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June 8, 2000

Geologist challenges belief about development of area rock formation

Geologist challenges belief about development of rock formation

The Masontown Kimberlite Dike is an undistinguished-looking stretch of gray rocks, sporadically exposed and less than a mile long, that Fayette County residents first noticed around 1860.

It was formed many millions of years ago when molten rock, erupting from deep in the earth, pushed its way up through existing sedimentary rock.

Until recently, scientists agreed that a single eruption created the dike, as such geological features are called. But Michael Bikerman, a Pitt associate professor emeritus of geology and planetary science, rocked a Geological Society of America Northeastern Section meeting in late March when he suggested that the Masontown dike was the product of two eruptions, not one.

It may not sound earthshaking to the general public, but geologists are intrigued by the hypothesis.

"If we actually have found that a very small dike like this can host two events 40 million years apart, and that this magma pooled up a second time and found it easier to break through already-existing kimberlite rather than coming up through other, surrounding fissures — well, from a geological point of view, that would be very interesting," Bikerman said.

Kimberlite is igneous — that is, formed by solidification of molten rock — and especially interesting because it can contain diamonds. It's the same type of rock found in the diamond-rich ground near Kimberley, South Africa, after which the rock was named. There are no diamonds in Masontown, alas, because the kimberlite dike there formed at too shallow a depth to create the necessary pressure.

Geologists had long known, based on geologic evidence, that the Masontown Kimberlite Dike was younger than 290 million years old, the upper limit of the age of surrounding rocks through which the dike intruded.

However, researchers who used potassium-argon (K-Ar) dating measured dates of 350 million years ago by testing large mica crystals imbedded in the dike. By examining the extent to which potassium in rocks has decayed and been replaced by the inert element argon, scientists can determine the rocks' age.

When K-Ar dating showed the dike was 350 million years old, scientists concluded that the dates merely indicated the presence of excess or inherited argon — argon not derived from decay of potassium within the mica.

Research by Bikerman and Pitt graduate students Nellie Pimental and Henry Prellwitz confirmed the 350 million-years-plus age of some of the rocks, but found that other minerals in the dike did not conform to these ages.

Similar findings resulted when Bikerman and his colleagues tested the rocks using two other methods, rubidium-strontium and samarium-neodymium dating, as well as a higher precision variant of K-Ar dating.

Evidence that younger kimberlite was mixed with older, along with the way the ground around the fault reacted to the eruptions, led Bikerman to conclude that the Masontown dike was created by one eruption 180 million years ago, followed by a second eruption 145 million years ago.

"An excess or inherited argon emplaced 145 million years ago could explain the difference in the disagreement we get with the potassium-argon dating, but there's no way to explain why the other tests came out as they did unless we're looking at two events," Bikerman said.

"This is an interpretation," he added, "and more study will either confirm it, change it or disprove it. My feeling is, we scientists are like turtles: If we don't stick our necks out, we don't make advances."

Bikerman plans to sail as academic dean on next fall's Semester at Sea voyage and won't pursue his theory on the Masontown dike until after he returns. He said he hopes, in the meantime, that fellow geologists will further investigate his findings.

Bikerman said he wouldn't be surprised if creationists seize on this latest dispute among geologists in an attempt to debunk evolution science.

"Their argument will be, 'See? These geologists can't even agree among themselves whether something is 145 million or 180 million years old!'"

Like some of his colleagues, Bikerman has been mildly harassed by creationists over the years. Just last semester, he said, one of his undergraduate courses was attended by two Christian fundamentalist brothers who repeatedly challenged assertions that life on Earth evolved over billions of years.

"They were always polite," Bikerman recalled, "but eventually I had to tell them, 'You're free to believe what you want to believe, but this is the material you'll be expected to learn in this class….' "Creationists believe they already have the answers. That's the big difference between them and scientists. We don't have the answers. We're searching."

— Bruce Steele

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