Visualizing the flow of molten rock through seabed mantle

Jul 29, 2011 By David Lindley
Visualizing the flow of molten rock through seabed mantle
X-ray microtomography images show the networks formed by molten rock in a cube of mantle material, 140 microns on a side, at four different melt fractions. Grey areas are melted material between solid olivine grains, represented by the white regions. Red indicates channels of melt slicing through the faces of the cube.

New information about how most of the Earth’s crust formed has been uncovered by investigators who utilized the Advanced Photon Source (APS) at Argonne National Laboratory to obtain unprecedented, three-dimensional x-ray images of melted rock. Their results, published in the journal Science, offer a more sophisticated picture of rock porosity and a resolution of the discrepancy between permeability and melt velocity.

Where the Earth's tectonic plates drift apart at ocean floor spreading centers, mantle rock partially melts and seeps upward, eventually solidifying to form new crust. Geologists have had difficulty reconciling estimates of the permeability of the partially molten mantle with analyses of the rate at which ascends. In microtomography experiments utilizing the X-ray Science Division beamline 2-BM at the U.S. Department of Energy Office of Science’s APS, a team of scientists from the Woods Hole Oceanographic Institution, the University of Western Australia, and Argonne has, for the first time, directly imaged the intricate network formed by the molten fraction within a mainly solid rock.

The Earth’s upper mantle consists mostly of olivine as well as other minerals with lower melting points. Using measurements of seismic wave speeds, geophysicists estimate that 1% to 2% of the mantle beneath seafloor locations at the East Pacific ocean ridge is in a molten state. This melt fraction implies that the permeability of these rocks is relatively low: The less permeable the mantle, the harder it is for buoyancy to drive the melted fraction upward, and the greater the amount of melt the mantle retains.

On the other hand, geochemical analyses of ocean-ridge basalts show that the relative abundances of uranium, thorium, and radium isotopes belonging to a single decay chain have not reached the long-term equilibrium values dictated by their half-lives. This contradicts the geophysical data, explains one of the Science article’s co-authors, Wenlu Zhu of the University of Maryland, since it implies that magma rose rapidly from great depths, which requires high permeability.

Adding to the confusion are analyses that predict, for given temperature and pressure and for known rock compositions, thermodynamically favored arrangements of solid and melted fractions. These arguments suggest melt fractions should be around 0.5% or lower.

“The weak link is using the thermodynamic model to guess rock structure,” said Zhu. Permeability depends on fine details of how the melted fraction within a rock connects together. Previous studies have attempted to infer the three-dimensional network of melted material from two-dimensional images of slices through a rock, but such analyses are ambiguous, especially at low melt fractions.

Zhu and her colleagues turned to x-ray studies of heated and compressed basalt to image directly the three-dimensional network formed at different melt fractions. Because the density contrast between the solid and melted fractions was small, they used an edge-enhancement technique to obtain Fresnel fringes from the liquid-solid boundaries in their millimeter-diameter sample. Rotating the sample through 180° in 0.12° increments allowed the team to build up three-dimensional microtomographic images with submicron resolution.

At four melt fractions, from 2% to 20%, the researchers see an interconnected network of melt channels running along the edges where three or more olivine grains meet, in broad agreement with thermodynamic predictions.

At higher melt fractions, however, the images show films of melted material coating the boundaries between adjacent grains, but these films are far less evident at lower melt fractions. “One has a nicely connected network of channels that controls the flow,” Zhu said, but “the structure is not as simple as models predicted.” That more complex structure changes the exponent in the power law relating permeability to grain size.

To understand why mantle rocks have a melt fraction of 2% rather than the lower figure predicted, Zhu suggests thinking of their structure as less like a sponge and more like a snowball.

“When you squeeze a sponge, it is very compliant,” she said, whereas a snowball is easy to squeeze at first but then becomes much harder as its solid particles are compacted together. In the same way, the matrix of highly compressed mantle grains resists compression so that the melt network between the grains doesn't feel so much pressure and therefore doesn’t flow as fast as expected. Geologists have sometimes wondered about this kind of behavior, Zhu says, but the new results show that it can't be ignored.

In the future, the researchers hope to image the rocks while they are heated and kept under pressure.

Explore further: Magnitude-7.2 earthquake shakes Mexican capital

More information: Wenlu Zhu1, et al. "Microtomography of Partially Molten Rocks: Three-Dimensional Melt Distribution in Mantle Peridotite," Science 332, 88 (2011) DOI: 10.1126/science.1202221

Related Stories

Helium-3 - all is not lost

Sep 03, 2005

Is there a reservoir of primordial rock deep within the Earth, left over from the birth of our planet? Geochemical data have traditionally indicated 'yes', but evidence from seismology seemed inconsistent with the survival ...

Carbonates make diamonds grow in the Earth's mantle

Apr 08, 2011

Diamonds that no-one ever sees can form deep in the Earth’s interior. This is due to the chemical conditions controlling the associated carbon cycle. Swiss researchers used laboratory experiments to show ...

Rare melt key to 'Ring of Fire'

Oct 07, 2010

(PhysOrg.com) -- Oxford University scientists have discovered the explanation for why the world?s explosive volcanoes are confined to bands only a few tens of kilometres wide, such as those along the Pacific ...

Oceanic crust formation is dynamic after all

Nov 25, 2009

Imagine the Earth's crust as the planet's skin: Some areas are old and wrinkled while others have a fresher, more youthful sheen, as if they had been regularly lathered with lotion.

Recommended for you

Magnitude-7.2 earthquake shakes Mexican capital

Apr 18, 2014

A powerful magnitude-7.2 earthquake shook central and southern Mexico on Friday, sending panicked people into the streets. Some walls cracked and fell, but there were no reports of major damage or casualties.

User comments : 0

More news stories

China says massive area of its soil polluted

A huge area of China's soil covering more than twice the size of Spain is estimated to be polluted, the government said Thursday, announcing findings of a survey previously kept secret.

UN weather agency warns of 'El Nino' this year

The UN weather agency Tuesday warned there was a good chance of an "El Nino" climate phenomenon in the Pacific Ocean this year, bringing droughts and heavy rainfall to the rest of the world.

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...