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kareinaThe easy part of the calculations I've been doing lately is just that--the calculations themselves; format the numbers correctly, save as a txt file, and push a button. Poof, an answer.
However, when doing any sort of calculation, it is important to start with the correct numbers. I've been attempting to determine the temperature and pressure at which the minerals in my samples grew based upon the composition of the minerals present. Specifically the rims of the minerals which are zoned (their composition changes from core to rim) and any any random analysis from those which are homogeneous.
However, "homogeneous" minerals when analysed on a microprobe do tend to show slight variation in their composition, and sometimes that variation is enough to give somewhat different P/T results if you try the calculations using each different version. The zoned minerals can also be problematic--sometimes the rims are nice and uniform in composition (as they should be, if the minerals achieved equilibrium), and sometimes they aren't.
Today's sample had two different garnets analysed, one line across each from edge to edge. One shows a distinct compositional change from core to rim (nice, normal u and n shaped graphs), the other looks more like it is two smaller garnets that happened to grow together, resulting in W or M shaped zoning graphs. In addition, all four analyses from the grain edges are quite different from one another, and deciding which one is a better representative of the actual "ideal" rim composition for the garnets in this rock is a difficult task. Therefore I decided to plug all four versions into the calculations, to see just how different the results might be. The answer is very different. Sigh. Having spent the whole day looking at the graphs of the composition & comparing them to the photos of the minerals, and then playing mix-and-match trying the calculations with many different combinations of analyses from this sample, I'm no nearer to having a single pressure/temperature estimate for this sample than when I began. Why do I care? Because this is the only sample from that particular region that was analysed, so if I don't come up with a number which I can feel reasonably confidant about, then I have nothing for that region at all.
So I just took an hour and read some fic, and now I'm going to go do my yoga and get some sleep. Perhaps my subconscious will come up with some logic I haven't considered for determining which one is best. Or, if all else fails, I can always speak to my advisor...
However, when doing any sort of calculation, it is important to start with the correct numbers. I've been attempting to determine the temperature and pressure at which the minerals in my samples grew based upon the composition of the minerals present. Specifically the rims of the minerals which are zoned (their composition changes from core to rim) and any any random analysis from those which are homogeneous.
However, "homogeneous" minerals when analysed on a microprobe do tend to show slight variation in their composition, and sometimes that variation is enough to give somewhat different P/T results if you try the calculations using each different version. The zoned minerals can also be problematic--sometimes the rims are nice and uniform in composition (as they should be, if the minerals achieved equilibrium), and sometimes they aren't.
Today's sample had two different garnets analysed, one line across each from edge to edge. One shows a distinct compositional change from core to rim (nice, normal u and n shaped graphs), the other looks more like it is two smaller garnets that happened to grow together, resulting in W or M shaped zoning graphs. In addition, all four analyses from the grain edges are quite different from one another, and deciding which one is a better representative of the actual "ideal" rim composition for the garnets in this rock is a difficult task. Therefore I decided to plug all four versions into the calculations, to see just how different the results might be. The answer is very different. Sigh. Having spent the whole day looking at the graphs of the composition & comparing them to the photos of the minerals, and then playing mix-and-match trying the calculations with many different combinations of analyses from this sample, I'm no nearer to having a single pressure/temperature estimate for this sample than when I began. Why do I care? Because this is the only sample from that particular region that was analysed, so if I don't come up with a number which I can feel reasonably confidant about, then I have nothing for that region at all.
So I just took an hour and read some fic, and now I'm going to go do my yoga and get some sleep. Perhaps my subconscious will come up with some logic I haven't considered for determining which one is best. Or, if all else fails, I can always speak to my advisor...
