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Princeton Geosciences Department opens world class geochronology laboratory

PhD student Brenhin Keller using the class 10 clean hood space to do rock and mineral dissolution for geochemistry and isotopic analysis.

Ph.D. student Brenhin Keller using the class 10 clean hood space to do rock and mineral dissolution for geochemistry and isotopic analysis.

 

A world class radiometric geochronology laboratory has now opened in Guyot Hall, with facilities equipped to date Earth’s oldest rocks. Assistant Professor Blair Schoene proposed the new laboratory when he joined the faculty in June 2009, and has since overseen its design and preparation.

The new space culminates a series of other smaller renovations throughout Guyot that Schoene has coordinated over the past several years to build a self-sustained geochronology program at Princeton. Sample processing that was previously contracted out to other institutions can now be completed from start to finish within Guyot’s walls, ultimately saving time and resources. Having the facilities onsite also allows Schoene’s group to pioneer advancements in fundamental techniques of geochronology.

The heart of the new laboratory is an instrument called a Thermal Ionization Mass Spectrometer (TIMS), which measures the ratio of parent-daughter isotopes in rocks or single minerals. Schoene chose the TIMS over other dating techniques because it is unarguably the most precise, arriving at precisions on dates better than 0.1% (±10,000 years in a mineral that is 10 millions years old). Schoene developed a passion for this type of work during his PhD research at MIT.  “TIMS work is hard. There are only a handful of labs doing it well,” he explained, “so there are plenty of opportunities to make advancements.”

The TIMS will primarily be used to measure the ratio of uranium-lead (U-Pb) parent-daughter isotopes within individual minerals as small as ~30 µm in diameter.  Such minerals commonly have <1 pg of Pb (that’s 10-12 grams), requiring intensely clean mineral preparation facilities with ultrapure air and reagents.  A sample age can be calculated by subtracting the amount of contamination and using the ratio of radiogenic Pb isotopes to U in combination with the well-established U-Pb decay constants.

Other isotope systems and minerals are useful in answering certain questions about Earth history, but U-Pb is best suited for Schoene’s work concerning deep-time and Earth’s oldest rocks. This is because radioactive U isotopes decay very slowly and zircon is extremely robust. Even when submitted to extreme changes in heat and pressure in the Earth’s crust, zircon crystals can remain a closed system — in essence becoming an ideal time capsule recording Earth processes.

High on Schoene’s list for upcoming TIMS analysis include zircons from rocks dating hundreds of millions of years back to the onset of animal life, and others dating even further back to the early oxygenation of the atmosphere. U-Pb geochronology is also ideal for calibrating the rates of magmatic and metamorphic processes in Earth’s crust, which control the timescales of mountain building and the resulting composition and structure of continents.  The demand for higher and higher precision dates makes U-Pb TIMS geochronology an increasingly important tool for understanding the Earth.

Defining these distant timescales is necessary in quantifying rates of biologic and geologic change through time. Ultimately, this helps us better understand the evolution of life, the cause of mass extinctions, and why Earth looks the way it does today.

“From a philosophical level,” Schoene explained, “understanding geologic time in general is one of the biggest contributions of Earth Science to humanity. It allows us to understand where we come from, and how our home has evolved.”

The chance to make such advancements in these cutting edge facilities has attracted a talented and committed group of graduate students and post docs to Schoene’s group. "I think we're at a point where a lot of geoscientists are realizing the importance of time constraints,” said Brenhin Keller, a second year PhD student who has been involved in the lab’s final preparations. “U/Pb TIMS is the most precise way to provide such constraints for almost all of geologic time, which makes setting up a new TIMS lab a really exciting opportunity."

 

PhD students Jon Husson (at computer) and Brenhin Keller (in labcoat) running the new IsotopX PhoeniX62 Thermal Ionization Mass Spectrometer.

Ph.D. students Jon Husson (at computer) and Brenhin Keller (in labcoat) running the new IsotopX PhoeniX62 Thermal Ionization Mass Spectrometer.

 

PhD student Jon Husson does ion exchange chemistry to separate Uranium and Lead from within dissolved zircon crystals recovered from volcanic ash beds.

Ph.D. student Jon Husson does ion exchange chemistry to separate Uranium and Lead from within dissolved zircon crystals recovered from volcanic ash beds.