Dr. Brent Miller Research
Jan 31, 2017
If the Earth had an operating manual that was carved in stone, then the page numbers of that manual would be found in the mineral zircon. Dr. Brent Miller, the department’s geochronologist, uses his research of zircon dating to “see those page numbers.”
Dr. Miller found interest in the nature of time as a child reading H.G. Wells novels. Zircon is one of Earth’s longest-running and most accurate minerals used for dating. Zircon incorporates uranium at the time it crystallizes, which over time decays to lead. By using a very sensitive measuring device called a thermal ionization mass spectrometer, the amounts of uranium and lead in zircon can be determined. It is then a simple calculation to determine how much time is required for the measured amount of lead to have decayed from the measured uranium.
Dr. Miller has most recently applied these methods to determining the ages of volcanic eruptive deposits in the Eagle Ford shale in west Texas. Zircon is used to very precisely determine the ages of volcanic ash beds interbedded with the sediments. Those ages help to constrain the timing of microfossil occurrences in the surrounding sediment which could otherwise not be dated directly. This information then provides a consistent timeline for testing ideas about past climate changes, sedimentary basin evolution, and correlations with other local, regional, or global sections.
The ultimate geological timekeeper, however, is to be found in the rhythms of the Earth, itself not in the laboratory. According to Dr. Miller, “We are currently limited to about 0.1 percent precision in the best of cases by dating zircon”. For Late Cretaceous ages that means each zircon age has an uncertainty of about plus-or-minus one hundred thousand years, and extrapolating the age into the surrounding sediments only increases the uncertainty. More importantly, the zircon ages can be used to calibrate the subtle signals preserved in the sediments caused by environmental changes tied to known periodicities in Earth’s orbital parameters.
Knowing the absolute ages of a few key points in a stratigraphic section, Miller uses that information to ‘tune’ the sediments in between the ash beds to orbital, or Milankovitch, periodicities.
Armed with this information Dr. Miller and his students and colleagues can estimate the age at any point in the section to about a five-fold higher precision. Mike Deluca, a recently graduated Masters student co-supervised by Dr. Miller and Dr. Mike Pope did exactly this type of work for his thesis entitled, “Ash bed analysis of the Cretaceous Eagle Ford shale using ID-TIMS U/Pb methods: Implications for biostratigraphic refinement and correlations within the Western Interior Seaway.”
But it’s not just volcanic deposits that can be dated by zircons. For the past seven years Dr. Miller has been involved in teaching field camp’s igneous and metamorphic rock mapping in Southwestern Montana where some of the oldest rocks in the world can be found. Each year several undergraduate students who have mapped these rocks in field camp also bring samples back to College Station as part of a “Topping off the Capstone” independent research project. Now a Masters graduate student in the department, Brandon Geddie recently completed an undergraduate honors thesis on Montana rocks entitled, “Constraints on the timing of deformational fabric development: ca. 1780 Ma Big Sky Orogeny, southwestern Montana”. Prior undergraduate researchers have used zircons to demonstrate that rocks in the field camp map area are as old as 3.27 billion years.
“The very sensitive instruments in the Williams Lab can be used for much more than just analyzing rocks”, claims Dr. Miller.
He has recently begun partnering with colleagues in Nuclear Engineering and the Cyclotron Institute to analyze uranium ore materials and reactor fuels for projects involving nuclear nonproliferation and safeguards. The elemental and isotopic composition of these materials can serve as a “fingerprint” of their sources, methods of refinement, and previous uses. This collaboration has led to the acquisition of a new robust mass spectrometer designed for rapid and accurate analysis of both nuclear and geological materials. Yet another mass spectrometer along with a laser ablation sampling instrument, recently acquired through a National Science
Foundation Major Research Instrumentation grant, will elevate the R. Ken Williams ’45 Radiogenic Isotope laboratory to among the best equipped labs in the world.
