Tuesday, May 19, 2009

The tool I learned to use today, and what I discovered when I applied it

My life recently has reduced itself to not much more than working on the thesis in progress, or taking the occasional walk for exercise; I’ve even chosen to miss out on social events I would have otherwise attended in my push to “finish”. However, as I work I keep discovering new things I need to learn in the process. I once heard that the learning curve on a PhD project is exponential, and, truly, I believe it—it seems to me that I am learning ever so much more every day now in this very late stage of the project than I did in the early months of working on it (and, for the record, during those early months, I was astounded by just how very much I was learning compared to my prior rates of information acquisition).

One of this week’s new lessons was how to apply a t-test in an Excel spreadsheet. When my advisor gave me back chapter five, with comments and suggestions for improvement, one of the things he said was that the variation of some of the abundances for some of the trace elements in the monazite for the grains which grew in the Cambrian looked on the graph like they might, perhaps, vary by age of the grains. The “vary by age” we are talking here is very minor—these crystals grew ~505 Million years ago, give or take a handful of million years. How long did that growth period take? If it was fast it might have been just a couple million years, but if it was slow it could have taken tens of millions of years. In the latter case, it would be possible for there to be changes in the growing conditions (temperature/pressure) over the time period. If there were changes in the temperature and pressure during the metamorphic episode it is possible that during that period different minerals would be stable at different times during the event. It is known that how much of any given trace element is incorporated into monazite is based, in part, on what other minerals are growing or breaking down at the same time the monazite is growing. Since monazite tends to be a small mineral (generally <> 1 mm, and sometimes > 1 cm)) tend to out-compete it for any elements they are fond of if they are growing at the same time, or if the large mineral is breaking down, they tend to release trace elements, which the monazite can then incorporate into its own crystal structure.

Therefore, to test this, part of my day was spent going back to the results of my monazite analyses, sorting them by age to get all of the ones from the ~505 Million year old growing episode together, then sorting them back out into their individual samples again, then sorting each sample by first one, and then another trace element, then doing a t-test for each element to compare all of the analyses from a single sample which are high in that element with those which are low in that element. If the “t Stat” number reported is larger than the “t Critical two-tail” number then I can say that, yes, there is a statically valid difference in the ages of the two groups. However, for the samples from the first region I’ve tested, I can’t say that. Statically, the ages for those analyses high in these elements are indistinguishable from those which are low in those elements. Therefore, the factor which is more likely to effect how much of each of those trace elements wound up in the monazite is probably just a simple “how close was each grain of monazite to the other minerals which were either giving off, or taking up, those elements?”.

Having done all of that for this region, I next need to repeat the process for the other regions, write up the results (in less general terms!) in the thesis, and move on to the next task on the list…

Monday, May 4, 2009

There will always be just one more sample you will want to analyze, no matter how close the deadline. When this happens, just say no.

Once upon a time, before I started my PhD project, my advisor did some analysis of the mineral monazite in various samples from across Tasmania, to determine the age of the grains. When I started my project some time there after I was unable to obtain training on the use of our microprobe the first year I was here, because our microprobe operator had been hired away to Edinburgh, and it took a while to find a suitable replacement and get him the appropriate visas to start work. So, to keep me out of trouble, in addition to looking at 100+ thin sections of the various Tasmanian rocks that had been collected by previous UTAS students and faculty over the years, to determine which ones would be good candidates for detailed petrological analysis, he also put me to work writing a paper summarizing the results of the monazite dating he’d done before I arrived. For most of the samples I wrote the sample description myself, since I had the thin sections in my office. However, one of them, which had been collected in 1962 from an area which is now under a dam, wasn’t in my office, and my advisor couldn’t find it in his office. Undaunted, I searched the 1962 thesis (which didn’t have a table of contents—so I created one for it) and found the original sample description, and adapted that information for our paper.

Years elapsed. UTAS hired a new microprobe operator. I learned to use the microprobe. I engaged in my own analytical work, both my own monazite dating of other Tasmanian samples, and analysis of other minerals to do calculations as to what temperatures and pressures these minerals must have grown. Eventually, I slowed down on my data acquisition and commenced writing up my results. During this process, I noticed a familiar sample number occurring in the list of “standards” being used for each and every session of monazite dating. Yes, that missing sample from the paper has been living in the microprobe lab all this time. It gives remarkably consistent results. It has had more than 170 different analyses (some samples have had 15), and they all give more or less the same age, and have pretty much the same composition. Indeed, the graphs for its monazite composition show considerably less variation than those for the other samples.

Today it occurred to me that I should look at the sample, to see if I can relate the pattern of its composition to any other details in the sample, so I went and borrowed it from the microprobe lab. Now that I’ve seen it, I so wish I’d have seen it years ago. It is full of lovely albite porphyroblasts with inclusion trails at an angle to the foliation of the matrix minerals. There are tiny garnet grains, and biotite is present. In short, all of the minerals needed to do the pressure/temperature calculations, plus plenty of structural details visible in the sample to make for an interesting story. And here I am, with only three weeks to get my thesis finish, printed, bound, and submitted. There is just no way that I am going to be able to analyze this sample and do the calculations, since I am out of time and budget with which to do it, which is a pity, because it looks as though it would have been a lovely addition to the story.

Sunday, May 3, 2009

the final push is underway

Not too long back I received an offer on a post-doctoral position in Europe. Before receiving that offer, I thought that I had already reduced my life to pretty much nothing but working on my thesis and basic “take care of myself” tasks like food, exercise, sleep, and checking in with my mother so that she knows I’m still alive. Since receiving that offer I’ve discovered that there is nothing like a solid job offer with a specific deadline when it comes to being a motivator. I did, in fact, have other places on my daily schedule that could be converted to time for “uni work”, and progress proceeds apace. Assuming that all goes well with my achieving my deadlines for submission, and with the paperwork for visas and everything else involved in such a move, July should see me in a new country, starting a new job, but there are a number of tasks yet to be completed to achieve that goal.

One word of advice for anyone else coming down to the end of a major project with a hard deadline: any time you need to take a break, be it to sleep for the night, or just to go grab a quick bite to eat, be certain you know what specific task you will be doing when you return. I find that is ever so much easier to procrastinate if I am thinking in vague terms of “lots to do”, and that it is ever so much easier to start working if I’m thinking in terms of “one specific, reasonably easy to accomplish” task to undertake next.