Grants to Fund New $1.1 Million Electron Microprobe in Geosciences

Calcium compositional map of a chemical reaction preserved in a rock from the Athasbasca Terrain, northern Saskatchewan, Canada. The reaction occurred between two minerals, feldspar, upper left, and pyroxene, lower right.
Calcium compositional map of a chemical reaction preserved in a rock from the Athasbasca Terrain, northern Saskatchewan, Canada. The reaction occurred between two minerals, feldspar, upper left, and pyroxene, lower right.

The geosciences department, internationally known as host to one of the top electron microprobe laboratories in the world, will add a new, state-of-the-art $1.1 million microprobe to its toolkit later this year, say professors Michael Williams and Michael Jercinovic, who direct the electron microprobe/scanning electron microscope facility in Morrill Science Center.

They are now in the process of purchasing it after having won a coveted Major Research Instrumentation grant from the National Science Foundation, plus a $300,000 grant from the campus.

The new instrument will replace an aging electron probe that was the workhorse micro-analytical tool at UMass for the past 27 years, they say. The electron probe micro-analyzer will provide non-destructive, in-situ analysis of mineral phases on the scale of a micrometer or less using a focused electron beam that generates characteristic X-rays from a target, usually a rock sample. By analyzing the wavelength and intensity of these X-rays, researchers can determine the specific elements present in a sample.

Williams says the electron microprobe is one of the essential research instruments of the geosciences today, with applications in petrology, geochemistry and climate science, as well as in biology, chemistry, physics materials and engineering.

Jercinovic teaches a course in electron microprobe analysis, and a large number of post-doctoral fellows, graduate students and undergraduates have been trained to use the instruments.

As Jercinovic explains, a rock is not just a lump of the Earth. Rocks in natural outcrops are often layered, folded or fractured and have complex histories. Rocks are composed of minerals with concentrations of elements and isotopes that reveal the story of the specific environment where each was formed. They evolve as they cool, heat or deform; their composition varies and some minerals grow while others break down.

“It can be a very complicated story, in some cases there might be multiple episodes of high pressure, heating and cooling that we need to detect. It’s like working a crime scene to decipher what happened and in what order,” Jercinovic says. “But once you do, it allows you to understand how deep the rocks were in the crust, what state they started out in, and how they changed over time, all of which can tell you a great deal about larger scale processes like mountain building or formation of mineral deposits. Looking back, I think we have been able to piece together some pretty remarkable things while gaining insight into Earth history in general.”

Williams says, “Geologists have gotten better and better at measuring the age of rocks. The specialty of the UMass lab is determining the age of geologic processes, such as when faults form and when folding took place.”

Geologists from around the world bring or send their specimens to the lab for compositional mapping. It is a dating technique to analyze and graphically display the composition and variations of minerals and elements inside ancient and modern Earth rocks, meteorites and other materials, says Williams. “We generate compositional maps of each specimen, rather than just the spot analysis that most labs do,” he points out. The technique, which they call “reaction dating,” has aided hundreds of scientists and their students who study crustal processes. Williams says, “We are recognized as one of the very top such labs in the world.”

An example of its use can be found in garnet crystals. Williams says, “We know that garnet soaks up manganese, essentially sucking it from the surrounding rocks as it grows. By carefully mapping and measuring the manganese content, we can trace how the garnet crystal grew. Using compositional maps, we can see the core or seed where the garnet crystal started growing and, using data from the probe, we can tell the temperature, the pressure and possibly the age of the garnet crystal. Garnet maps are spectacular to look at, and each one records a chapter in the history of a rock.”

Jercinovic adds, “This mapping informs us about how reactions took place in the rock and relates them to the structural features in the rock.” This can tell scientists where and when the material developed, how deep or how hot it was and how it has changed through time. “This information can help researchers to solve geologic problems from all over the world,” he notes. “Our probe has played a fundamental role in a number of geological discoveries and breakthroughs, especially concerning the tectonic history of North America.”

The new electron microprobe will be able to generate compositional maps much faster and with higher resolution than the older instrument it will replace. However, the researchers say, the biggest advantage of the new instrument will come in the second, analysis, stage, where the composition of minerals, or even small parts of minerals, can be accurately measured without destroying or changing the rock in any way. It is this accurate analysis that can be used to illuminate the depth in the earth, temperature or history of the rock sample.