New Electron Microprobe Can Determine the Ages of Rocks
The new method offers greater efficiency, and access to a much more detailed geologic record than current dating methods, the scientists say. The successes of the early phases of the research have led to funding for development of a new electron microprobe that will significantly enhance the potential of the technique.
The new microprobe, to be housed in the University's department of geosciences, will have applications throughout Earth and planetary science, as well as in fields as diverse as microelectronics, forensic science, fiber optics, and even microbiology, according to the researchers.
Reading the record of materials the Earth provides, and particularly the record of plate tectonics, is critical, explains UMass geologist Michael Williams, because the phenomenon governs much of our understanding of issues such as the development of Earth's crust, the evolution of climate throughout Earth's history, and how human beings and different species of animals spread across the planet.
"If we can read the past, we have a better chance to understand the future," he said. Williams and his colleague Michael Jercinovic are co-investigators on the project.
Currently, scientists using conventional methods can determine the age of a rock sample within several million years, explains Williams. But, he suggests, there are limitations when using the traditional methods of obtaining isotopic ratios (radiometric dating) to determine ages.
First, most current methods are labor-intensive, requiring tens of hours to examine a single sample. Second, different parts of a rock may have different ages because rocks typically record a progression of events which may involve melting, deformation, or reheating within the Earth's crust.
Thus, dating a whole rock leaves scientists with an average age, rather than pinpointing the ages of specific events in a rock's history. And third, the most common and precise dating methods require the rocks to be ground into a fine powder.
There are some samples that scientists are understandably reluctant to destroy such as moon rocks, or rocks containing fossils. The electron microprobe has the advantage of being able to chemically analyze very small areas (1/1000 of a millimeter) in a rock without having to crush the rock and separate out the constituent minerals and, furthermore, does not damage the sample during analysis.
This allows scientists to relate mineral chemistry to other aspects of the rock (sequences of mineral growth, folds, fractures, etc.) in developing a complete picture of the evolution of a rock sample, he says.
Being able to date specific parts of rocks, and pinpoint their constituent elements, offers a new perspective to scientists as they consider the movement of the Earth's tectonic plates, a crucial issue in Earth science.
"This will better enable us to read records of how the continents (actually tectonic plates) have grown and collided with one another," Williams explained.
"Some rocks that were once at the Earth's surface were later buried, their minerals were changed by the heat and pressure (called 'metamorphism'), and then they were returned to the Earth's surface. "We can get a better sense of how deep within the Earth the rock traveled, how hot it was, and importantly, the time at which it was at a particular location, as we try to rebuild its history."
Williams and Jercinovic recently received a $450,000 grant from the National Science Foundation (NSF) as part of a $1.2-million project to develop and construct a new specialized electron microprobe for geologic dating.
Funding for the collaborative project has also been provided by Cameca Inc., a developer of analytical instrumentation, and the University. Electron microprobes have been workhorses in scientific labs for some time, Williams noted, but until recently they could not have been used for precise geologic dating.
The new instrument will be optimized for trace-element analysis and geologic dating, and should allow a significant improvement in the precision of the technique.
Uncertainties in rock ages produced using the current technique are on the order of 5-10 million years, but the UMass researchers hope to chip that number down to 1-2 million years using the new microprobe.
Development of the new microprobe has already begun at Cameca's laboratory in France. It is expected to arrive on the UMass campus within the year.
The new techniques, dubbed "high-resolution age mapping and microprobe dating," involve the analysis of monazite, a mineral that is present in many rocks but typically in such small quantities that it is rarely noticed in geologic studies.
It is widely used in radiometric dating because it contains significant amounts of the elements thorium and uranium, which decay to the element lead at a known rate.
"Monazite essentially provides us with a 'stopwatch,' for timing geologic events in different areas of rocks," Williams said.
One of the project's major breakthroughs is the recognition that even single crystals of monazite, as small as several hundredths of a millimeter, can grow in increments over time, adding new material when mineral-forming events occur in the Earth's crust.
Using the electron microprobe, the age of each layer can be determined and interpreted in terms of the sample's history.
According to Williams, "looking at the entire lifespan of a sample, rather than just tagging it with an average age, is critical."
"It's the difference between having a single snapshot, or a lengthy, detailed videotape of a series of events. We don't want to know just where and when a rock was formed, but also when and where it was buried, deformed, heated, melted, and eventually exhumed to the Earth's surface. "We want to determine the life history of each rock and then combine these histories to understand the geologic history of a region of the Earth," said Williams.
Williams and Jercinovic have already used the basic technique with the existing electron microprobe at UMass to investigate topics including the growth of the North American continent, its interaction with other continents to make supercontinents, the eruption of volcanoes, and the history of the Appalachian Mountains.
University of Massachusetts-Amherst
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