In an attempt to do something productive and work on my science communication skills (I need all the practice I can get...qualifying/comprehensive exams are looming!) I have decided that one of the projects I'm finally going to tackle during my lab work lull is to write about the main instrument that I use in my research. Plus, I often learn best by teaching! I've been meaning to do this for a while and if you've seen references in my posts to "the instrument" or "the TIMS" and wondered what on Earth that was..this is the series for you! Hopefully, today's post will provide necessary background for those of you with little or no exposure to isotope geochemistry. Next post I'll show you how the samples get loaded onto the machine and eventually we'll talk about how the machine actually goes from the samples to a useful result and what running the TIMS is really like.
The primary instrument we use in my research lab is a "thermal ionization mass spectrometer," which we refer to as "the TIMS" for short. [Which, incidentally, has lead to several comical instances where people overheard me telling someone that I "date garnet" or "spend a lot of time with the TIMS" and then inquired about the romance in my life after assuming I was talking about a boy! lol] Breaking down the full name of the instrument actually gives a pretty solid overview of the instrument's purpose. The TIMS uses high temperatures to ionize (break down into individual charged atoms) the sample, separates the ions by mass using an electromagnet, and allows us to measure the relative abundance of ions at specific masses to determine isotope ratios. If that just went over your head, don't despair yet! I promise upcoming posts will have pictures and likely use household objects as analogies to break this overview down into manageable parts.
First...a little general background on geochronology and why I use this crazy instrument. As an 'isotope geochemist,' my goal is to figure out the age of each of my garnet grains and then use those ages to examine bigger questions about how mountains form and the surface of the Earth changes. Figuring out the age of the garnet is possible because garnet includes trace amounts of a radioactive element called samarium (Sm) that decays to the element neodymium (Nd) at a known rate. It takes 1.06x10^11 (106 billion) years for enough Sm to decay so that only half of the original amount is left (called the half-life). Also, garnet is basically never 100% pure...it usually contains little bits of other minerals that we refer to as inclusions. To borrow an example from my adviser...you can think of it as a chocolate chip cookie. The cookie is the garnet and the chocolate chips are the bits of other minerals, or inclusions, inside the garnet. Many of these inclusions contain a different ratio of Sm to Nd than the pure garnet (just like the chocolate chips and the cookie base would have different ratios of sugar to milk). Therefore, I use the TIMS to measure the Sm and Nd ratios in the pure garnet and in the inclusions. Since the ratios are different and the rate of decay is known, when I plot the ratios for the pure garnet and the inclusions I can define a line between two points, called an isochron.
The slope of the isochron = The age of the garnet!
By the time I am ready to walk into the TIMS lab and start an analysis, I have already been working through sample preparation and clean lab chemistry on a single garnet grain for several weeks. Those week of work result in a set of 4 beakers, each containing a single bead of dried goop about the size of a ballpoint pen's tip. The four beakers contain the Nd from the pure garnet, the Sm from the pure garnet, the Nd from the inclusions, and the Sm from the inclusions. Each isochron that results in an accurate age requires all four samples to run successfully on the TIMS, usually with only one shot at getting each one right.
This is a very simplistic overview of geochronology and mass spectrometry, especially since there are entire courses on the principles of these areas of study. Basically, once you understand that by measuring isotope ratios of Sm and Nd we can figure out the age of garnet, you understand the purpose of using a TIMS and one of my basic research goals. The remainder of my research examines ways that we can apply these methods to new materials that have never been dated or how the ages that I get tell us about the rates of metamorphism, when sedimentary basins open and close, how metamorphic conditions change over time, and much more.