Big money machines: A day of geochemical analysis at USGS Reston

I really thought that I’d be more nervous using the 27-year-old $1.5 million Microprobe. The monitors’ flickering should have been enough to induce an epileptic episode and the combined age/cost knowledge was making my stomach turn. But, there I was, programming in >100 points on my petrographic thin section for the machine to zap and measure the chemical properties of the minerals from my samples.

I was fortunate to spend a day in the USGS National Center in Reston, VA where I learned about (and used) their Field Emissions Scanning Electron Microscope (FE-SEM) and Microprobe. My research requires more than one day in the center, and more than my knowledge of the machines and their uses, but the goal of the day was more to experience the machines and meet my new colleague. Ryan McAleer is a research scientist for the USGS, and the newest co-author to my thesis. He’ll be operating the FE-SEM and Microprobe for chemical analysis, and helping with the geochronology once the Denver nuclear reactor is up and running again: fingers crossed.

The chemical analysis is the first component to my geochronological analysis. I’m using 40Ar/39Ar dating of potassium to constrain the deformation age of my rocks. When a rock is metamorphosed by heat and/or pressure, certain minerals are dissolved while others are precipitated, and still others are altered slightly. A large number of the hard rock clasts within my diamictite have “beards,” or pressure shadows where the original minerals have changed orientation. In addition to that movement, we also hypothesized prior to our Reston trip that they would be chemically different from the rest of the matrix, but couldn’t be sure.

The FE-SEM confirmed that the minerals in the beards were indeed sericite, a fine grained white mica that’s rich in potassium, and therefore compatible with 40Ar/39Ar dating. It also confirmed that the sericite minerals in the beards had higher Si:Al ratios than the sericite in the matrix, indicating that the beard sericite could have crystalized as the result of deformation. The logic follows that we could date the sericite from the beards for a deformation age, and the sericite from the matrix for comparison. If the sericite in the matrix is younger than the accepted depositional age, but older than that of the beards, we could potentially use that as an indicator of multiple deformation events.

Ryan will continue mineral chemical characterization, and I will continue strain analysis, and hopefully the reactor will be available soon.

Note: This post is for the week of June 17, 2019

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