New Picture Of Earth's Lower Mantle

Science Daily ^ | June 22, 2007 | Arizona State University

[SunkenCiv]

Most of Earth's lower mantle is made of a magnesium silicate mineral called perovskite. In 2004, earth scientists discovered that under the conditions of the lower mantle, perovskite can change into a high-pressure form, which they dubbed post-perovskite. Since its discovery, post-perovskite has been geophysicists' favorite candidate to explain the composition of a mysterious layer that forms the bottom of Earth's lower mantle. Known to earth scientists as D" (dee-double-prime), this layer averages 120 miles thick and lies directly above Earth's core. D" was named in 1949 by seismologist Keith Bullen, who found the layer from the way earthquake waves travel through the planet's interior. But the nature of D" has eluded scientists since Bullen's discovery. "Our team found," says McNamara, "that while post-perovskite has some properties that fit what's known about D", our laboratory measurements and computer models show that post-perovskite doesn't fit one particular essential property." That property is seismic anisotropy, he says, referring to the fact that earthquake waves passing through D" become distorted in a characteristic way... "Down in the D" layer, the horizontal part of earthquake waves travel faster than the vertical parts. But in our laboratory measurements and models, post-perovskite produces an opposite effect on the waves." ... "It's possible that post-perovskite does exist in the lowermost mantle, and another mineral is causing the seismic anisotropy we see there."

[SunkenCiv]

Allen McNamara. (Credit: Image courtesy of Arizona State University, Photo by Tom Story)

[SunkenCiv]

Scientific maverick’s theory on Earth’s core up for a test
SF Chronicle | Monday, November 29, 2004 | Keay Davidson
Posted on 12/05/2004 2:17:28 PM EST by SunkenCiv
http://www.freerepublic.com/focus/f-chat/1294934/posts

[SunkenCiv]

Thanks Steve.

[SunkenCiv]

· join · view topics · view or post blog · bookmark · post new topic ·

[SunkenCiv]

Mineral Physics Illuminates Lower Mantle Hypothesis
Science Daily
March 24, 2007

Scientists have shown increased interest in a mantle layer known as D", presumed to be just above the core-mantle boundary, since laboratory experiments in 2004 revealed a possible new high-temperature, high-pressure, crystal packing structure derived from the common mantle mineral perovskite.

Noting that recent research has also hypothesized the existence above and below the D" layer of discontinuities in the velocities of seismic waves, Hernlund and Labrosse investigate whether such discontinuities are theoretically predicted by mineral physics.

Using independent constraints for a lower bound on temperature in the Earth's deep mantle and for the temperature of the Earth's inner core boundary, the authors examine the nature of the transition between perovskite phases, as hypothesized by the existence of the D" layer and the observed seismic velocity anomalies. They find it consistent only with part of the range of uncertainties in current knowledge of the pressure-temperature behavior of minerals at such depths.

[raybbr]

ping for later