SAN FRANCISCO--Researchers probing Earth's deep interior are celebrating what could be a startling new find: fresh, seemingly unaltered minerals from hundreds of kilometers deeper than any found before. The rocks, on one of the Solomon Islands in the Pacific Ocean, may divulge much about the nature and origin of a part of the planet once accessible only remotely, through seismic imaging. "[They're] potentially the most important rocks ever" for probing the deep Earth, says petrologist Harry Green of the University of California, Riverside.
The new treasures come from a jungle-covered island called Malaita, which lies just northeast of Guadalcanal of World War II fame and farther east of Papua New Guinea. Researchers have known that 34 million years ago, deep-seated volcanic eruptions blasted through a massive pile of 120-million-year-old submarine lavas called the Ontong Java Plateau. Carried by the Pacific Plate, the plateau eventually collided with the Australian Plate about 23 million years ago, pushing up Malaita and exposing the tops of the volcanic pipes.
Recently, a mining exploration company sent a group led by petrologist Kenneth Collerson of the University of Queensland in Brisbane, Australia, some "funny-looking garnets" from Malaita, unlike any seen before. Collerson and his colleagues found 10- to 20-micrometer crystals of diamond, majorite, and other minerals embedded in 10- to 20-centimeter chunks of deep rock. The crystals indicated the rocks had been subject to high pressure, he says, and thus came from great depths; squeezing so much silicon into majorite, for instance, would have taken pressures between those at 260 and 570 kilometers, near the traditional upper mantle/lower mantle boundary at 660 kilometers. The team also found some crystals with the composition of silicate perovskite--a mineral that would be produced from the basalt of ocean crust once it has sunk to 770 kilometers, as seismic imaging shows it sometimes does (Science, 31 January 1997, p. 613).
The Queensland group's early findings, announced here last month at a meeting of the American Geophysical Union, have created quite a buzz. Minerals have been reported before from such depths (Science, 10 September 1993, p. 1391), but they had either returned to their low-pressure crystal forms or been too tiny for complete analysis. "The prospect of having natural samples [of the deep mantle] that appear so unaltered is very exciting," says Catherine McCammon of the University of Bayreuth in Germany.