University of Pittsburgh
May 16, 2000

PITT GEOLOGIST SAYS TWO ERUPTIONS, NOT ONE, CREATED PENNSYLVANIA IGNEOUS FORMATION

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PITTSBURGH, May 16 -- It's just a slab of gray rocks, and the difference between it and other slabs of gray rocks in Fayette County, Pa., is barely perceptible, except to veteran geologists who've spent years identifying rocks, especially rare and unusual varieties.

But the Masontown Kimberlite Dike is the center of a debate between researchers in the international geology community over its origin.

Michael Bikerman, emeritus professor of geology and planetary science at the University of Pittsburgh, caused the row at a Geological Society of America Northeastern Section meeting in late March when he presented his hypothesis that the Masontown Kimberlite Dike was a product of two separate events, rather than a single event, as had been previously supposed.

Previously, on geologic grounds, the dike was known to be younger than

290 million years old, the upper limit of the age of the surrounding Pennsylvanian sedimentary rocks through which the dike intruded. However, researchers using potassium-argon (K-Ar) dating had measured dates of 350 million years ago by testing large mica phenocrysts—large crystals imbedded in the dike—and interpreted these dates to indicate the presence of excess argon—argon not derived from decay of the parent potassium within the mica.

Earlier research by Bikerman and Pitt graduate students Nellie Pimental and Henry Prellwitz confirmed the 350 million-plus age of some of the stones, but found that others in the dike did not conform to these ages.

Bikerman used two other dating methods, rubidium-strontium (Rb-Sr) and samarium-neodymium (Sm-Nd). Using the alternate methods Bikerman dated stones such as garnet and phlogopite at 145-188 million years old. The Rb-Sr technique produced results about 180 million years old, and the Sm-Nd, an imprecise 145 million years old.

Using a higher precision variant of K-Ar dating, Bikerman and colleague David Phillips of the Australian National University, determined dates on micas to be between 180 and 145 million years old, leading Bikerman to the conclusion that two events, not one, created the Masontown dike.

Evidence that younger kimberlite was mixed with older, along with the way that the ground around the fault reacted to the eruptions, gave Bikerman the clues that he was looking at one eruption 180 million years ago, followed by a second eruption

149 million years ago.

"An excess or inherited argon emplaced 149 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."

Disagreements like this are common, Bikerman says, especially when new research challenges old assumptions.

"My presentation was well-received, but since then I've gotten a few calls from colleagues who disagree," Bikerman said.

As if to reinforce the idea, Bikerman points to a framed motto on his office wall: "Geology has several versions of reality."

"That's what happens when you get an educated group of people into a field like geology. We're like detectives, only our evidence is millions of years old."

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BACKGROUND

Geologic Dating Systems

Geologists use a dating system that incorporates the fact that radioactive elements decay over time. The time during which the strength of an element decays to half its original value is called a half-life, and examining the level the element has decayed helps to determine the age. But these measurements are not precise in terms that humans could relate to, however: The half life of potassium is 1.25 million years, for instance!

Kimberlite

Kimberlite is an igneous rock, formed by magma, or molten rock, erupting from deep in the earth. Kimberlite is especially of interest because it can contain diamonds. There are no diamonds in Masontown, unfortunately, because the kimberlite dike there apparently formed at too shallow a depth to create the necessary pressure.