Scrambled Earth; researchers look deep to learn how the planet cools its heart - includes related article about use of diamonds in high-pressure physics

Science News, April 9, 1994 by Richard Monastersky

For these experiments, the researchers assumed that the lower mantle contains the same basic ingredients as the upper mantle but that the rocks assume a more compact crystalline structure in the lower mantle. Geologists know that this type of phase transformation does indeed occur at a depth of 670 km.

In the models, subducting rock sinks to the boundary but cannot penetrate at first. Only after enough cool material accumulates atop the boundary does it finally break through to the lower mantle, according to two simu!ation studies reported last year (SN: 2/27/93, p. 133).

Taken at face value, these results present a picture akin to that emerging from seismology. But many regard the models as too simplistic to offer much help in resolving the convection debate.

Modelers thus far have fudged their simulations by ignoring the stiffness of the lithospheric plates, a characteristic that, if included, would increase the cost of computing. According to some researchers, if the models represented the plates more realistically. these stiff slabs might break through the boundary instead of pooling above it. Tackley is currently trying to include the rigidity of plates in a three-dimensional simulation of mantle convection.

Despite the problems inherent in the models, Tackley believes the recent computer simulations, along with the seismic tomography results, have helped push the debate forward. He sees geophysicists now embracing the idea that the mantle does not mix as either one layer or two layers, but rather exhibits some intermediate type of flow.

"The debate is shifting to a question of the degree of the stratification in the mantle instead of whether the mantle convects as one layer or two," says Tackley, who sees some drift toward agreement between the opposing camps. "Even people who in the past have held extreme views are moving in favor of a compromi se."

They may not admit it, however. Both Cal techs Anderson and MIT's Jordan regard their positions as unchanged.

Anderson thinks the debate has converged recently by shifting toward the idea that he has long championed - namely. that the mantle is stratified. The recent model results, he says, show at least partial layering within the mantle because sinking material gets delayed at the boundary. Anderson suggests that the models would produce even more complete stratification if they included differences in chemistry between the upper and lower mantle- a possibility supported by highpressure experiments.

Jordan, not surprisingly. reads the situation differently. He contends that the new modeling studies don't contradict his original conclusion that plate material sinks all the way into the deep mantle. Whether the plates get delayed at the 650 km boundary is less important, he says. In fact, Jordan, Tackley. and others reported last year that seismic records do not show evidence of major pools of cold material sitting at the boundary. That study. he says, indicates that slabs cannot linger there for too long before breaking into the lower mantle.