Wednesday, 31 March 2010

Quality is better than quantity

The section of the paper I read today is just under 1000 words, but the next section of the paper is a long one—if I read on it will be over 2000 words of reading for the day. While that is not necessarily a bad thing in and of itself, I do have other things I need to do with my day, so I will choose to cut today’s total # of words a bit short, knowing that today and tomorrow will combine to meet the goal of reading “1000 words a day”. It is the sprit of the law which is more important than the letter in this case, and I feel that the “active reading” I’ve been doing, summarizing the contents of each paragraph as I read it, and before going on to the next paragraph is more than good enough in terms of quality of reading comprehension and retention to make it reasonable to break at a section break, rather than at an exact word count.
So, without further adieu, here follows my summary of the paper I’m reading. See the past few days for the earlier sections, and stay tuned tomorrow for the next section.
The next section covers Thermometry and Barometry. They present thermometry first.
Paragraph 44 lists the three techniques they used to calculate metamorphic temperatures. They specify that unzoned garnet and matrix biotite compositions were used for one set of calculations, that garnet core + green matrix hornblende and garnet rim + blue-green hornblende was used for the other, and they don’t specify here what minerals were used for the Thermocalc compositions.
Paragraph 45 reports temperatures of 700-800◦ C for the grt-bt schists of Ruby Mt. and 675-730◦ C for the grt-hbl in the amphibolites. They say that the amphibolites above and below the contact between the Skagit Gneiss and the Naeequea give results within error of one another.
Paragraph 46 reports for Elijah ridge 650-700◦ C for both ky-st & and-crd schist and 570-670◦ C for the amphibolite. They specify that Elijah Ridge gives lower temps than Ruby Mt., but point out that errors are ~50◦ C so it is hard to say if difference is real. They also point out that the andalusite is a late phase, so the T pre-dates that mineral’s growth.
That concludes the thermometry section, the next section is barometry
Paragraph 47 lists the three barometers used, and specifies that garnet core and matrix plag gave max P. They also state that they usually assumed that kyanite is the stable Al2SiO5 polymorph, and named the one exception to that.
Paragraph 48 states that most samples from this study yield pressure estimates of 8-10 kbar, which agrees with previous studies in the area. They state that while Elijah Ridge gave slightly lower results, they are still within error of the others.
Paragraph 49 addresses garnet zoning, pointing out that only along the eastern margin near the RLFZ can one find garnet that has discontinuous zoning in the grossular component (see pretty maps of fig 8)—they state that this pattern could relate to GASP reaction of coexisting grt and plg, in which case the increase in Ca towards garnet rims equals an increase in pressure as they grew. They go on to specify that structurally deeper samples have homogeneous garnet or only a thin retrograde rim zoning. They further state that the above mentioned zoned(Ca) garnet eastern margin samples saw lower T than the ones with homogenous garnets, and say that the zoned ones might be a better record of the prograde history and they also seem to have a more complex later history given magmatism and faulting associated with RLFZ. (This paragraph was the hardest to summarize yet, being packed full of lots of information and not flowing from one thought to the next as well as it might.)
This ends the section on Barometry. The combined section on Thermometry and Barometry includes a figure showing all of the above P/T results—the individual error squares overlap from one to the next, giving a range that is visually continuous, despite the fact that the lowest and highest individual samples don’t overlap. The next section is on P-T paths.
Paragraph 50 mentions three lines of evidence for different stages of the PT path: 1) evidence for pre-andalusite conditions for the andalusite-cordierite schist with 2) garnet rims showing increase in P for those rocks. 3) the fact that kyanite used to be present in all samples, so max P had to be in kyanite zone. They go on to state that the max P/T reported in last section was followed by near isothermal decompression to less than 5 kbars while T was still high.
Paragraph 51 lists textures which point to metamorphic reactions continuing during decompression. Two of these textures are observed in this study, and they mention that their observations are in agreement with a previous study, and list other lines of evidence from that study.
Paragraph 52 states that the P-T results from the eastern side of Elijah ridge are for conditions which preceded a low-P metamorphic overprint. It also equates the P with depths of ~25-33 km, which is equivalent to results from farther to the west in Skagit Gneiss.
This ends the P-T paths section. Their proposed path focuses on the max P/T conditions followed by decompression while T was still high. The next section is on microstructural analysis.

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