UW Scientists Discover Accurate Method to Date Oceanic Crust |  
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Oct. 28, 2005 -- Scientists from the University of Wyoming, the United States Geological Survey and Woods Hole Oceanographic Institution reveal in the Oct. 28 issue of "Science" the most accurate method ever discovered to determine the age of oceanic crust.
Michael J. Cheadle, UW associate professor of geology and geophysics and third author on the research paper, says the work is the first of its kind. Now that the UW team has unlocked the door to the 60 percent of Earth's surface covered by water, Cheadle predicts the geological community will be quick to emulate the study and expand on the findings.
"This research is significant in that it helps advance our understanding of how oceanic crust is formed and the processes involved," he says. "People have tried previously, but they haven't had the resolution; they couldn't date it as precisely as we can now."
The key to the scientists' success was the confirmation of the presence of tiny crystals, called zircons, in oceanic crust. U-Pb dating of zircon is widely regarded as the best technique for providing the absolute age of rocks on land, according to Barbara E. John, the paper's second author and professor of geology and geophysics. The zircon dating technique has been used extensively to answer fundamental questions such as when and how fast the Earth's continental crust forms. Until now, scientists have relied on geophysical methods based on magnetism to date oceanic crust.
"As the Earth's tectonic plates spread apart through time, new crust is created at mid-ocean ridges," John explains. "Magnetic minerals in the rocks that make up the crust are magnetized in the direction of the Earth's magnetic field as they cool and freeze. Because the field flips through time from normal to reversed polarity, the rocks record the polarity, creating alternating stripes on either side of a mid-ocean ridge.
"To learn the age of the ocean's crust, traditionally, instruments aboard ships have predicted the age of the ocean's crust below by mapping the magnetic stripes and then calculating an age using distance and time between polarity reversals within the crust. But this method cannot reveal all the complexity involved in the growth of ocean crust."
Cheadle says one of the reasons zircon dating hasn't been conducted previously is because some scientists believed that rocks in ocean crust don't contain zircon.
"People didn't realize the same types of minerals used to accurately date continental crust also exist in ocean crust," he says. "Of course, the other reason is that it is difficult and expensive to retrieve these rocks that are buried under water at mid-ocean ridges."
With the aid of a $250,000 grant from the National Science Foundation, the group of scientists collected the 17 zircon-bearing samples of lower oceanic crust discussed, using three different techniques. Cheadle and John, along with Elena A. Miranda, from Plano, Texas, fourth author and UW Ph.D. candidate in geology, had the opportunity to dive to the bottom of the sea in a scientific-manned submarine to collect selected samples from the sea floor.
"It's a very tight fit inside a 1.8 meter diameter sphere with two pilots," John says, adding this is the ideal way to collect samples.
Samples also were collected by a remotely-operated submersible, similar to the one that found the Titanic, and by a process called dredging, in which rocks are gathered by dragging a bucket across a section of the ocean floor.
After collecting their samples, the scientists used a Sensitive High Resolution Ion Micro Probe (SHRImP) to determine the absolute ages of 17 samples from Atlantis Bank (about 75 miles south of the Southwest Indian Ridge in the southern Indian Ocean). Using the U-Pb zircon dating method, they found they could determine the absolute age of oceanic crust with an error of less than 1 percent of the age. The dating was conducted at the U.S. Geological Society-Stanford Micro Analysis Center.
Furthermore, the scientists discovered that 25 percent of the samples they dated are up to 2.5 million years older than predicted by conventional models of crust generation at mid-ocean ridges.
Joshua J. Schwartz of Belfast, Maine, the paper's first author and a UW Ph.D. candidate in geology, says the team's research offers another tool to understand the complex processes occurring beneath the Earth's surface.
"Our finding that some of the zircons are older than they should be relative to the magnetic age changes what we've previously thought about oceanic crust. Our ability to date the zircons in ocean crust offers another and better way to determine how ocean crust is formed," he says.
Cheadle agrees, adding that one of the key premises of geology is "the idea that the present is the key to the past."
"Our findings about today's ocean ridge help us to better understand how the Earth has worked in the past," he says.
Photo: Tiny crystals called zircons, used to date oceanic crust, are relatively common in rocks known as gabbros.
Credit: Michael John Cheadle, University of Wyoming
Posted on Friday, October 28, 2005
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