Geosciences student uncovers micr When rocks move along a fault plane, they often grind along each other as solids in what’s known as “brittle deformation.” When they’re moving against each other deep beneath the Earth’s surface, however, the enormous pressure and heat causes the rocks to squish together instead. It’s called a “shear zone.” But along the Willard Thrust Fault in Utah, there’s evidence of rock shearing even though the rocks weren’t deep enough to be in a shear zone. Chad Martin wants to know why, and he thinks the answer lies in water and microscopic grains of quartz. “What we’re thinking is that microfractures formed in these grains. There was normal brittle deformation. There was enough pressure for some these grains to start cracking and fracturing,” he said. “That allowed water to get into these grains and become part of their crystal structure.” When minute amounts of water – on the scale of parts per billion – enters
those cracks, it can cause hydrolytic weakening. Instead of cracking, the grains of quarts deform, acting like PlayDoh as they move. Martin is working toward his Master’s degree in geosciences at UWM. As part of his research, he’s spent the last year collecting quartz samples from the Willard Thrust Fault and examining them for evidence of microfractures. If he can find them, and find signs of water, he’ll be well on his way to proving his theory. There are three steps Martin follows to look for fractures. First, he grinds the rock down until it’s 30 microns thick so he can look at it under a petrographic microscope to identify strain features which show how the quartz grains have deformed. The second step is to look at those grains under a scanning electron microscope with a cathodoluminescence attachment. “It allows you to see the microfractures that you couldn’t see any other way,” Martin says as he points to an image of a quartz grain on his computer. It’s crisscrossed by tiny black lines, which are microfractures that opened in the rock but subsequently “healed” and disappeared from normal view.
Geosciences graduate student Chad Martin collected rocks in Utah to Chad Martin.
The last step is a doozy: Martin has to travel to California to use a Fourier-transform infrared (FTIR) beam microscope at Berkeley National Lab’s Advanced Light Source (ALS). Berkeley’s FTIR microscope is extremely powerful because it uses light from a synchrotron.
Geosciences graduate student Chad Martin used three different microscopes to search for evidence of water inside tiny pieces of quartz. 1. Chad Martin bends over the FTIR microscope at Berkeley National Lab the third image is a microfracture. 4. Martin used the FITR microscope to look for OH particles along the microfracture. The arrow points to a thin blue line that indicates where the OH particles apear - right alo
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