Monday, December 29, 2008

A glossary for my last post

In my last post I shared photos and asked for comments from those with sufficient background knowledge to understand the post without a translation. However, that isn’t terribly fair, as even if I assume that only my fellow geobloggers will ever see my posts, still we are a diverse lot, with backgrounds from all areas in the earth sciences, and metamorphic petrology isn’t always required for students wishing to specialize in one of the other sub disciplines. Therefore I’ll type up some definitions of the terms used in the last post, and an explanation of what prompted me to ask for comments.

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.

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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!

2 comments:

ScientistMother said...

I'm a bit behind on blog reading, but I guess the fact that I have no idea what these definition mean, I probably won't understand the previous post.

A Life Long Scholar said...

lol! Well, the previous post *was* aimed at any readers who have sufficent background in metamorphic petrology *and* determining the timing of metamorphism via the composition of the mineral monazite, so nope, if you don't understand the definitions, you wouldn't have the background to answer the questions I asked. But if you'd like to understand the definitions, let me know which parts (if any) you follow, and what needs more work on my part to explain, and I'll try again...