Rare Rocks Offer A Unique Glimpse Of The Earth's Core

In a study published in the Aug. 19 issue of the journal Science, researchers from Stanford and the University of Copenhagen used state-of-the-art technology to measure 13 millimeter-sized grains of rock collected from slabs of oceanic crust and the lower mantle that have been pushed up on the American continental shelf in the Klamath Mountains of Northern California and Southwestern Oregon.

The samples are of particular interest to geologists because they contain large amounts of the element, osmium (Os). Though rarely found on the surface, osmium is believed to occur in relatively high concentrations in the Earth's core some 1,700 miles below the crust. The core is too deep for scientists to observe directly, so the discovery of osmium-laden rocks provides tantalizing hints about what lies below.

"Osmium loves iron," said Stanford Research Associate Anders Meibom, lead author of the Science study. "We know that osmium is found in iron meteorites," he added. "Since the Earth's inner core is also predominantly iron, we believe that osmium has a strong tendency to go into the core instead of remaining in the surrounding mantle, which contains little iron."

Volcano studiesEarlier studies of lava from volcanoes in Hawaii and Siberia revealed high concentrations of two very rare osmium isotopes: 186-Os and 187-Os. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei.

Scientists suspect that the Hawaiian and Siberian lava samples were produced by hotspots - stationary sources in the mantle that feed hot magma to the volcano on the surface. Some researchers maintain that hotspots originate just 60 miles below the crust, while others contend that the source is much deeper – 1,700 miles down at the core-mantle boundary.

To resolve the controversy, Meibom and Science co-author Robert Frei of the University of Copenhagen decided to analyze the rock samples from California and Oregon.

"It is very likely that these osmium-rich grains will provide clues to the chemical makeup of the core-mantle boundary itself," Meibom said.

The researchers' goal was simple: First they would determine the 186-Os and 187-Os isotope composition of the California-Oregon rocks. Then they would compare those levels to the known isotope concentrations in the Hawaii and Siberia lava samples. If the results were similar, it would lend support to the idea that the Hawaiian and Siberian hotspots originate at the core-mantle boundary instead of near the surface. <