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Stanford Report, April 24, 2002

Rare rocks offer a unique glimpse of the Earth's core

BY MARK SHWARTZ

Rare grains of metal from California and Oregon are providing new clues about the origin of the Hawaiian Islands -- and fueling old controversies about the evolution of the Earth's core.

In a study published in the April 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.

Geologists believe that the Hawaiian Islands were created by a hotspot deep in the Earth. Some researchers say the hotspot is located in the mantle, while others say it is at the core-mantle boundary. Image courtesy of USGS

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 studies

Earlier 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.

Rare osmium isotopes found in California and Oregon may have originated in the Earth's outer core, 1700 miles below the surface, according to a new study. Courtesy of USGS

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.

The isotope analysis required the use of two powerful instruments located in two countries: the SHRIMP (Sensitive High Resolution Ion MicroProbe) operated by Stanford and the U.S. Geological Survey; and the N-TIMS (Negative-Thermal Ionization Mass Spectroscope) at the University of Copenhagen in Denmark.

"These are cutting-edge technologies," observed University of California-Santa Cruz scientist Quentin Williams, an expert on the geology of the inner Earth. "These types of osmium grains are harder than the devil to find. I'm quite impressed that they found some."

The SHRIMP and N-TIMS analyses revealed that osmium isotope levels in the California-Oregon rocks are remarkably similar to those in the lava samples from Hawaii and Siberia. A likely explanation is that all of the samples came from hotspots located at the core-mantle boundary, according to Williams.

"All of these sites -- from Hawaii in the middle of the Pacific to Asia to North America -- show grossly similar osmium isotope characteristics," Williams concluded. "Therefore, they are actually a signature of the Earth's core."

Meibom and Frei were more cautious, noting that reservoirs of osmium isotopes could exist higher in the mantle. Their findings also rekindled a long-standing dispute between geochemists, who believe that rare osmium isotopes must have been established in the liquid outer core as the solid inner core was forming when Earth was less than 100 million years old, and geophysicists, who maintain that the inner core formed much later.

"We've never had a sample of the core and we never will," Williams noted. "The best we can hope for results like these: a geochemical hint of the core's perfume -- a faint scent of the core, if you will."

Anders Meibom