Earth's crust as a sponge

Cold, fast-sinking tectonic plates diving deep into the Earth's interior are carrying up to one trillion kilograms water per year into the mantle, double the amount than earlier estimates suggested, according to a new study from a team led by South Korean researchers.

A research team led by Yongjae Lee, professor at the Department of Earth System Science at Yonsei University, used several methods, including X-ray techniques, to show that the water-bearing rock serpentinite carries significant amounts of water as it sinks into the Earth's interior.

The findings have implications on how seismic activity is interpreted in these highly volcanic areas, as well as our understanding of the distribution of the Earth's water, and were published in the journal Nature Communications.

Subduction zones as a transporter of water into the interior of the Earth

Subduction zones are the areas where two sections of tectonic plates in the Earth's crust overlap, and one sinks below the other. These extremely geologically active areas are associated with large numbers of volcanoes and earthquakes. Not all subducting plates behave the same way. Older sections of oceanic crust have had more time to cool and thicken since they formed at a mid-ocean ridge, making them denser and less buoyant than younger oceanic crust. This lets them sink faster and at a steeper angle - a phenomenon known as a "cold" subduction zone. This is the case at the two deepest trenches in the world, the Mariana Trench and the Tonga Trench in the Pacific Ocean, where old, dense oceanic plates plunge steeply into the mantle.

For decades, the scientific consensus held that as serpentinite in these crust sections sinks deeper into the Earth, rising pressure and temperature would force the rock and its constituent mineral serpentine to release its stored H2O. The release of the water initiates the development of arc-like chains of volcanoes via melting of the overlying continental crust.
Yet researchers have suspected for some years that water-rich minerals could survive deeper into cold slabs than classical models predicted. The current study provides additional experimental evidence pinning down exactly where and under what pressure and temperature conditions this is possible.

Extreme conditions in the lab

To replicate these extreme deep-Earth conditions in the lab, the research team conducted several high-pressure and high-temperature X-ray diffraction and ultrasonic velocity experiments starting in 2019. They used different synchrotron radiation facilities around the world for their study, including PETRA III at DESY (Germany), the Advanced Photon Source at Argonne National Laboratory (USA), and the Pohang Accelerator Laboratory (South Korea).

At DESY, the research team used both of the extreme-states beamlines at PETRA III, P02.2 and P61B. Both experiment stations can fine tune pressures and temperatures and use the PETRA III X-rays to examine the changes taking place in the sample at the atomic scale.

"This study illustrates the strength of combining research in the diamond anvil cell and the large volume press targeting different facets of this long-lasting research project," says Hanns-Peter Liermann, who leads the extreme conditions beamline P02.2 optimized for research in the Diamond Anvil Cell. "Here, the team performed most of the X-ray diffraction experiments at high pressure and temperature with varying water contents to test the stability of serpentine."

At P61B, the large volume press beamline, the team performed ultrasonic measurements on serpentine. "These measurements are essential to estimate the seismic properties at the same pressure and temperatures," says P61B leader Robert Farla.

Water remains trapped in the depths

The researchers' analyses revealed a stark contrast. At the relatively warmer edges of the sinking tectonic plate, water is indeed released at a depth of approximately 220 kilometres, causing the rock's density to increase by about 16 percent and affecting its transmission of seismic motion.

However, in the relatively cooler environment in the middle layers of the sinking tectonic plate, a reaction takes place that had been suspected but, until the team's experiments, had not experimentally observed. At around 280 kilometres beneath the Earth's surface, the serpentine mineral suddenly takes on one of two different forms. In a water-rich environment, one form develops that can take on an extra six percent of its weight in water. Where water is scarcer, the form that develops there just hangs onto the water it has, rather than taking on more.

Altogether, this translates to serpentinite hanging onto roughly three times the amount of water as previously believed. The research team argues that the extra water is channeled into the subduction zone by fault lines developing as the crust bends while it sinks into the mantle. Both kinds of water-absorbing reactions can happen simultaneously, depending on the amount of water available, the pressure, and the temperature. These insights affect the interpretation of seismic data that is relevant to studying earthquakes and volcanic eruptions, alongside other seismic and tectonic events.

Up to a billion kilograms of water in a year

The team then modeled what these reactions could mean at a global scale. Of the 56 subduction zones worldwide, 16 are classified as cold or ultracold. Combining the newly observed reactions with existing models for these 16 zones, the researchers estimate that as much as 1 trillion kilograms of water per year could be carried into the mantle - double previous global estimates.

"Research performed over the past several decades has focused almost exclusively on the dehydration of serpentinite," states Yonsei University professor Yongjae Lee, who led the study. "However, our research provides clear evidence that as a cold subducting plate sinks, it can act like a giant sponge, retaining massive amounts of water and carrying it even deeper. This finding suggests that a revision of our deep Earth models might be necessary regarding the circulation of water within tectonic plates and how its interior has evolved."

» Original publication

Source: Deutsches Elektronen-Synchrotron (DESY)