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Meteor linked to rich ores at Sudbury.

Take a nickel out of your pocket and chances are that the metal came from an unusual geologic structure in Ontario, Canada, called the Sudbury Igneous Complex. This 60-kilometer-long structure is made up of zones of different igneous (cooled from a molten state) rock types stacked like a series of elliptical-shaped bowls. Not only is it the world's largest single source of nickel, but it is also rich in copper and other elements.

In spite of a long history of mining, the geologic origin of the complex has remained in question for more than a century. Some scientists have argued that the complex formed when magma(molten rock) from the mantle moved up through the crust. But now three researchers present geochemical evidence in the Oct. 25 SCIENCE that fortifies another idea, originally suggested in 1964, in which the structure was formed when a meteorite slammed into the earth 2 billion years ago with enough energy to melt crustal rocks.

Asish R. Basu and Billy E. Faggart Jr. at the University of Rochester(N.Y.) and Mitsunobu Tatsumoto of the U.S. Geological Survey in Denver measured the concentrations of neodymium and samarium in 16 rock samples representing each rock type in the complex. The researchers chose to look at neodymium and samarium because these elements provide a powerful tool for differentiating between crustal rocks and material that comes from the mantle. For example, neodymium atoms are larger and somewhat lighter than samarium atoms, so they have a greater tendency to migrate into plumes of magma, which then rise to the crust, leaving proportionately more samarium in the mantle. And since samarium-147 radioactively decays into the isotope neodymium-143, one also expects to find a higher ratio of neodymium-143 relative to neodymium-144 in mantle-derived rocks than in crustal material.

Regardless of the rock type or the sample location, Basu's group could find no neodymium isotope ratios that reflected that mantle material. Instead, "we find a very strong signature of the upper crust," says Basu. "The only viable explanation is that the Sudbury Complex formed from the melting of crustal rocks by way of a meteoritic impact." The other main line of evidence pointing to an impact are previously discovered shatter cones--distinctively striated conical rocks that have been fractured in a way thought to result from shock waves.

While they have yet to work out the details, the researchers think the impact was responsible for the unusual abundances of ores at the complex. Basu suggests that the meteorite triggered the melting of very large amounts--much more than are typically associated with impacts -- of both basaltic and granitic crustal rocks at Sudbury. When molten basalts and granite combine, he says, sulfur tends to separate out from the mixture just as oil and water separate after mixing. In so doing, the researchers think, the sulfur took with it nickel, copper and other elements from the melt to form the ore-rich, concentrated sulfide layer found at the bottom of the Sudbury Complex.

"That kind of process doesn't easily take place in ordinary volcanism in the crust," says Basu. Because the basaltic magma from the mantle cools constantly as it moves through and heats the crustal granites and sandstones, it doesn't have a chance to assimilate enough crustal rocks to produce the sulfide ores in such quantities, he says.

In addition to neodymium and samarium, Basu's group measured the concentrations of rare earth elements. They found that for light rare earth elements, the compositions of the Sudbury Complex rocks were essentially the same as those measured in the North American Shale Composite, an eroded sediment thought to represent the average upper continental crust.

The researchers also dated the Sudbury rocks using the samarium-neodymium radioactive clock to obtain an age of 1.8 billion years -- in excellent agreement with previous dating using uranium and lead isotopes. These dates make the Sudbury Complex one of the earth's oldest known "astroblemes," or eroded impact craters. The group will present its findings at the upcoming Geological Society of America meeting in Orlando, Fla.
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Title Annotation:unusual geologic structure in Ontario, Canada
Author:Weisburd, Stefi
Publication:Science News
Date:Oct 26, 1985
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