Tiny Bubbles A Storehouse Of Knowledge

But the realization of the scientific value of fluid inclusions isrelatively recent. At the Centennial Celebration Symposia for theSociety of Economic Geologists, Virginia Tech Distinguished Professorof Geosciences Robert Bodnar will review the history of the scientificuses of fluid inclusion, identify the big questions that remain to beaddressed, and predict the state of the field in 10, 50, and 100 years.

The symposium is Saturday, Oct. 15, during the GeologicalSciences of America national meeting in Salt Lake City Oct. 16-19. Thesociety has asked the world's experts in areas of economic geology togive talks summarizing the state of their fields. As the Society ofEconomic Geology Distinguished Lecturer, Bodnar was asked to give thekeynote address.

The title of his paper is "Fluid inclusions, from H.C. Sorby to Edwin Roedder and Beyond."

Fluid inclusions were considered curiosities until Sorby saw themineral-incased bubbles as clues to processes that createrocks andminerals. Sorby, a pioneer in the use of the microscope to examinerocks, was the first to describe inclusions from a scientificperspective. In 1858, he used them to determine the temperature andpressure at the time the rock was formed.

"Not much else was done until Ed Roedder applied inclusions to widerange of geological problems," Bodnar said. Roedder showed thatinclusions could be tapped to determine the pressure and temperature atthe time of formation. Roedder's definitive book, Fluid Inclusions(1984), described what fluid inclusions are and how they can be used.Now 86 and still active, Roedder was Bodnar's mentor at the U.S.Geological Survey and continues to provide valuable advice and supporttoday.

Fluid inclusions are a timely topic. They are used by the oil industryto predict where oil deposits have occurred and to track where oil hasflowed through the subsurface in order to discover new fields. Chevronhired Fang Lin, one of Bodnar's recent Ph.D. students, to help withexploration because of her experience with fluid inclusions.

But fluid inclusions are really most valuable to explorationfor metal deposits, such as copper, gold, and lead. Certain types offluid inclusions are located with certain types of ore.

"We slice the rock, and if you see inclusions with certaincharacteristics, it tells us what we are likely to find," Bodnar said.Which leads to the questions that remain to be addressed. "The biggestuncertainties are how long it takes for an ore or petroleum deposit toform," Bodnar said. "Does it take hundreds of years, thousands ofyears, or millions of years?

"We know how long some processes are active. We know some metaldeposits formed in an environment similar to an active volcano. Two orthree miles under Mount St. Helens is the kind of environment wherecopper deposits form," Bodnar said. "The Bingham Canyon Mine, one ofthe world's largest surface mines, was once under a volcano."

But how does ore form in the volcano? "Volcanoes can be dormanthundreds or thousands of years. Mount St. Helens has been active only afew months in the last 25 years, for example," Bodnar said.

He is part of a group that is studying Mt. Vesuvius. "Writtenrecords that go back thousands of years provide a historical recordregarding when eruptions occurred, how long they lasted, and how longthe volcano was dormant. We are able to extract fluid from theinclusions and use isotope techniques to determine the age when it wastrapped," Bodnar said. "We can compare that with the written recordsand then know what the volcano was doing as the ore formed.

"The better we understand how deposits form, the better able wewill be to develop techniques to look for new deposits. There aremillions of old, dead volcano sites. They didn't all form ore deposits.The inclusions from the ones that did will tell us what to look for,"Bodnar said.

But his favorite future use of fluid inclusions is to study theuniverse. The rock-bound bubbles are clues to whether there was oncewater, and possibly life, among the stars. "If we have rock samplesfrom Mars and we find inclusions with water, it increases the chancesthat there could have been some form of life," Bodnar said. Fluidinclusions will be an important tool to study meteorites and rocks fromthe moon, asteroids, and the planets we will visit to look for evidenceof water and the possibility of life."

Bodnar will present his talk at 4 p.m. Saturday, Oct. 15, aspart of the Centennial Celebration Symposia for the Society of EconomicGeologists in the Salt Palace Convention Center, room 250 AB.

Virginia Tech faculty members and students are presenting more than 30 papers at the GSA national meeting.

Learn more about Bodnar's research at http://www.geol.vt.edu/research/fluidslab/.