Monday, December 29, 2008
For Kim, a photo of the Tasmanian whiteschist. It is located along the Collingwood River, just up from its intersection with Scarlet Creek. It is the resistant stuff surrounding the water (and cobble) filled hollow.
Some terms used in last post (feel free to skip down to the ones which interest you, some of the explanations are wordy!):
BSE photo. A back-scatter-electron image (in this case taken on a scanning electron microprobe). In this sort of photo the relative levels of brightness or darkness communicates information as to the composition of the minerals in the photo. The brightest areas correspond to areas with heavy elements, and the darkest areas correspond to areas containing light elements (and the in-between shades, oddly enough, to areas which are in between). Therefore monazite, which contains uranium (U), thorium (Th), and lead (Pb) (amongst other stuff), which are all well along on the periodic table (and so heavy) comes out a nice, bright white, quartz, which is (mostly) just silicon (Si) and oxygen (O) (both nice and early in the periodic table, and so light) is such a dark grey it is bordering on black, and the garnet, which contains iron (Fe), magnesium (Mg) (and other stuff) in addition to Si and aluminum (Al) averages out to somewhere in between, and is the medium grey shade. I can see at least two other shades of grey in that photo, and while I didn’t happen to analyze those minerals, I could guess that they are apatite and a titanium oxide. This guess is made because those are common in other samples from this area and look about like that in the BSE images from the samples wherein I did analyze them. However, when doing BSE images the only safe time to compare the shades of grey to determine which mineral is which is when both images are set to the same level of contrast. It is common to search for monazite with the contrast set such that almost everything on the screen is black, save the monazite and zircon. Therefore just because I remember those minerals being that shade of grey doesn’t guarantee that I’ve correctly identified them.
High levels of Y: Although not a primary ingredient in monazite or garnet, the element yttrium (Y) occurs in both minerals to a limited extent. Both of these minerals like Y better than do the other minerals in a metapelite, and so whatever Y is available in a rock is likely to be found in one or both of them. However, garnet is (usually) much larger than monazite; therefore it is able to take up more Y by virtue of having more room for it. As a result many workers consider changes in the amount of Y in monazite to be an indicator of what was happening with the garnet in that sample. When both garnet and monazite are growing at the same time the garnet hogs the Y, leaving the monazite to be low in Y. When garnet is breaking down whilst monazite is growing the Y that had been stored in the garnet becomes available for the monazite, and it winds up with higher levels of Y (there are, of course, several other possible scenarios).
Inclusion: when one mineral grows fast enough to surround (an)other mineral(s) the surrounded grains are said to be “inclusions”. When this happens we know that the included mineral must have already existed at the time it was surrounded, and therefore we have learned something about the order in which the minerals crystallized. (note: when one mineral is included within another it could be because mineral A grew first and quit growing, then mineral B started growing and eventually surrounded it, or it could be because they were growing at the same time but mineral B grew so much faster that it managed to surround A. Therefore, while we know that the one existed before it got surrounded, we don’t necessarily know if it was old or new when it happened).
Metapelites: metamorphic rocks which were comprised of mud before they were metamorphosed. They tend to be high in Si and Al and common minerals in metapelites include quartz, feldspar, mica (biotite and/or muscovite or other micas), garnet, kyanite, and, as an accessory mineral, monazite.
Monazite generation: Monazite is able to grow under a variety of conditions, and from a variety of metamorphic reactions. In areas which have seen multiple episodes of deformation it is not uncommon for there to be more than one generation of monazite. Sometimes a single monazite grain contains zones from different generations, and each zone will give a different calculated age, and often, have a noticeably different chemical composition from the other zone(s).
Monazite: Monazite is a rare-earth phosphate with the general formula of (Ce, La, Th)PO4. It is very common in metamorphic rocks, particularly metapelites (used to be mud). It is often used for chemical U-Th-Pb dating wherein the concentrations of those three elements are measured and calculations done to work out how much lead has been created by the radioactive decay of the U and Th, and therefore how much time has elapsed (since the half-life of U and Th are known, it is possible to figure out how long it would have taken to make that much lead from those minerals, assuming no starting lead was present in the mineral). Monazite tends to be an “accessory” mineral, which means that it is rarely more than 1% of the total rock, and is often quite tiny. So long as it is at least 10 microns in diameter, it is large enough to do the analysis needed for dating (remember there are 1000 microns in every millimeter). Some of my monazites are more than 100 microns in length, and some are even bigger. The large ones can be seen without using a hand-lens or microscope, especially because if they happen to be surrounded by biotite (a brown mineral which is also common in metamorphic rocks) there is usually a darker discoloured “halo” around the monazite due to damage to the biotite crystal lattice as the radioactive elements in the monazite decay.
Whiteschist: a fairly unusual metamorphic rock which contains talc and garnet and other minerals. It is very high in magnesium compared to other sorts of metamorphic rocks (which is part of why it contains talc) and its garnet tends to be higher in Mg than is normal for garnets in metapelites. It is thought to form under rather high pressure.
So there you have it, a brief glossary to help you understand my last post. Let me know if I missed anything I should have defined, or if you want a reading list of sources for any of the above information (I typed it up off the top of my head, but I can find the sources I’ve sited in my in-progress thesis if anyone wants to see them).
Those of you who have been paying attention may not need them pointed out, but the main reasons I’ve asked for comments from others are 1) it is not common to see so much monazite all together in one location like that, so thoughts of what could have been there before the monazite grew to cause the concentration are appreciated 2) the pattern of high Y/low Y monazite grains established in the rest of the sample is quite different than occurs in this cluster, so thoughts of why it is different are appreciated. It is quite likely that the answers to 1) and 2) are related!
Sunday, December 28, 2008
This BSE photo shows (part of) a large cluster of monazite grains included within a single garnet in a whiteschist. This sample has only one generation of monazite (metapelitic samples from the same region show two generations of monazite, the younger of which is the same age as the ones in this sample). The other garnets in this sample have few (or none at all) monazite inclusions. The monazites included within the other garnets of this sample show different levels of Y based upon where in the garnet they are located. The ones in the garnet cores (there is a circle of quartz inclusions at the core-rim boundary for these garnets) are low in Y (0.07 to 0.4 wt%), while the ones in the rims and in the matrix are higher in Y (0.99 to 2.7 wt%). However, this cluster, which is located within the core of its garnet, as defined by the circle of quartz inclusions (see below ppl photo of the garnet which hosts the cluster of mnz), has the same high levels of Y as seen in the monazite included within the rims of the other garnets.
I've had some thoughts about this cluster and its implications for this sample, but I would welcome hearing what others have to say about it.
Tuesday, December 23, 2008
When I did my Master’s all of my maps were done by first scanning in a topographic map, then tracing it into CorelDraw so that I could change the size without distorting it, and then adding in my sample locations by hand by looking at my field map and making a corresponding mark onto the map on the screen. These days I use ArcMap for my sample collection maps—what a joy it is to take the GPS coordinates recorded in the field, type them up, import the list into ArcMap, and with the push of a couple of buttons they show up on the map, in exactly the location I expected them to be. Add a layer from the published maps showing the geology of the region, and it is good to go! (Or, if needed, create my own layer showing my interpretation of the distribution of rock types—slightly more effort, but ever so much easier than drawing it by hand on paper, or in CorelDraw.) And if I wish to know how far apart two samples were collected I can use the handy measuring tool within the program to look it up in a few seconds.
But today’s high-tech advantages come with a price. ArcMap, in particular is not freely available. My university has a very specific, limited number of users at once, license for that program. This means that even though it is installed upon my personal computer, it is necessary for me to connect via the internet to the geology department’s computer (I, like many other graduate students these days, work from home) so that the program can access its license and check to be certain that not too many of us are trying to use it simultaneously. This was all well and fine at my old house, where we had a broad-band, reliable, internet connection. I tended to make that connection in the morning, and leave it on all day (it also being necessary to download my university e-mail or to access other things on the geology departments shared hard drive), and unless something happened to disconnect me whilst I was using it, I tended to forget that ArcMap actually required the connection.
Alas, with our recent budget-saving move to the home of my partner’s parents we’ve entered the world of unreliable, low-speed, satellite internet connection. It took nearly a week of being here before we even managed to get the router communicating with the modem to permit more than one computer in the house access to the internet at once, and now that we’ve got that working, the connection is still not reliable enough to keep my remote connection to the geology department engaged sufficiently for ArcMap to open. I didn’t realize just how often I use that program, until I couldn’t! We are currently attempting to obtain a reliable, high-speed, internet connection for the house. But we are located fully 25 minutes drive away from the capitol city of this state, in a country neighborhood where each house has six or more acres of fields, vineyards, and/or horse paddocks to call its own, and the phone company is proving less than helpful in providing the connection to such a “distant” location. Perhaps in the New Year I’ll get the reliable connection I need. In the mean time, I think I’ll go pick some raspberries from the garden and rejoice that not everything in my life requires technology.
Sunday, December 14, 2008
There comes a time in the life of most graduate students when one’s funding is about to expire, but one’s writing is not yet complete. When faced with this situation we students take a variety of options. Some choose to seek additional funding, some seek employment, and others of us seek options which negate the problems entailed by lack of funding. Each of these has their advantages and their pitfalls. For those of us enrolled in an Australian University the end of funding deadline appears much sooner than we might like in that the official time limit is three years from start to finish of a PhD program (note: Australian PhD students do not enroll in courses—rather it is a research based degree program). Those of us who can demonstrate complications or interruptions to our progress which were outside of our control can often obtain an additional six months of funding, but the university is pretty clear that it doesn’t wish to extend funding beyond that point.
Some of my cohorts choose to obtain employment at that point, with the intention of completing the write-up of their results in their “spare” time in the evenings and on the weekends. Some of them manage this goal, but it seems to extend the process by many more months than they would have liked. I am one of the fortunate few to have another alternative. When my Master’s funding ran out years ago, I moved in with my mother & step father and enjoyed free room and board in exchange for some basic housework and managed to complete my thesis in only one more semester’s worth of time. Now that my PhD scholarship has ended my own mother’s home is no longer an option, as she lives on another continent. However, my husband’s parents are local and have a spare room and have made us welcome in their home.
The process of packing up and moving all of our belongings, some for storage until I complete my degree, find employment in academia, and move to our new location, and some to be kept out and available in our room in their home and the final cleaning of our rental house, has cost me fully 12 days wherein I could have been working on completing my degree. However, it is my hope that by thus avoiding a major time commitment, such as a full time job, I shall be able to complete my writing and submit my thesis (note: in Australia both Master’s and PhD students write a “thesis”) within the next month or two.