NEWS FROM CARNEGIE INSTITUTION OF WASHINGTON

Contact Sean Solomon at 202-686-4370, ext. 4444 or solomon@dtm.ciw.edu or Tina McDowell in the Carnegie news office at 202-939-1120

ICELAND PLUME HAS A LOWER-MANTLE ORIGIN, SCIENTISTS REPORT

The origin of plumes - the thin cylinders of hot, upwelling magma that emerge from stationary hotspots around the world-has long been a mystery. While most scientists believe that plumes originate deep in the mantle, possibly as deep as the core-mantle boundary, there has been little evidence to support this view.

Now, in this week's Nature magazine, a group of seismologists from the Carnegie Institution, Woods Hole Oceanographic Institution, and the University of Iceland provide the first seismic evidence suggesting that at least one plume - the Iceland plume - originates in the lower mantle. "It appears that the Iceland plume penetrates the upper-to-lower mantle transition zone," says Sean Solomon, director of Carnegie Institution's Department of Terrestrial Magnetism (DTM) in Washington, D.C. and one of the paper's authors. "Our result resolves a debate that has been raging for more than 25 years: do plumes originate in the lower mantle or are they the result of convective instabilities in the upper mantle? For the Iceland plume, the answer is now in." The other authors of the paper are Yang Shen, the lead author (previously of Woods Hole and recently relocated to the University of Rhode Island), Cecily Wolfe (Woods Hole), and Ingi Bjarnason (University of Iceland's Science Institute).
The Iceland plume is one of the most extensively studied plumes in the world. It erupts onto an exposed part of the mid-Atlantic ridge, where two tectonic plates are pulling apart. Last year, Solomon and collaborators (including Wolfe and Bjarnason) reported the first high-resolution seismic image ever produced of the Iceland plume, indeed of any active mantle plume. Their map of the upper mantle beneath the Iceland hotspot, based on seismic data gathered from an array of portable seismometers on the surface, showed that a cylinder of hot, upwelling material directly beneath Iceland occupies a slender zone about 300 km wide.

While the researchers determined that the plume extends more than 400 km in depth, they were unable to resolve structure below that depth, and so could not determine the plume's depth of origin. Instead, they took another tack. Rather than concentrate on the plume, they chose instead to image the mantle's transition zone, which the plume penetrates, again using data from the portable seismic array. The transition zone is an area about 250-km thick sandwiched between two seismic discontinuities, one at 410-km depth, the other at 660-km depth. (The lower depth corresponds to the boundary between the upper and lower mantle.) At each of these discontinuities, pressure-induced changes in mineral molecular structure yield successively denser crystalline arrangements. Also at each, an upward traveling seismic wave can be converted to another type, for example, from a compressional (P) wave to a shear (S) wave.
By measuring variations in the depths of P-to-S wave conversions, the researchers were able to determine the thickness of the transition zone, as well as its relative temperature. (The thinner the transition zone, the hotter it is.) The results indicated that the transition zone is thinner (by about 20 km) and thus hotter (by about 150 K) within the area containing the circular plume than it is in those areas immediately adjacent to the plume, or, indeed, in any average slice of Earth. The plume thus extends below the transition zone and so must originate in the lower mantle. Just how deeply it extends is unknown.
An alternative scenario-with the plume originating instead from an instability at the base of the upper mantle (as a result of regional heating from below)-is not tenable, write the authors. If it were, the horizontal extent of the instability layer supplying the hot material for the plume would be much greater than the diameter of the plume conduit. The scientists found, however, that the thin transition zone occupies an area only as wide as the plume itself.
Thin transition zones are not general features of mid-ocean ridges. Indeed, the researchers found that the boundaries of the transition zone are completely different from those of the ridge structure beneath Iceland. Most ridges are evidently the products of passive upwelling from shallow mantle depths in response to spreading of the tectonic plates. Plumes, if Iceland is representative, are part of a distinct mantle flow system involving exchange between the lower and upper mantle.

The work was supported by the National Science Foundation.

The Department of Terrestrial Magnetism (DTM) is one of five research departments of the Carnegie Institution of Washington, a nonprofit science research organization founded by Andrew Carnegie in 1902. Led by the biologist Maxine F. Singer, the institution is today devoted to advanced research and education in the physical and biological sciences. Sean C. Solomon has been director of DTM since 1992.

Seismology has been a part of DTM's research for many years. Researchers there have pioneered the development of the borehole strainmeter device and took the lead in developing portable seismological instruments of the type used in this study. In 1994, Solomon and his colleagues emplaced an array of 15 portable seismometers on Iceland's surface, directly above the hotspot site on the Vatnajokull glacier. For three years, before their removal in 1996, the instruments gathered valuable seismic information about the Iceland plume, leading to the group's first paper in Nature, published in January 1997 (Nature 385, 245-247), and to the second, published this week. Shen, Wolfe, and Bjarnason are all former students or postdoctoral fellows of Solomon.