percent of the volume of rocks.

"Melting occurs because hydrated rocks are carried from the transition zone, where the rocks can hold lots of H₂O, downward into the lower mantle, where the rocks cannot hold as much H₂O."

"Melting is the way to get rid of the H₂O that won't fit in the crystal structure present in the lower mantle," says Jacobsen.

He adds:
"When a rock starts to melt, whatever H₂O is bound in the rock will go into the melt right away. So the melt would have much higher H₂O concentration than the remaining solid. We're not sure how it got there."

"Maybe it's been stuck there since early in Earth's history or maybe it's constantly being recycled by plate tectonics."

Seismic Waves
Melt strongly affects the speed of seismic waves, the acoustic-like waves of energy that travel through the Earth's layers as a result of an earthquake or explosion.

This is because stiff rocks, like the silicate-rich ones present in the mantle, propagate seismic waves very quickly.

According to Schmandt, if just a little melt, even 1 percent or less, is added between the crystal grains of such a rock it causes it to become less stiff, meaning that elastic waves propagate more slowly.

"We were able to analyse seismic waves from earthquakes to look for melt in the mantle just beneath the transition zone," says Schmandt.

Brandon Schmandt (University of New Mexico, left) and Steve Jacobsen (Northwestern University, right) combined seismic observations from the US-Array with laboratory experiments to detect dehydration melting of hydrous mantle material beneath North America at depths of 700-800 km. 

Credit: University of New Mexico/Northwestern University

"What we found beneath the U.S. is consistent with partial melt being present in areas of downward flow out of the transition zone."

"Without the presence of H₂O, it is very difficult to explain melting at these depths. This is a good hint that the transition zone H₂O reservoir is not empty, and even if it's only partially filled that could correspond to about the same mass of H₂O as in Earth's oceans," he adds.

Jacobsen and Schmandt hope that their findings, published in the June issue of the journal Science, will help other scientists to understand how the Earth formed and what its current composition and inner workings are, as well as establish how much water is trapped in mantle rock.

"I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades," says Jacobsen

Schematic representation of seismometers placed in the US-Array between 2004 and 2014 and used in the study by Schmandt and Jacobsen to detect dehydration melting at the top of the lower mantle beneath North America. 

Credit: NSF-Earthscope

Crystals of laboratory-grown hydrous ringwoodite, a high-pressure polymorph of olivine that is stable from about 520-660 km depth in the Earth’s mantle. 

The ringwoodite pictured here contains around one weight percent of H2O, similar to what was inferred in the seismic observations made by Schmandt and Jacobsen. 

Credit: Steve Jacobsen/Northwestern University