What does research about the early Earth, the tectonics of the Alps and the collapse of ancient mountains have in common? Understanding of all these important Earth processes can benefit from an advanced mineral dating technique conducted in a new Penn State facility for the first time.
Using equipment in a new laboratory funded by Penn State, geoscientists are able to measure the age of rocks using a process that analyzes the atomic proportions of uranium and lead in the mineral zircon.
“This is exciting because this technique really supports a significant part of our research,” said Andrew Smye, assistant professor of geosciences. “A majority of what geoscientists know about how old the Earth is – the geological time frame, the evolution of Earth and its history – really is pinned to the absolute timing of geological events. And so having the capability to determine the age of key samples in-house is a real step forward.”
The new geochronology lab uses a laser ablation unit to drill into individual mineral grains much smaller in thickness than a human hair. Those samples are then fed into two mass spectrometers, which measure the isotopic composition of the minerals found in the rock. Zircon is typically used because it can accurately and reliably be dated using the uranium-lead decay scheme.
To determine the approximate age of a rock, samples are crushed and then mounted onto epoxy pucks. The samples are then blasted with lasers and heated to temperatures as hot as the surface of the sun to break them down into their constituent atoms. The chemical makeup and age are then determined using the mass spectrometers in the lab.
The laboratory will specialize in measuring the age and composition of individual mineral grains using a depth-profiling technique whereby measurements are made layer-by-layer on sub-micron length scales. It’s similar, Smye said, to understanding the growth history of a tree by studying its rings.
The lab recently started to process unknown samples after working on reproducing standard measurements and benchmark tests for calibration purposes since January. The results so far, they said, have been very promising.
For Smye, the lab is a chance to better understand the geological history deep within the Earth’s crust, greater than 10 miles below the surface. Within these layers are clues to how the planet formed and evolved.
“The information we can generate from the lab is really key to understanding the history of rocks from the deep crust, analogous to the way in which black box flight recorders can tell the history of a plane’s flight pattern,” Smye said. “We can use this lab to examine these minerals and probe key processes for how the Earth’s crust evolves.”
Garber shows the path atoms take through the mass spectrometer (a Thermo Element XR inductively-coupled plasma mass spectrometer, or ICPMS). The large gray piece is a large magnet that separates different elements based on their different masses.
Joshua Garber, a postdoctoral scholar and lab manager, plans to use the lab to study subduction zones – where oceanic plates move below continental plates and is absorbed into the mantle – and particularly the Samail Ophiolite of Oman, an obduction zone where the oceanic plate instead veered towards the Earth’s surface.
When the lab gets a sample, Garber said, they’ll examine it to see the best course of action to take. They can take a sliver, put it under a microscope, and then move on to the crushing phase. The rocks are crushed and the minerals are sorted – just like panning for gold at a much smaller grain size – and the desired minerals are captured. They’re then mounted in epoxy, blasted with a laser, and moved to the mass spectrometer, which identifies the isotopic properties of the sample. The sample is then compared with blast standards to determine its age and composition.
“When we determine a number in millions of years for how old something is, let’s say it was 300 million years old, it’s going to be plus or minus 3 to 6 million years, or 1 to 2 percent,” Garber said. “It’s that precise.”
Jesse Reimink, an assistant professor of geosciences at Penn State who has used zircon dating to advance his early Earth research, said the lab is also a great a chance to bring collaborations into the fold. In terms of equipment and expertise, the lab is a unique setting for interdisciplinary research as well as an educational tool for students.
Because it’s such a common technique in geosciences research, he envisions Penn State being a hub for external collaborators.
“This will benefit diverse research interests because it’s such a widely used technique in all of geosciences,” Reimink said. “Lots of people will want to use this technique, including others who are studying very different processes.”
It’s also a great way for students to learn because the process is nearly instantaneous. Students can be in a field one day, preparing their samples the next and watch before their eyes as the results are determined.
“It’s such a great teaching tool because it’s so visual and immediate,” Reimink said. “It’s a great resource to show students of all levels.”
To determine the approximate age of a rock, samples are crushed and then mounted onto epoxy pucks. The samples are then blasted with lasers and heated to temperatures as hot as the surface of the sun to break them down into their constituent atoms. The chemical makeup and age are then determined using the mass spectrometers in the lab.IMAGE: David Kubarek