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Mica Technique Yields Valuable Geological Infomation

A single crystal of rock can provide a wealth of information about a mountain's history, two MIT scientists have shown.

Professor Kip V. Hodges and Research Specialist Willis B. Hames, both of the Department of Earth, Atmospheric and Planetary Sciences (EAPS), discovered that crystals of mica-a mineral common in mountainous regions-often contain ring-like distributions of argon 40. These distributions, in turn, can be used to date a variety of geologic events, including the time of crystallization of the mica and the ages of subsequent thermal disturbances.

Although theoretical models had predicted the existence of such gradients, the MIT work "is the first to document these distributions and to show that they can be used to get detailed information about mountains' cooling histories," Professor Hodges said. The work, which was reported in the September 24 issue of Science, will be expanded upon in an upcoming issue of Geology (with Associate Professor Samuel A. Bowring of EAPS).

Drs. Hodges and Hames made their observations using a laser technique originally used to study meteorites and lunar samples. "We applied an existing technology to a different problem," Dr. Hames said.

And because the new application provides much more information about mountain-building events than conventional techniques, Dr. Hodges predicts that "within the next 10 years this application will become very important. It could become one of the most powerful ways to look at the cooling histories of mountains."

To study a mountain

Scientists interested in how mountains formed use radiometric dating to determine when various tectonic events occurred. This method takes advantage of the radioactive decay of certain elements to others over time. Researchers who study the formation of mountains commonly track the decay of radioactive potassium 40 to argon 40.

Because the decay rate of potassium 40 to argon 40 is well known, scientists can determine the dates for certain geologic events based on the amount of argon 40 in a sample or the ratio between argon 40 and potassium 40 (the more argon 40 present or the higher the ratio, the older the sample).

In conventional argon dating, geologists melt hundreds of crystals of rock in a furnace or with a laser to extract the gaseous argon. But such a procedure tends to "homogenize" data. As Dr. Hames explained, "when a sample is heated in a furnace, the [argon] gas is liberated from all areas of a crystal at the same time, so any gradients in distribution would tend to be evened out."

About three years ago, Professor Hodges assembled the Cambridge Laboratory for Argon Isotopic Research at MIT (which he continues to direct), one of five labs in the world that use a laser to melt samples for radiometric dating. Most often, however, people using the facility (which is open to scientists around the world) simply melt bulk samples of material-the conventional approach to argon dating.

Drs. Hodges and Hames decided to apply the laser in a new way: they used it to melt tiny points all over the surface of a single crystal of mica. And they found that the argon was indeed distributed in ring-like gradients, with more argon at the center of the crystal and less at the rim.

Such a distribution occurs because as a sample slowly cools, it becomes harder and harder for the gaseous argon to escape until it is effectively "frozen," or locked, into the crystal. But the argon isn't all locked in at once-argon at the center of the crystal is locked first, because it is the longest distance from the surface, while argon at the rim is locked last. The specific pattern of the argon gradients in a given crystal is determined by such factors as the size of the crystal and that crystal's geologic history (the multiple heating and reheating events characteristic of mountainous terrains).

What they found

Using radiometric dating, the scientists therefore found that the center of the crystal they studied "locked" about 462 million years ago while the edge locked much more recently, about 420 million years ago. The date for the center means, in turn, that the sample itself must have first crystallized during a major geologic event sometime close to 462 million years ago. (Dr. Hames noted that larger crystals have older ages in their cores, providing better estimates of the crystal's age.)

The gradients also told the scientists much more about other metamorphic events that affected the sample over the years. For example, two anomalous areas inside the crystal showed ages much younger than the outer rim. These, it turned out, corresponded to cracks in the crystal that must have happened when the crystal was folded or bent. By analyzing the argon present in these areas, the scientists were able to determine that the cracks must have been introduced around 400 million years ago.

Finally, by analyzing the patterns of the argon gradients themselves, Professor Hodges said, "we extracted important information about the rate of cooling of the sample after crystallization."

Drs. Hodges and Hames demonstrated the power of the new laser application by comparing their data on the sample described above to data collected 20 years ago for the same sample. The scientists who conducted that first analysis used conventional argon dating, and according to Professor Hodges they could determine only a minimum age for when the sample was formed-they couldn't determine its cooling history, or when other geologic events occurred.

"So the laser application we're using provides a more continuous record of the dynamic process of mountain building," Dr. Hames concluded.

Educational mission

Professor Hodges noted that the Cambridge Laboratory for Argon Isotopic Research is fully automated. As a result, it's easy to train new users of the facility, and the lab has become a major resource for UROP projects. He encourages students to get in touch with him or Research Specialist William Olszewski (who supervises the daily operation of the lab) for more information. "We're always interested in getting other undergraduates involved in both scientific studies that use argon data and engineering projects that lead to new analytical methods," he said.

The facility has also become an important regional resource for students, Professor Hodges said. "We have several students from undergraduate colleges in New England who are doing their senior theses using the machine."

The lab is supported by the National Science Foundation and EAPS, with some funds coming from outside industrial and academic users. It was established with funds from MIT, the NSF and Harvard University.

A version of this article appeared in the October 27, 1993 issue of MIT Tech Talk (Volume 38, Number 11).

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