Professor emeritus M.E. (Pat) Bickford of the Department of Earth Sciences in The College of Arts and Sciences at SU has determined that rocks in southwestern Minnesota are very ancient and have long and complex histories -some as old as 3.5 billion years. Bickford is the lead author of an upcoming article in the January-February issue of the Geological Society of America Bulletin, published by the Geological Society of America.
Although it has been known since the 1970s that rocks exposed in the Minnesota River Valley are among the oldest in North America, only recently have modern techniques of uranium-lead dating revealed how truly ancient and complex they are. Using the sensitive high-resolution ion microprobe (SHRIMP) at Stanford University, Bickford and his colleagues Joe Wooden (U.S. Geological Survey) and Bob Bauer (University of Missouri) have shown that the oldest components of these complex rocks are 3.5 billion years old and that episodes of emplacement of new molten material occurred at 3.42 billion, 3.38 billion, 3.14 billion and 2.6 billion years ago.
The SHRIMP instrument allows measurement of uranium-lead ratios, enabling the research team to determine the age of portions of complexly zoned zircon crystals less than 0.020 mm in diameter.
By using the SHRIMP instrument, Bickford and his research team are developing a clearer picture of the very early history of the formation of Earth's continental crust. "Study of the most ancient parts of the Earth's crust is important for understanding of the mechanisms by which the continents formed and grew," says Bickford. "And all geologists are fascinated with very ancient things!"
Interest in such ancient parts of Earth's crust has intensified since the discovery of rocks more than 4 billion years old in northern Canada and Australia. Newly discovered ancient rocks, 3.5 to 3.8 billion years old, occur in northern Manitoba, in the Assean Lake area, and in the Porpoise Cove area of northern Quebec. If these and the Minnesota River Valley rocks are the disrupted parts of an ancient continent, intervening younger rocks of the Superior Province of Canada, Minnesota and Michigan-only 2.7 to 3.2 billion years old-may have formed as the ancient continent drifted apart.
