Deep rocks offer a glimpse into mantle.

Deep rocks offer a glimpse into mantle

In a South African diamond mine, geologists have discovered rocks that originated in the heart of Earth's upper mantle, some 300 to 400 kilometers below the planet's surface. Scientists have hotly debated the makeup of this region, but until now they have lacked direct evidence to constrain their theories.

"These are, to date at least, the deepest known samples to come out of the Earth," says Stephen E. Haggerty of the University of Massachusetts in Amherst.

He and Violaine Sautter of the University of South Paris in Orsay collected the rocks from an extinct volcanic pipe called the Jagersfontein kimberlite, about 230 km southeast of Kimberley, South Africa. Kimberlites represent ancient conduits through which diamonds and other pieces of upper mantle are driven toward Earth's surface.

In a mine at the Jagersfontein, Haggerty and Sautter discovered two highly unusual rock fragments, containing numerous specks of the mineral clinopyroxene scattered within garnet. The specks are oriented along specific planes, indicating the mineral was once dissolved in the garnet under extreme pressure but has since come out of solution, they report in the May 25 SCIENCE. Laboratory experiments indicate that clinopyroxene dissolves in garnet only at pressures higher than about 130 kilobars, which corresponds to a depth of 300 to 400 km below the surface. The researchers say the evidence for the clinopyroxene-garnet solution suggests the rocks came from at least that depth--near the bottom of the asthenosphere, the partially molten section of the upper mantle.

The asthenosphere supports the more buoyant and rigid lithospheric plates that drift across Earth's surface. Never before have scientists found kimberlite rocks bearing telltale marks of asthenospheric pressures, although some finds may have originated at such depths.

The newly discovered rocks support a controversial theory concerning the Earth's structure. Seismic evidence collected from earthquake waves traveling through the mantle indicate that a transition zone beneath the asthenosphere, between depths of 400 and 670 km, separates the upper mantle from the lower mantle. Most scientists believe both the upper mantle and the transition zone share the same composition, consisting largely of olivine-rich rocks.

But Don L. Anderson at the California Institute of Technology in Pasadena contends the transition zone holds both olivine and a high concentration of eclogite -- a rock made of garnet and clinopyroxene. This arrangement would tend to seal off the lower mantle from the upper mantle, preventing mixing between the two. A corollary of this theory holds that subducting pieces of oceanic crust do not sink into the lower mantle but instead stop in the eclogite-rich transition zone.

Haggerty says the Jagersfontein samples support Anderson's theory that the transition zone contains abundant eclogite, lending credence to the idea of a stratified mantle.

Geologist Joseph R. Smyth of the University of Colorado at Boulder calls the Jagersfontein rocks "exciting and very intriguing," but maintains they will not end the debate on the structure of the mantle.