03/13/1997 08:00:00

Researchers Establish Upper Limit of Temperature at the Core-mantle Boundary of Earth

PASADENA— Researchers at the California Institute of Technology have determined that Earth's mantle reaches a maximum temperature of 4,300 degrees Kelvin. The results are reported in the March 14, 1997, issue of the journal Science.

According to geophysics professor Tom Ahrens and graduate student Kathleen Holland, the results are important for setting very reliable bounds on the temperature of Earth's interior. Scientists need to know very precisely the temperature at various depths in order to better understand large-scale processes such as plate tectonics and volcanic activity, which involves movement of molten rock from the deep interior of the Earth to the surface.

"This nails down the maximum temperature of the lower-most mantle, a rocky layer extending from a depth of 10 to 30 kilometers to a depth of 2900 kilometers, where the molten iron core begins," Ahrens says. "We know from seismic data that the mantle is solid, so it has to be at a lower temperature than the melting temperature of the materials that make it up."

In effect, the research establishes the melting temperature of the high-pressure form of the crystal olivine. At normal pressures, olivine is known by the formula (Mg,Fe)2SiO4, and is a semiprecious translucent green gem. At very high pressures, olivine breaks down into magnesiowüstite and a mineral with the perovskite structure. Together these two minerals are thought to make up the bulk of the materials in the lower mantle.

The researchers achieved these ultra-high pressures in their samples by propagating a shock wave into them, using a high-powered cannon apparatus, called a light-gas gun. This gun launches projectiles at speeds of up to 7 km/sec. Upon impact with the sample, a strong shock wave causes ultra-high pressures to be achieved for only about one-half a millionth of a second. The researchers have established the melting temperature at a pressure of 1.3 million atmospheres. This is the pressure at the boundary of the solid lower mantle and liquid outer core.

"We have replicated the melting which we think occurs in the deepest mantle of the Earth," says Holland, a doctoral candidate in geophysics at Caltech. "This study shows that material in the deep mantle can melt at a much lower temperature than had been previously estimated. It is exciting that we can measure phase transitions at these ultra-high pressures."

The researchers further note that the temperature of 4,300 degrees would allow partial melting in the lowest 40 kilometers or so of the lower mantle. This agrees well with seismic analysis of wave forms conducted in 1996 by Caltech Professor of Seismology, Donald Helmberger, and his former graduate student, Edward Garnero. Their research suggests that at the very lowest reaches of the mantle there is a partially molten layer, called the Ultra-Low-Velocity-Zone.

"We're getting into explaining how such a thin layer of molten rock could exist at great depth," says Ahrens. "This layer may be the origin layer that feeds mantle plumes, the volcanic edifices such as the Hawaiian island chain and Iceland. "We want to understand how Earth works."

Written by Robert Tindol