Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

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

19.10.2010
Commonly held view in geophysical community that water is carried deep into Earth's mantle is false, says UC Riverside's Harry Green

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.

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.

Iqbal Pittalwala | EurekAlert!
Further information:
http://www.ucr.edu

More articles from Earth Sciences:

nachricht Novel method for investigating pore geometry in rocks
18.06.2018 | Kyushu University, I2CNER

nachricht Decades of satellite monitoring reveal Antarctic ice loss
14.06.2018 | University of Maryland

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Overdosing on Calcium

Nano crystals impact stem cell fate during bone formation

Scientists from the University of Freiburg and the University of Basel identified a master regulator for bone regeneration. Prasad Shastri, Professor of...

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

Im Focus: Photoexcited graphene puzzle solved

A boost for graphene-based light detectors

Light detection and control lies at the heart of many modern device applications, such as smartphone cameras. Using graphene as a light-sensitive material for...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Carbon nanotube optics provide optical-based quantum cryptography and quantum computing

19.06.2018 | Physics and Astronomy

How to track and trace a protein: Nanosensors monitor intracellular deliveries

19.06.2018 | Life Sciences

New material for splitting water

19.06.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>