cutting rocks!
Feb. 29th, 2008 01:59 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
kareinaSo, in theory, I am meant to have completed all of my analytical work for my PhD project by the end of 2007 so that this year I've only to finish crunching numbers and working out what they all mean and writing up the thesis. However,
In talking to my advisor last week we decided that it would be helpful to do a bit more monazite dating. (Monazite is a mineral which is rich in the elements uranium, thorium and lead, and as such it is possible to work out how old it is by using a microprobe to determine the abundance of each these elements and then performing some calculations to work out how much time has elapsed between the crystallization of the mineral and the formation of that much lead due to the radioactive decay of the others.)
Setting up the microprobe to do these particular analyses is a complex process, so they won't do it unless there are enough people wanting to do it to keep the probe busy for a full week. Therefore I contacted the probe operator expressing interest, and while speaking with him suggested that we could do the MLA Maps well in advance of the actual dating. The MLA maps are a wonderful, time saving tool--one puts the sample into the MLA and tells it to find all of the grains of monazite (which shows up as a very, very bright spot on the screen due to the radioactive (and therefore heavy) elements it contains). The machine then takes a photo of all of them, and also works out what mineral(s) surround each grain. (Monazite tends to be very small, rarely larger than 100 microns (there are 1000 microns in a milimeter!)). It then creates a report which I can go through and decide which grains I wish to probe--the best ones, for my purpose, are those included within the garnet crystals, as they had to exist before the garnet grew around them, and therefore we also get an estimate of the age of the garnet. Making the decisions of which ones to analyze with the microprobe takes about an hour using the MLA maps. Before we started using this technique it took me 12 hours of time on the microprobe (an expensive instrument to use!) to find all of the monazites by hand, and then decide which ones to probe.
Therefore, when Karsten suggested that I could come in and do the MLA maps on Wednesday of next week, I agreed to the appointment. But then I looked at the samples I've got in a form that is ready to go into the MLA and the microprobe from the area we wish to date this time, and realized that only one of them has the correct chemical composition for it to be possible for it to contain monazite. sigh. This means that I need to create more thin sections of other samples. I had one other from the ones I collected this summer that would be suitable, but it is a good idea to have four ready to do, in case one of them doesn't contain monazite (the microprobe can take three samples at a time, and monazite being so small, it is really hard (if not impossible) to tell if it is present with an ordinary microscope). So I returned to the rock-storage basement and found some samples collected from the same area back in 1964, and in that collection found two more with the appropriate minerals, giving me a total of three that need to become polished thin sections. By next Wednesday.
Of course, the lapidary room in the Earth Science Department, which spent most of the summer with not many samples to process, is currently got a bit of a backlog, and they can't make my thin sections for me! So today has been a day of running up and down stairs. Down to the basement & use the big rock-saw to cut small rectangles off of the three rocks, then place them in the automatic grinding machine till they have one very polished surface (~10 minutes), clean them and leave them on the hot plate to dry. Run back up to my office and do a half an hour's work. Back down stairs, discover that they aren't quite flat & polished enough (it is easier to tell when they are dry!), so set them back in the machine for more polishing & head back up stairs. Then clean them and leave them to dry again. Then back downstairs to glue them to glass slides & leave the glue to dry.
When that finishes, I'm to go slice most of the rock off of the glass, and put it in a different machine which will polish/grind it till the part of the rock still glued to the glass is starting to be translucent. Then I'll need to take the weekend off, because by then the person who can show me how to use the final machine, which grinds it down to the 3 microns thickness it needs to be will be gone for the day.
Oddly enough, even though it is making it hard to make progress on my other tasks with all of these breaks to run downstairs and do the next step in the rock-cutting room, I am very much enjoying the process. Perhaps this is a sign that I've not left myself enough time for art projects recently, if cutting & polishing rocks is so much fun!
In talking to my advisor last week we decided that it would be helpful to do a bit more monazite dating. (Monazite is a mineral which is rich in the elements uranium, thorium and lead, and as such it is possible to work out how old it is by using a microprobe to determine the abundance of each these elements and then performing some calculations to work out how much time has elapsed between the crystallization of the mineral and the formation of that much lead due to the radioactive decay of the others.)
Setting up the microprobe to do these particular analyses is a complex process, so they won't do it unless there are enough people wanting to do it to keep the probe busy for a full week. Therefore I contacted the probe operator expressing interest, and while speaking with him suggested that we could do the MLA Maps well in advance of the actual dating. The MLA maps are a wonderful, time saving tool--one puts the sample into the MLA and tells it to find all of the grains of monazite (which shows up as a very, very bright spot on the screen due to the radioactive (and therefore heavy) elements it contains). The machine then takes a photo of all of them, and also works out what mineral(s) surround each grain. (Monazite tends to be very small, rarely larger than 100 microns (there are 1000 microns in a milimeter!)). It then creates a report which I can go through and decide which grains I wish to probe--the best ones, for my purpose, are those included within the garnet crystals, as they had to exist before the garnet grew around them, and therefore we also get an estimate of the age of the garnet. Making the decisions of which ones to analyze with the microprobe takes about an hour using the MLA maps. Before we started using this technique it took me 12 hours of time on the microprobe (an expensive instrument to use!) to find all of the monazites by hand, and then decide which ones to probe.
Therefore, when Karsten suggested that I could come in and do the MLA maps on Wednesday of next week, I agreed to the appointment. But then I looked at the samples I've got in a form that is ready to go into the MLA and the microprobe from the area we wish to date this time, and realized that only one of them has the correct chemical composition for it to be possible for it to contain monazite. sigh. This means that I need to create more thin sections of other samples. I had one other from the ones I collected this summer that would be suitable, but it is a good idea to have four ready to do, in case one of them doesn't contain monazite (the microprobe can take three samples at a time, and monazite being so small, it is really hard (if not impossible) to tell if it is present with an ordinary microscope). So I returned to the rock-storage basement and found some samples collected from the same area back in 1964, and in that collection found two more with the appropriate minerals, giving me a total of three that need to become polished thin sections. By next Wednesday.
Of course, the lapidary room in the Earth Science Department, which spent most of the summer with not many samples to process, is currently got a bit of a backlog, and they can't make my thin sections for me! So today has been a day of running up and down stairs. Down to the basement & use the big rock-saw to cut small rectangles off of the three rocks, then place them in the automatic grinding machine till they have one very polished surface (~10 minutes), clean them and leave them on the hot plate to dry. Run back up to my office and do a half an hour's work. Back down stairs, discover that they aren't quite flat & polished enough (it is easier to tell when they are dry!), so set them back in the machine for more polishing & head back up stairs. Then clean them and leave them to dry again. Then back downstairs to glue them to glass slides & leave the glue to dry.
When that finishes, I'm to go slice most of the rock off of the glass, and put it in a different machine which will polish/grind it till the part of the rock still glued to the glass is starting to be translucent. Then I'll need to take the weekend off, because by then the person who can show me how to use the final machine, which grinds it down to the 3 microns thickness it needs to be will be gone for the day.
Oddly enough, even though it is making it hard to make progress on my other tasks with all of these breaks to run downstairs and do the next step in the rock-cutting room, I am very much enjoying the process. Perhaps this is a sign that I've not left myself enough time for art projects recently, if cutting & polishing rocks is so much fun!
