Water-bearing minerals reveal that Earth’s mantle could hold more water than all its oceans. Researchers now ask: Where did it all come from?
A couple hundred pebble-size diamonds, plucked from Brazilian mud, sit inside a safe at Northwestern University. To some, they might be worthless. “They’re battered,” said Steve Jacobsen, a mineralogist at Northwestern. “They look like they’ve been through a washing machine.” Many are dark or yellow, far from the pristine gems of jewelers’ dreams.
Yet, for researchers like Jacobsen, continues Marcus Woo in Quanta Magazine, these fragments of crystalline carbon are every bit as precious — not for the diamond itself, but for what is locked inside: specks of minerals forged hundreds of kilometers underground, deep in Earth’s mantle.
These mineral flecks — some too small to see even under a microscope — offer a peek into Earth’s otherwise unreachable interior. In 2014, researchers glimpsed something embedded in these minerals that, if not for its deep origins, would’ve been unremarkable: water.
Not actual drops of water, or even molecules of H20, but its ingredients, atoms of hydrogen and oxygen embedded in the crystal structure of the mineral itself. This hydrous mineral isn’t wet. But when it melts, out spills water. The discovery was the first direct proof that water-rich minerals exist this deep, between 410 and 660 kilometers down, in a region called the transition zone, sandwiched between the upper and lower mantles.
Since then, scientists have found more tantalizing evidence of water. In March, a team announced that they had discovered diamonds from Earth’s mantle that have actual water encased inside. Seismic data has also mapped water-friendly minerals across a large portion of Earth’s interior. Some scientists now argue that a huge reservoir of water could be lurking far beneath our feet. If we consider all of the planet’s surface water as one ocean, and there turn out to be even a few oceans underground, it would change how scientists think of Earth’s interior. But it also raises another question: Where could it have all come from?
Without water, life as we know it would not exist. Neither would the living, dynamic planet we’re familiar with today. Water plays an integral role in plate tectonics, triggering volcanoes and helping parts of the upper mantle flow more freely. Still, most of the mantle is relatively dry. The upper mantle, for instance, is primarily made of a mineral called olivine, which can’t store much water.
But below 410 kilometers, in the transition zone, high temperatures and pressures squeeze the olivine into a new crystal configuration called wadsleyite. In 1987, Joe Smyth, a mineralogist at the University of Colorado, realized that wadsleyite’s crystal structure would be afflicted with gaps. These gaps turn out to be perfect fits for hydrogen atoms, which could snuggle into these defects and bond with the adjacent oxygen atoms already in the mineral. Wadsleyite, Smyth found, can potentially grab onto lots of hydrogen, turning it into a hydrous mineral that produces water when it melts. For scientists like Smyth, hydrogen means water.
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