Ever wonder what the Earth’s mantle looks like?

This green olivine meteorite from the mantle of an alien world might be the closest we’ll come to holding one. While its isotope ratios are unique to its patent body (the asteroid Vesta; more on that below), the minerals of its mantle and molten metal core are believed to be very similar to Earth.

On Earth, uplifted mantle tends to be ancient and heavily weathered. The olivines gelatinize in weak acids and offer little resistance to attack by weathering agents or hot hydrothermal flows. The greenish magnesium leaches out from hydration, oxidation, and carbonation.

For 60 years researchers have tried to drill to the Earth’s mantle to get a pristine sample. The Soviets still hold the record for the deepest drill in 1979 (the Kola Superdeep Borehole goes down 7.6 miles), but they only got through 1/3 of the crust.

While the mantle makes up 85% of Earth by volume, we know very little about it. The mantle is like “a planet-sized lava lamp where material picks up heat at the core-mantle boundary, becomes less dense and rises in buoyant plumes to the lower edge of Earth’s crust, and then flows along that ceiling until it cools and sinks back toward the core. Circulation in the mantle is exceptionally languid: a round-trip might take 2 billion years.” — Smithsonian Magazine

THE LURE OF GREEN SPARKLE

Recently, geologists recognized an outcropping in Maryland as mantle, part of the seafloor of a now-vanished ocean. “Mantle rock is generally full of sparkly green crystals of the mineral olivine, but these are surprisingly unremarkable: mottled yellow-brown stone occasionally flecked with black. “Those rocks have had a tough life,” says George Guice, a mineralogist at the Smithsonian’s National Museum of Natural History. Guice has long chased after sparkly green rocks, known to geologists as ultramafic. They’re rich in magnesium and make up the majority of our planet as the mantle. Ultramafic rocks form deep underground at high temperatures and pressures, so their minerals are not stable near Earth’s surface. In this shallow environment, they’re often exposed to hot fluids rushing through cracks, transforming their mineral makeup, as seen in the rocks strewn around Baltimore. Understanding the rock’s history through these changes, Guice says, is like looking through thick fog.” — National Geographic

SHINE BRIGHT LIKE AN EMERALD, FROM VESTA

I recently procured this ultramafic beauty from a collection in Germany (procured from nomads in Mali). It is one of only 12 meteorites that are classified as diogenite-olivine or harzburgitic (greater than 40% olivine, and the orthopyroxene is magnesium-rich in contrast to conventional iron-rich orthopyroxene diogenites). It is the main mass of NWA 7370, and is also high in chromite (15%), which can be seen as black notches protruding from the crust. Due to its higher melting point, the chromite resisted atmospheric ablation during Mach 32 entry better than the matrix.

Vesta is the largest and brightest asteroid in the asteroid belt and the second largest body overall (after the dwarf planet Ceres), with an average diameter of about 525 km (326 miles). That is pretty close to the size of the State of Colorado.

A billion years ago, two massive impacts ejected a large part of Vesta’s mass, and some of that material eventually was ejected from the asteroid belt by the resonant effects of Jupiter’s orbit, randomly, and it happened to land here on Earth. It’s astonishing to consider that when you look at photographs of Vesta, or even through a powerful telescope, the actual craters from which the HED meteorites were blasted out can easily be seen, such as Rheasilvia crater which is over 300 miles wide with conical residue at the center forming the tallest mountain in our solar system. The DAWN spacecraft orbited Vesta for a year, gathering data to match the “HED” meteorites on Earth. From the DAWN mission, we now know that Vesta is the only intact, layered planetary building block surviving from the very earliest days of the solar system, forming within the first 10 million years, long before Earth.

The “swirl” texture that can be seen on the interior and exterior graphically displays the flowing and mixing of the olivine and orthopyroxene grains, with intermingled chromite crystals. “Vast underground chambers of magma churned and roiled, eventually cooling to the point at which orthopyroxene crystals froze out of the liquid rock. Insulated by kilometers of overlying rock and magma, they cooled slowly, growing into enormous sizes before settling downwards into vast piles of large crystals at the base of the underground caverns.” — Tim Gregory’s recent book Meteorite, p.102.

Meteorites offer a preserved time capsule from the mantle’s depths, albeit from an alien world.