Early Earth got much of its water from relentless bombardment by water-rich asteroids and icy comets. Now, scientists say the young planet had a way to hold onto much more of that water than once thought: Rocks deep in Earth’s belly may have contained up to 100 times more water than previously estimated, researchers report December 11 in Science. That adds up to perhaps a whole ocean’s worth of water once stowed away in the ancient mantle rocks.
Using laboratory experiments that re-created the extreme conditions of Earth’s deep mantle, geochemist Wenhua Lu of the Chinese Academy of Sciences in Guangzhou and colleagues investigated how much water bridgmanite, one of Earth’s first minerals, could actually hold. As the heat increased, the bridgmanite was able to incorporate more and more water into its crystal structure.
“The findings … add another vital piece to an intricate and multifaceted puzzle,” writes petrologist Michael Walter of Carnegie Science in Washington, D.C., in an accompanying commentary in Science. Understanding how water was incorporated into these most ancient minerals, he says, provides new clues to the earliest origins of Earth’s water cycle, the key to our planet’s habitability.
Long before Earth’s surface was two-thirds ocean, there was an abundance of water locked in the deep rocks of the planet’s lower mantle. Nearly 4.4 billion years ago — during Earth’s earliest era, known as the Hadean Eon — the mantle began to form, as the magma ocean that covered the planet slowly cooled and crystallized into rock.
Bridgmanite was the first and most abundant mineral to form; today, it makes up about 60 percent of the mantle. Bridgmanite forms in conditions of intense heat and pressure — such as inside the planet. In the deepest parts of the mantle, which extends to 2,890 kilometers below the surface, temperatures can be hotter than 4000° Celsius and pressures can be up to 700,000 atmospheres.
As the magma ocean cooled, some molecules of water that had dissolved in the melted rock made their way into the newly formed bridgmanite and became tucked away in the mineral’s crystalline structure. That still happens today: Water is carried into the deep Earth along with subducting tectonic plates. It gets locked in minerals like bridgmanite for a time and eventually returns to the surface via volcanic eruptions.
But how much water was there in the depths in Earth’s earliest days? The answer largely depends on how much water the bridgmanite crystals could hold under those intense heat and pressure conditions. So Lu and colleagues set out to re-create the extreme conditions of the lower mantle using a laser-heated diamond anvil. The tool presses a rock sample between two diamonds to generate intense pressures and then scorches the sample with focused lasers.
The results revealed that heat increased bridgmanite’s capacity to hold water — which, in turn, suggests that the lowest part of the mantle held more than the somewhat cooler upper mantle. Previous estimates had suggested the bridgmanite was nearly dry, holding less than 220 parts per million of water by weight; instead, the new study suggests, there was a substantial deep reservoir. Over time, the researchers say, the churning of tectonic plates and upward spurt of mantle plumes helped redistribute the water, bringing much of it to the surface. But some of that primordial water may still be down there.