My First Garnet Age

It's been way too long since I've written a post.  I've saved up a nice long list of topics that I can write about and adventures from the last month and half but just haven't gotten around to writing the posts yet.  I'm hoping that this post will be the first of several over the next couple of days to get more or less caught up and whittle down my idea list.  Instead of going chronologically, I'm going to start with a science update...since that's the cause of my delay in writing anyway. :-)

There has been a lot of exciting science in the past month and a half since I wrote about my first trip to WHOI to use the SEM.  I'm balancing two classes (Geodynamics as well as "Thermodynamics and Kinetics of Tectonometamorphic Processes," which is taught by my adviser) and finally getting pretty close to being independent on most of my research procedures after learning several new steps these past couple of weeks.

The biggest push for research has come over the past two weeks.  This past summer when I did fieldwork in Scotland (see earlier posts if you missed the beautiful Scotland scenery!) I collected several samples and shipped them back here.  In November I sent two of those samples off for crushing and mineral separation and they came back roughly three weeks ago.  As I was in a meeting with my adviser just over two weeks ago, we were talking about research priorities and he mentioned that he would love to have an age for garnet from one of the samples ahead of his upcoming talk at the University of St. Andrews, Scotland on his European speaking tour but he wasn't sure if it was possible.  Normally taking garnet from raw mineral to an age takes us three to four weeks, with two weeks being the absolute fastest speed possible if everything goes according to plan.  I agreed to try and plotted out every step on a calendar, knowing that I would make it with just about 12 hours to spare if I succeeded.  It was also complicated by the fact that I have never completed some of the final steps, because I just hadn't gotten there yet with test samples.

So, basically the last two weeks were two solid weeks of lab work.  It included a second trip to Woods Hole to do SEM work on the Scotland samples (and incidentally learning how to change an SEM filament...), about a week and a half of 12+ hour workdays in the clean lab in my superhero get-up, two days of learning to load samples and how to load the mass spec., and two days of learning how to run our instrument.  (Eventually I'm hoping to write a series of posts to show you some of the basic methods and our instrument, the TIMS.)  Amazingly, my sample survived and I can finally say I have my first garnet age!!  Not only was this the first time I generated an age by doing the procedures from beginning to end, I made it within two weeks (with 11.5 hours to spare), it was a ridiculously small sample (I started with only 4mg of garnet...4 sand grains basically), many of the procedures I was doing independently for the first time or I was learning with supervision for the first time, and the result was right in the expected ballpark!  If you can't tell, I have been quite pleased for the last day or two.

Now that I have my first age under my belt, I can start testing and implementing some of my ideas for changes to the procedure to improve how our lab works with very tiny samples and for further method development on the new garnet dating system I am developing.  While I'm excited about my first data point, there are many improvements to be made and many more points to be gathered.  Onward in the pursuit of science.

Grains under the Microscope: A WHOI Adventure

About a week and a half ago I had the opportunity to visit Woods Hole Oceanographic Institute (WHOI) on the southern part of Cape Cod in Massachusetts.  I took a day trip away from my usual research at BU in order to gather some test chemical data on a couple of my detrital garnet samples.  I received an invitation from a research scientist at WHOI offering me the opportunity to try out a piece of equipment we do not have in our department at BU.

Before I describe what my day at WHOI looked like, let me give you some general background on the goal of the trip and the method that I was using for those of you who are unfamiliar with the SEM.  I traveled to WHOI to use a tabletop scanning electron microscope (SEM).  A scanning electron microscope uses a beam of electrons that bombard the sample.  When the electrons collide with the material being analyzed, different types of detectors can be used to provide information about the electrons that are scattered or energy that is emitted to learn more about the sample being analyzed.  The instrument I was using is smaller than most standard SEMs and is designed to be user-friendly, to take up less space, and to allow for analysis not just of polished, coated samples (which is the standard procedure) but also of rough, uncoated samples using a lower vacuum setting.  This makes this instrument an excellent candidate for analysis of my samples, since the grains do not have to be coated or polished, which would destroy them.  The tabletop SEM is equipped with two different detectors- a BSE detector and an EDS detector.  BSE stands for backscattered electrons and measures the electrons from the beam that are scattered and reflected by the object being analyzed.  It allows for the differentiation between heavy elements and light elements, since heavy elements scatter electrons more strongly.  From this differentiation, an image of the material being analyzed is generated using grey scale that can show surface detail or different mineral phases.  Below is an example from one of the grains that I analyzed at WHOI, though this particular grain is zircon rather than my usual garnet subject.

BSE image showing a zircon grain, analyzed at WHOI 1/20/2012

Besides BSE, the SEM can also do EDS analysis, which stands for energy-dispersive X-ray spectroscopy.  The electron beam hitting the sample can trigger the release of x-rays and the energy of the x-rays released is characteristic of the major elements that compose the object being analyzed.  This makes EDS useful for preliminary chemical characterization and mineral identification.  EDS results in a spectrum showing major element peaks and can also generate atomic and weight percentages for major elements using spot analysis.  Basically, we can get a general idea of the composition of the material being analyzed.

While I was at WHOI, I analyzed twelve different detrital garnet grains.  I used grains from two different locations in Vermont that we have been using as test samples throughout the method development process.  Three of the grains were from one location and the other nine were from a second location.  The sand collected at the second location contains detrital garnet of different colors, so I was able to analyze three grains of each color (red, orange, and pink) hoping to test the ability of the instrument to differentiate between garnet with different compositions.  When doing detrital work, it is important to be able to identify populations of similar grains that likely originated from the same general area and should be roughly the same age, since a single detrital sediment can contain different populations of garnet grains that originated from different locations and were affected by different conditions.  In addition to the garnet analyses, I also analyzed many grains of any other mineral that could remotely resemble garnet under the microscope from one of my other field areas to verify that garnet is not present in the sample.  Here is a picture showing how the grains were mounted for analysis.  The grains are still attached to this mount- they are just too small to see easily in the picture and blend into the carbon sticky tape.  The grain mount is sitting in a small weigh boat to minimize contamination.

Grain mount for SEM using carbon sticky tape

Just for fun, let me give you a look at what my day of research looked like.  My commute to WHOI started around 6:30am and involved two different subway lines, a Peter Pan bus, getting picked up at the bus station and a short drive to WHOI's Quissett campus.  I arrived around 10:00am and started learning how to run the instrument and by 11:00am I was running the instrument independently.  I then spent the next five to six hours (minus a lunch break) analyzing as many grains as possible and accumulating roughly 90 computer files between BSE images, EDS analyses, etc.  Then I took a shuttle bus to Woods Hole, where I ate dinner in an awesome little cafe while waiting for my bus to Boston.  (If you ever find yourself in Woods Hole, MA..I highly recommend eating at Pie in the Sky. It was delicious!)  After a bus ride and two more subways, I arrived home around 10:30pm.  It was a long and tiring day...but definitely worth it.

Here is one final picture, showing me running the SEM at WHOI.  Not only did I learn a new instrument and gather helpful information on some of my samples, I am learning how to be an independent scientist!

Me running the tapletop SEM at WHOI