Geophysicists claim conventional understanding of Earth's deep water cycle needs revision

October 18, 2010

RIVERSIDE, Calif. - A popular view among geophysicists is that large amounts of water are carried from the oceans to the deep mantle in "subduction zones," which are boundaries where the Earth's crustal plates converge, with one plate riding over the other.

But now geophysicists led by the University of California, Riverside's Harry Green, a distinguished professor of geology and geophysics, present results that contradict this view. They compare seismic and experimental evidence to argue that subducting slabs do not carry water deeper than about 400 kilometers.

"The importance of this work is two-fold," Green said. "Firstly, if deep slabs are dry, it implies that they are strong, a major current question in geophysics that has implications for plate tectonic models. Secondly, even small amounts of water greatly reduce the viscosity of rocks; if water is not cycled deep into Earth, it means that mantle convection has not been as vigorous over time as it would have been with significant water."

Study results appear in the current issue of Nature.

The Earth's lithosphere is formed at mid-ocean ridges where magma upwells and freezes to form new oceanic crust. Interaction between cold water of the deep ocean and the extreme heat of magma results in widespread cracking of rocks and a hydrothermal circulation that drives sea water several kilometers below the surface.

Away from the mid-ocean ridges, the lithosphere moves along under the ocean until it reaches an oceanic trench, long topographic depressions of the sea floor. Here, the lithosphere bends sharply and descends back into the mantle. Near the trench, numerous faults are created that provide a pathway for additional water to enter the down-going lithosphere. Subsequent dehydration results in large amounts of this water leaving the subducting slab and migrating upwards. The ensuing instability leads to seismic activity.

Geophysicists have long suspected but only recently established that at depths less than about 250 kilometers earthquakes occur through dehydration of minerals like serpentine. But when Green and his colleagues studied the data for deeper earthquakes, they found that the subducting slabs are essentially dry, providing no pathway for significant amounts of water to enter the Earth's lower mantle.

Further, the researchers cite evidence for olivine in the slabs at these depths, despite the fact that it is not stable below about 350 km.

"At these depths, olivine should transform to the stable phase, spinel," Green said. "The very cold temperatures deep in the downgoing slabs inhibit this transformation. Experiments show that even extremely small amounts of water, if present, would cause the olivine-to-spinel transformation to run. But we see no spinel here, just olivine, which confirms that the slabs are dry."

Green explained that the olivine found below 400 kilometers is "metastable," meaning it is physically present as a mineral phase even though this is not its "right phase" at such depths - akin to a diamond, which forms only at the kind of high temperatures and pressures found very deep in the Earth's crust, being brought to the Earth's surface.

"At such depths, the olivine should undergo a phase transformation," he said. "A different crystal structure should nucleate, grow and eat up the olivine. If it is very cold in the center of subducting slabs, the reaction won't run. This is exactly what is happening here."

According to Green, the presence of the metastable olivine provides an alternative mechanism to initiate deep earthquakes - a mechanism he discovered 20 years ago - and also to cause them to stop at around 680 kilometers, where they are seen to stop.

"Does this mean that Earth's deep interior must be dry? Not necessarily," he said. "It is possible there are other ways - let's call them back roads - for water to penetrate the lower mantle, but our work shows that the 'super highway,' the subducting slabs, as a means for water to enter the lower mantle can now be ruled out."

Green and his colleagues cite the evidence for the existence of metastable olivine west of and within the subducting Tonga slab in the South Pacific and also in three other subduction zones - the Mariannas, Izu-Bonin and Japan.
-end-
Green was joined in the study by seismologists Wang-Ping Chen of the University of Illinois, Urbana-Champaign; and Michael R. Brudzinski of Miami University, Ohio.

Grants from the National Science Foundation to the researchers supported the study.

The University of California, Riverside (www.ucr.edu) is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment of about 18,000 is expected to grow to 21,000 students by 2020. The campus is planning a medical school and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Graduate Center. The campus has an annual statewide economic impact of more than $1 billion.

A broadcast studio with fiber cable to the AT&T Hollywood hub is available for live or taped interviews. To learn more, call (951) UCR-NEWS.


University of California - Riverside

Related Water Articles from Brightsurf:

Transport of water to mars' upper atmosphere dominates planet's water loss to space
Instead of its scarce atmospheric water being confined in Mars' lower atmosphere, a new study finds evidence that water on Mars is directly transported to the upper atmosphere, where it is converted to atomic hydrogen that escapes to space.

Water striders learn from experience how to jump up safely from water surface
Water striders jump upwards from the water surface without breaking it.

'Pregnancy test for water' delivers fast, easy results on water quality
A new platform technology can assess water safety and quality with just a single drop and a few minutes.

Something in the water
Between 2015 and 2016, Brazil suffered from an epidemic outbreak of the Zika virus, whose infections occurred throughout the country states.

Researchers create new tools to monitor water quality, measure water insecurity
A wife-husband team will present both high-tech and low-tech solutions for improving water security at this year's American Association for the Advancement of Science (AAAS) annual meeting in Seattle on Sunday, Feb.

The shape of water: What water molecules look like on the surface of materials
Water is a familiar substance that is present virtually everywhere.

Water, water everywhere -- and it's weirder than you think
Researchers at The University of Tokyo show that liquid water has 2 distinct molecular arrangements: tetrahedral and non-tetrahedral.

What's in your water?
Mixing drinking water with chlorine, the United States' most common method of disinfecting drinking water, creates previously unidentified toxic byproducts, says Carsten Prasse from Johns Hopkins University and his collaborators from the University of California, Berkeley and Switzerland.

How we transport water in our bodies inspires new water filtration method
A multidisciplinary group of engineers and scientists has discovered a new method for water filtration that could have implications for a variety of technologies, such as desalination plants, breathable and protective fabrics, and carbon capture in gas separations.

Source water key to bacterial water safety in remote Northern Australia
In the wet-dry topics of Australia, drinking water in remote communities is often sourced from groundwater bores.

Read More: Water News and Water Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.