a very good day
Nov. 3rd, 2008 08:26 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png)
I managed to wake up early this morning and head in to uni straight away. Had a good talk with my advisor, who suggested another technique I might try. Then met with S, who has been helping me with the zircon analysis. Apparently my zircons are as complex as he's had to deal with in ages. One does zircon dating by comparing the ratios of isotopes of uranium and of lead. Some zircons are easy--the only lead present was formed in the process of radioactive decay from the uranium which has been in the zircon since it crystallized. In these, ideal, crystals, the ratios of the different isotopes are a function of time, and were one to date a bunch of them with different ages, the ages would all plot upon a nice, smooth, curve. However, it is possible to have several different complications. One complication is a second episode of zircon growth some millions of years after the initial crystallization of the mineral. In this case the ages you get don't plot on the curve at all, but instead plot along a straight line, which intersects the curve at two points--one of those points is the age of the original zircon growth, and the other is the age of the new zircon growth. If there are only two ages of zircons and no further complications then the resultant graph is straightforward to read/understand. Another possible complication is the presence of "common lead", which is to say lead that did not form as a result of radioactive decay from uranium. In pure zircon this is rarely a problem, because the crystal structure of zircon doesn't really fit lead into it very well, so as it grows it tends to automatically become lead-free. However, it is possible to have "inclusions" of other minerals within a crystal. If the zircon contains inclusions of a "common" lead-bearing mineral then the ages you'd get from it will be shifted off of the curve heading in a direction whcih will never intersect another portion of the curve. Yet another possible complication is that of "lead loss". If the zircon happens to be quite high in the isotope of uranium which decays by fission the process leaves "tracks" in the zircon--places where the crystal structure has been destroyed and filled in by some glassy stuff that had been zircon before being bombarded by the tiny bit of radiation that caused the damage. In a crystal with lots of these "fission tracks" it is possible for the lead created by the decay of one of the other uranium isotopes to leach out of the crystal via those tracks. When this happens the ages plot off of the curve in a different direction, but one which, again, won't intersect the curve again. And, last but not least, those fission tracks are also a place wherin lead can get into the crystals, which also causes problems with the "apparent ages".
As it turns out, the zircons from one of my samples, appears to have not two, but three distinct ages. There appears to be a quite old core which is "detrital" (crystals which grew in some other rock, survived the process of erosion, got deposited into some mud somewhere, and are still here now that that mud has been metamorphosed into a rock), with a metamorphic over-growth from ~ 1200 million years ago, and another metamorphic overgrowth from ~510 million years ago. This complication, in conjunction with an occasional example of each of the other complications I listed, meant that it took S three full days to convert the raw data into usable format and make graphs that show what is going on, and I am exceedingly grateful to him! After he explained everything to me (in much more detail than lies behind that cut--for instance he specifies which isotopes are involved!) he commented that if we do some monazite dating on this sample to corroborate our results that this could easily become a publishable paper. I pointed out that I'd love to do the monazite dating, but that my funding is pretty much out. At which point he gave me an account number from one of his accounts which still has enough in it to do some monazite dating. I guess this means that he is serious about wanting to do this paper with me! So I've started the process in motion to make it happen. My advisor points out that my thesis will be fine without this data, so take my time in making the arrangements and this can be a project for once the thesis is done. If it turns out that I land that teaching job, it will be very good to arrive with such a project to work on, so that I can be publishing not long after I start work, and before I've found local projects with which to involve undergrads. However, if I wind up getting a post-doc somewhere working on something specific, it could be harder to make time for this project. Nonetheless, I am actually looking forward to it, long term though it will be.
Writing up my draft of the results as they stand now gave me another 400 words for the thesis. Then I tried the suggestion my advisor made this morning. Alas, it doesn't seem to help. He's already concerned about the low temperature estimates I've been getting--this change drops them even lower. So I've sent him an e-mail asking if it worth doing any more with this thought, or just abandoning it.
In other news, I made it to my favourite food store today. Eumarah is one of those grocery stores which carries bulk grains, nuts, dried fruits, etc. I had an hour to spare between meetings, and that was enough time to drive into the city centre, find a place to park, buy to very full bags worth of bulk food, return to uni, and even pick up my samples which were still at the microprobe lab. While it took well under an hour to go shopping, mixing up my muesli and packaging all of the other grains, nuts, seeds, dried fruit, and spices into various jars and other containers took just over an hour! However, it will be another month or two before I need to do this again--I've now got enough muesli to last awhile. Yum!
It is now only 21:00, but I think I'll put the computer down for the night, go for a walk, do some yoga, and see if I can get to sleep early.
As it turns out, the zircons from one of my samples, appears to have not two, but three distinct ages. There appears to be a quite old core which is "detrital" (crystals which grew in some other rock, survived the process of erosion, got deposited into some mud somewhere, and are still here now that that mud has been metamorphosed into a rock), with a metamorphic over-growth from ~ 1200 million years ago, and another metamorphic overgrowth from ~510 million years ago. This complication, in conjunction with an occasional example of each of the other complications I listed, meant that it took S three full days to convert the raw data into usable format and make graphs that show what is going on, and I am exceedingly grateful to him! After he explained everything to me (in much more detail than lies behind that cut--for instance he specifies which isotopes are involved!) he commented that if we do some monazite dating on this sample to corroborate our results that this could easily become a publishable paper. I pointed out that I'd love to do the monazite dating, but that my funding is pretty much out. At which point he gave me an account number from one of his accounts which still has enough in it to do some monazite dating. I guess this means that he is serious about wanting to do this paper with me! So I've started the process in motion to make it happen. My advisor points out that my thesis will be fine without this data, so take my time in making the arrangements and this can be a project for once the thesis is done. If it turns out that I land that teaching job, it will be very good to arrive with such a project to work on, so that I can be publishing not long after I start work, and before I've found local projects with which to involve undergrads. However, if I wind up getting a post-doc somewhere working on something specific, it could be harder to make time for this project. Nonetheless, I am actually looking forward to it, long term though it will be.
Writing up my draft of the results as they stand now gave me another 400 words for the thesis. Then I tried the suggestion my advisor made this morning. Alas, it doesn't seem to help. He's already concerned about the low temperature estimates I've been getting--this change drops them even lower. So I've sent him an e-mail asking if it worth doing any more with this thought, or just abandoning it.
In other news, I made it to my favourite food store today. Eumarah is one of those grocery stores which carries bulk grains, nuts, dried fruits, etc. I had an hour to spare between meetings, and that was enough time to drive into the city centre, find a place to park, buy to very full bags worth of bulk food, return to uni, and even pick up my samples which were still at the microprobe lab. While it took well under an hour to go shopping, mixing up my muesli and packaging all of the other grains, nuts, seeds, dried fruit, and spices into various jars and other containers took just over an hour! However, it will be another month or two before I need to do this again--I've now got enough muesli to last awhile. Yum!
It is now only 21:00, but I think I'll put the computer down for the night, go for a walk, do some yoga, and see if I can get to sleep early.