Tuesday, April 27, 2010

When you don’t understand something, look it up, even if it takes multiple sources

I mentioned recently that I had a particularly good conversation at the last conference I attended. In follow up emails exchanged with the famous scientist in question he inquired if I’d complied a list talc-garnet occurrences and natural partitioning data. I actually had to ask him what he meant by that (the part about making a list of all of the published instances where talc-garnet occur (preferably in equilibrium) together was straightforward (answer: no, but I think that I will do so, it could be very useful), but I didn’t really understand what he meant by “natural partitioning data”. His reply further specified that he “was thinking of Fe/Mg (and possibly Mn) partitioning data between garnet and talc, in order to better constrain the HP phase relations”.

Ok, the word “partitioning” is English, and I kind of understand what he is saying, but I still wasn’t completly clear. Therefore I grabbed the dictionary of geologic terms I purchased back when I was an undergrad and looked it up. Not listed. Then I checked the index of Spear’s 1995 book. Not listed there either.

My next step was to look in an on-line dictionary of geologic terms. This one does list "partitioning method" ("A resistivity method in which a special electrode configuration is used, consisting of five electrodes, instead of the usual number of four, to provide a check on the observations"), but this is clearly not what he meant. It also lists "partition curve" ("A curve indicating, for each specific gravity (or size) fraction, the percentage that is contained in one of the products of the separation; e.g., the reject. Syn: distribution curve"), which is also not what he meant. Neither were any of the several definitions in the OED of any use.
Clearly, though the term is in general enough use amongst the metamorphic petrology community that he expected me to understand it, it isn’t yet showing up in the dictionaries.
Therefore I did a Scopus search for "Fe/Mg partitioning", and downloaded an article*, which might shed some light on the concept. Stay tuned for updates once I think I really understand what I think he meant…

*Sakai, T., Ohtani, E., Terasaki, H., Miyahara, M., Nishijima, M., Hirao, N., Ohishi, Y. and Sata, N. (2009). "Fe-Mg partitioning between post-perovskite and ferropericlase in the lowermost mantle." Physics and Chemistry of Minerals (volume in-press) 1-10.

Friday, April 23, 2010

New, improved list of mineral abbreviations

I just found out about a newly published list of mineral names. A fairly high percentage of the geologic papers I’ve read have cited the 1983 paper by Kretz, Symbols For Rock-Forming Minerals, as a quick and easy way to state what they mean by the various abbreviations they used for mineral names, rather than wasting words in the paper stating that garnet is “Grt”, etc. However, 1983 is a rather long time ago, as far as papers are concerned, and that article listed only 193 minerals. Therefore I was delighted to hear about the newly published Abbreviations For Names of Rock-Forming Minerals by Whitney & Evans 2010. Their list expands on the 1983 list, giving us 371 mineral names to choose from (still only a drop in the bucket compared to the over 5,000 which are known, but this list includes the major rock-forming minerals). Like Kretz before them, they chose a format wherein all abbreviations consist of two or three letters (or rarely four if truly necessary to distinguish it from another). Unlike Kretz they also required that none of the mineral names conflict with the abbreviations used for elements of the periodic table (therefore, while they mostly keep the forms suggested in 1983, occasionally they changed them).
This expanded list will be very handy, and I’ve already copied it into an Excel spreadsheet and adjusted the formatting such that I can see the complete list on a single page.

Tuesday, April 20, 2010

inspiration found in someone else's adventure

I try to maintain a positive approach to all that I do, but this lady is an inspiration to me. The author of that blog is a friend of a friend of mine (though we've never met), and I'm quite glad my friend pointed it out to me.

Word of the day

While reading my 1000 words of Geologic Literature today I encountered a word with which I was unfamiliar: palimpsest.
According to the on-line Oxford English Dictionary the orginal sense of the word (when used as a noun) is as "Paper, parchment, or other writing material designed to be reusable after any writing on it has been erased. Obs."—this form dates back to 1616. By 1825 the word had evolved to mean "A parchment or other writing surface on which the original text has been effaced or partially erased, and then overwritten by another; a manuscript in which later writing has been superimposed on earlier (effaced) writing." By 1845 the word had also expanded to apply to "a thing likened to such a writing surface, esp. in having been reused or altered while still retaining traces of its earlier form; a multilayered record."
It has also been used (as early as 1876) specifically to refer to "A monumental brass plate turned and re-engraved on the reverse side. Cf. A. 2. Obs.". However, the most interesting uses to my mind are the geological senses of the word: "A structure characterized by superimposed features produced at two or more distinct periods." (1914) and "Of a rock structure: partially preserving the texture that existed prior to metamorphism." (1912). There are also geographical senses of the word: "Of a landscape or landform, esp. a glaciated topography or a drainage pattern: exhibiting superimposed features produced at two or more distinct periods."(1922).
It isn’t often that I encounter a word I don’t already know, and I find I rather enjoy it. There is something nice about being able to open a good on-line dictionary, paste in the new word, and be given not only the meanings, but dates for the earliest use of the word in that context that they could find. In this case I particularly like the development of the word over time first being limited to something humans make/use and re-use and then expanding to other categories. The rock record is full of instances where information is written upon pre-existing information to a greater or lesser degree. Our challenge, as geologists, is to correctly interpret each layer of information and take care not to confuse different layers of information with one another. Particularly when the layers of information may be recorded in so many varied manners.

Thursday, April 15, 2010

Inspiring conversations

I gave a talk at the EMPG conference this afternoon. As luck would have it, my talk was scheduled just before lunch. After I spoke one gentleman* came up with an additional question. We got to talking about my current research and my PhD research, and we wound up going to lunch together and then looking at photos from my thesis. He was interested in the Tasmanian whiteschist, having read about them years ago, and he enjoyed seeing my photos of them, and the graphs of the difference between the composition of the two different appearances of the garnet in that unit. The conversation lasted for the entire 1.5 hour lunch break. It was really enjoyable, and quite inspiring. I've already downloaded a handful of papers as a result of that conversation, and I look forward to reading them.

*Yes, his name is well-known in my field, no I hadn't met him hitherto, and yes, I have read and cited his papers.

Friday, April 9, 2010

another example of Geology in fiction

I recently picked up a copy of Ender in Exile, by Orson Scott Card. Card is a writer I enjoy due to the quality of his writing, though I’m often infuriated by the philosophies espoused by some of his characters. This time the experience was, once again, mixed, with large parts of the story sucking me in and causing me to forget time was elapsing (and as a result staying up way too late at night/morning one evening), and the occasional narrow-minded pronouncement on the part of one of the characters causing me frustration that I wasn’t actually present for the conversation, and therefore am unable to explain to the speaker which points (s)he’s failing to take into consideration and why I think that their statement is in error.
However, one of the highlights for me with this book is the below quote, from
page 148 of the paperback edition (Tom Doherty Associates Publishers). To understand the quote you need to know that it is describing the observation of a place where a planet used to be, before it was completely destroyed by a “MD” field breaking apart all of its constitution atoms.

“Since the MD field broke everything into its constituent atoms, it is coalescing with remarkable quickness. Our observer ship has recently been in a position to see the dust cloud with the star directly behind it, and during the passage sufficient spectrometry and mass measurements were taken to assure us that the vast majority of the atoms have re-formed into the common, expected molecules, and that the gravity of the cloud was sufficient to hold most of the material in place. There has been some loss from escape velocity and further loss to solar gravity, solar wind, etc., but our best estimate is that the new planet will be at no less than 80 percent of the original mass, and perhaps more. At that size there will still be atmosphere, potentially breathable. There will also be molten core and mantle, ocean, and the probability of tectonic movement of thicker areas of crust-i.e. continents.

In short, while no artifacts of the former civilization can possibly be found, the planet itself will be back in a nice wad, in stellar orbit, within the next thousand years, and perhaps cool enough to explore in ten thousand years. Colonizable in a hundred thousand, if we seed it with oxygenating bacteria and other life as soon as the oceans are fully formed.”

Isn’t that a wonderful quote? Clearly the man has spent some time researching theories on the formation of planets, to have been able to write such a thing. It pleases me when part of the “Science” in a science-fiction book includes Geology!
What other fiction books have you read that do a reasonable job including geology (or in this case, I suppose, planetary science, since there aren’t any rocks, yet)?

Sunday, April 4, 2010

Rest does wonders for one’s attention span, but you already knew that

Rest does wonders for one’s attention span, but you already knew that

Having learned (once again) from yesterday’s mistake of putting off reading my 1000 words till I was sleepy, today I sat down and did it only four hours after I woke up. This is a much more sensible time of day to approach such a task. It felt much easier to do the reading and typing up of my notes, and what I read felt far more interesting than I remember yesterday’s reading, on the exact same topic, to have been. With luck I will remember this one for quite a while. However, the reality is that there are many, many things I wish to do with my time, and sometimes I choose to do one of them, and sometimes I choose to do one of the others. As a result it will eventually happen that I will choose to do other important tasks with my awake-energy and will once again have to choose between reading when I am sleepy or letting my run of consecutive days reading the 1000 words lapse. I’m at 87 days in a row, this time. Every time I manage to get that number up to these levels I become even more vested in continuing. My record thus far is 118 days in a row—that is only one more month. I would so very much like to pass that record. It would be fabulous to manage it for an entire year.

The below are my notes for the final 1000 words of Gordon et al. (2010)*. . It has been interesting, to me, to do this exercise, and to post my progress on a daily basis. I don’t expect anyone to have actually read my notes themselves, but perhaps my thoughts on the process are interesting to someone. Tomorrow’s 1000 words will be on a new paper

Paragraph 71 describes the fabrics (constrictional) on Elijah Ridge and mentions that the temperature for these rocks is slightly lower than on Ruby Mt. though they have similar pressures. It reiterates that before this study no evidence for high-P metamorphic conditions had been found east of the western end of this ridge, but this study found evidence on the eastern part of this ridge for an early high-P (8 kbars) consistent with Ruby Mt., the western part of this ridge, and with the deeper structural levels of the Skagit Gneiss. They conclude therefore that part of the Methow terrane in this region was buried to mid-crustal depths. They say that the late cordierite-andalusite overprint is likely due to contact metamorphism from the Golden Horn batholith (exposed 1< to the east) or other nearby plutons.

Paragraph 72 states that mapping failed to find the location of the RLFZ on Elijah Ridge or Ruby Mt., but that there is clearly a tectonic boundary between the oceanic Napeequa unit and the clastic meta-Methow rocks. The contact between these two units and the Skagit Gneiss is also tectonic, and in places where it is exposed to the north and the south is steeply dipping (> 70 degrees ), but here is it more gently dipping (35-45 degrees)

Paragraph 73 explains the above observations with a overlapping step-over zone in this region, with the units defining a duplex structure within the step-over zone. They indicate that the western fault zone is expressed as the shear zone on Ruby Mt. and the eastern part of the step-over is obscured by the Golden Horn Batholith and Ruby creek plutonic belt to the east. They relate the duplex with the NW end of the stepover and cite another paper which previously defined the stepover.

Paragraph 74 indicates that either the Napeequea originally (pre 65 Ma?) overlay the Skagit orthogneiss along a tectonic contact or the orthogneiss intruded into it, with some evidence for this early boundary showing in the shear zone on Ruby Mt. They provide two alternatives 1) the orthogneiss overlying the Napeequa unit on Ruby Mt. isn’t related to the Skagit Gneiss, and the deformation affected both the Napeequa and this orthogneiss after its emplacement or 2) it is related to the Skagit magmatism, intruding at the same time as the shear zone was active.

Paragraph 75 starts of with “in this model”, with nothing to indicate which of the many choices above is intended with the word “this”. It goes on to say that the step-over zone started during a transpressional regime, bringing together the Napeequa unit and some of the Methow basin rocks into a duplex with much crustal thickening, burring the lot, giving all three units a similar burial and exhumation history. The gradient of metamorphic grade in this region supports the notion that the Methow terrane was tilted, and the western end may have been structurally deep before the duplex (i.e. the strike-slip fault isn’t what buried these rocks to ~25 km, but it probably helped with some of the extra depth.

Paragraph 76 cites sources arguing for a change in the North Cascades at ~57 Ma from a transpressional to transtensional regime, which they say caused the extensional step-over and facilitated the exhumation of the mid-crustal rocks. They then cite another source which mentions imbrication along a pre-existing contact, and state that if that is the case then the Skagit gneiss had to have intruded into the Napeequa unit (and the orthgneiss above the Napeequa unit is part of the Skagit gneiss, just in a different spot because of that imbrication). The imbrication model includes the Methow basin rocks above the imbricated Skagit Gneiss and Napeequa units, which is compatible with the east dips of major contacts and foliation on Elijah Ridge.

Paragraph 77 States that both the step-over and imbrication models agree that there is a tectonic boundary on between Methow basin and Skagit Gneiss, and states that movement of the RLFZ is important to the relationship of these units. They cite sources saying that high T-deformation and activity continued till the early Tertiary.

Paragraph 78 states that since the Napeequa unit was not likely to have been in the upper crust at the time of deformation that they disagree with the hypothesis that the Skagit orthogneiss which is now directly under the Napeequa unit on Ruby Mt. was a flowing ductile crust underlying the non-flowing Napeequa unit. The then list again the evidence for both the Napeequa and Skagit rocks sharing metamorphic, deformational, and exhumation histories.

The final section break of the paper is here. The above section reviewed what was already known of the tectonometamorphic history of this area and indicated how their results add to that knowledge. The next section discusses the Exhumation of the Skagit Gneiss.

Paragraph 79 States that crustal thickening (> 30 km) and near isothermal decompression affected the Skagit Gneiss. They list the evidence for rapid cooling on exhumation around 57 to 45 Ma, when the North Cascades were in a transtensional regime and the RLFZ was dextral strike-slip combined with components of both reverse (pre 57 Ma) and normal (post 57 Mal) slip. Since the timing matches, they figure that the fault movement enhanced the exhumation.

Paragraph 80 states that the RLFZ incorporates different slices of the high grade and low-grade rocks in this region, and in some areas (specifically this field area) the basin rocks which are low-grade elsewhere were metamorphosed and deformed. This area is also the closest to the magmatic core of the Skagit Gneiss, so the partial melting could have weakened the crust and permitted the development of the step-over zone in this region. They state that this area is a good example of the relationships (they specify four) which develop in transtensional orogen. They state that the RLFZ is a strike-slip fault which accommodated and may even have initiated burial of sediments to mid-crustal depths and subsequent exhumation and the metamorphic rocks provide evidence which indicates a significant vertical component to strike-slip fault zones.

*Gordon, S.M., Whitney, D.L., Miller, R.B., McLean, N., and Seaton, N.C.A., 2010, Metamorphism and deformation at different structural levels in a strike-slip fault zone, Ross Lake fault, North Cascades, USA: Journal of Metamorphic Geology, 28, 117-136.

Saturday, April 3, 2010

Active reading can even overcome sleepiness and result in comprehension

I know better than to put off reading my 1000 words a day from the geological literature until late I the day, yet I chose to do so anyway. Today the impulse to do so was triggered by Spring. There were very few people in our geology department today, as most people chose to leave for their Easter long weekend as early as they could. Some of the general staff were here, but were only scheduled to be here till about 2:00 pm. Around the time that they were leaving I noticed that it was a beautiful day, nice and sunny, yet not hot at all, and I felt restless. Therefore I decided to head out on an adventure, and went into the city center to admire the street artists and musicians who perform for the crowds there. I also stopped in the American Book store and picked up a new book in a favourite series.

After these adventures I managed to limit myself to only an hour of reading in the new book before crossing the street to my office again around quarter to nine, intending to finish the work I’d interrupted hours before. Instead I procrastinated by catching up on reading blogs/e-mail/etc, updated my financial records to show today’s spending, and chatted with a friend on line. As a result I didn’t start reading my 1000 till 11:30 pm, and it was difficult to find the discipline I needed to actually complete the process.

However, hard as it was to make myself do, I am now a total convert to this “active reading” thing. The need to type the below notes meant that I had to actually pay attention to what I was reading, and not just sleepily skim over the words catching one in ten. I am positive that focusing on each paragraph one at a time and typing the notes as I go is the only thing that made it possible to understand what I was reading tonight. Perhaps I can understand and retain what I read the “normal” way when I’m high in energy, but this late in the day “active” reading is required. So, without further adieu, I give you the second-to-last installment of my paragraph-by-paragraph summary of the paper I’ve been reading (see a few days back for the citation and the link to the article itself).

The next section of the paper is on the 40Ar/39Ar results.

Paragraph 63 introduces the two pegmatites from which muscovite was obtained for the dating, describes the location of the pegmatites, gives the age results (~47.1 Ma for one and ~46.9 for the other). They state that the samples were obtained from above and below the contact, and are within error of one another. They state that this means that these numbers represent the time of cooling, rather than crystallization. This ends the section on the dating, and also concludes the section on their results. The next section is the Discussion.

Paragraph 64 points out that previous studies described a fault in this area and described the rocks on either side as high-grade on the west and low-grade the east, but this study indicates that what had been called “low grade” has actually undergone sufficient burial to achieve >650 C at 8-10 kbars, or largely the same as the “high grade” Gneiss. They go on to mention that the microstructures near the contact between the two units reveal progressive deformation & a high-T constrictional shear zone between the units, & subsequent overprinting by low-T deformation. They say that therefore this area is complex & important in context with the entire Cascades region, and for strike-slip dynamics in orogeny.

This concludes the lead-in portion of the discussion; the next section is on the Previous interpretations of Ruby Mt-Elijah Ridge tectonic history.

Paragraph 65 names three different studies which have addressed the tectonic history of this area. The first called this area a suture of Insular and Intermontane belts since it puts ocean rocks structurally above continental arc rocks. The second calls it a tectonized intrusive contact without a through-going fault, saying tilting is enough to expose the largely intact Mid-Cretaceous section, and add in one more fault to account for the change in pressure across the area. The third described this area as a major tectonic boundary, since there are mylonites present in addition to the difference in metamorphic pressures.

Paragraph 66 gives one other possible interpretation (calling this area the uppermost part of the flowing orogenic crust with the Napeequea unit acting as a rigid lid of a layered crust. They describe this interpretation as analogous to migmatite-upper crust relationships observed in the hinterland of the orogen, and cite a source (but from the sentence itself it isn’t clear if the source proposes this interpretation or simply describes the relationships in the analogy). They then state that all four models can be considered in light of their new data. This concludes the section on previous interpretations. The next section is the Re-evaluation of Ruby Mt-Elijah Ridge tectonometamorphic history.

Paragraph 67 describes the units in this area (orthogneiss dominating, Napeequea over it on Ruby Mt. and part (west end) of Elihah ridge, and Methow rocks above it on the east part), the dip (35-45 to the east) of the planar fabrics of bother Napeequea rocks and orthogneiss on Elijah Ridge while the far east side of the ridge is folded (= different deformation style across the ridge).

Paragraph 68 specifics that the low-T fabric overprinting migmatite seen in the microstructural analysis occurs only in the structurally deepest part of the exposed orthogneiss at the base of Ruby Mt. the only other units to show such overprint is the tonalitic amphibolite at the Skagit Gneiss-Napeequea contact .

Paragraph 69 contains too much information for one paragraph. It states that the strain gradient near the summit of Ruby Mt. displays more constrictional fabrics near the contact between the Skagit orthogneiss and Napeequea units (both of which are L-tectonites). It mentions the pegmatite which intrudes both that was deformed and cooled at ~47 Ma. It lists the clues which lead them to conclude that the shear zone is a medium- to high-T feature. It acknowledges some lower T-overprinting in a few of the units. It suggests rheologic contrast as the cause of the shear zone, but doesn’t decide between shear zone formation during or after emplacement of the orthogneiss. It suggests that the shear zone is an old contact which preserves the early high-T deformation of both units, further suggesting that the deformation either developed along or transposed the intrusive contact.

Paragraph 70 mentions the contacts between these units in other areas, the fact that the quartzite and schist of the Napeequa unit is generally more deformed than is the orthogneiss, suggesting that the composition of each is likely responsible for that difference. It then states that the contact exposed on Ruby Mt. is the one showing the most intense high strain fabrics.

This is not a section break. However, there are no more section breaks in this paper, save for the one which precedes the final paragraph, and I’m at 1000 words read today, and 1000 more left before the end of the paper. Therefore I’m going to stop here and pick it up with the next paragraph tomorrow.

Friday, April 2, 2010

getting faster

I just read and took notes on the next section of the paper I’ve been “actively reading” for my 1000 words a day from the geologic literature, and am pleased to note that while I read well over 1000 (bringing yesterday’s and today’s combined total over 2000 words), it took just under 35 minutes to do so. I don’t know if this is because I’m becoming more comfortable with the process of active reading and typing up notes on each paragraph I read as I read them, or if these notes are less extensive than those for the earlier section, but I’m hopping it is just a matter of practice making me more efficient. I am willing to make 30 minutes a day, every day for this goal, but the over an hour I spent the first time I tried this technique might be asking a bit much.

I don’t know if anyone is actually reading the notes I’ve been posting, but I shall continue to post them till I reach the end of this article. If it should happen that someone tells me that the notes are interesting and/or helpful, I could easily be talked into continuing to post them for future articles I read, too. However, I suspect that for most people who are looking for interesting blogs to read one article summarized one paragraph at a time will be plenty. Feel free to let me know if I’m mistaken on that point.

Today’s notes are from the section on microstructural analysis:
Paragraph 53 introduces the locations from which the samples were collected for microstructural analysis and gives the goals for this portion of the study (understand mechanisms and relative timing of juxtaposition of the units)
Paragraph 54 mentions which samples were chosen for electron back scatter diffraction, gives details of what was found and compares and contrasts the results from the leucosome and mesosome layers of the sample. They go on to offer a tentative (since there weren’t enough samples analyzed to be confidant) interpretation (possible late shear zone affected the structurally deepest exposed rocks)
Paragraph 55 names a sample chosen for EBSD, reiterates that it is from the Skagit Gneiss), gives published U/Pb zircon age for this sample (which is younger than for other Skagit Gneiss samples), and describes the areas in this sample for which they made EBSD maps.
Paragraph 56 lists the results of the EBSD analysis for this sample (including strong crystallographic preferred orientation, prism slip for quartz (both regions of sample), sub grain rotation and recrystallisation (in shear zone), different grain sizes in different regions.
Paragraph 57 describes an effect (some qtz not deformed adjacent to larg plg grain) which only shows up in EBSD map, not via traditional optical methods. It also points out that such variety in qtz textures could indicate qtz deformed at all T and may have preserved evidence for more than one condition.
Paragraph 58 gives EBSD results for a new sample, from a new location.
Paragraph 59 mentions that the above sample has overprinting which doesn’t show up in higher parts of Ruby Mt. (instead the higher parts preserve high-T fabric with not much qtz recrystalization).
Paragraph 60 discusses textures in the structurally highest regions of Skagit Gneiss, and interpret them to indicate that the shear zone is a high-T feature.
Paragraph 61 more EBSD results this time for another couple of samples, comparing and contrasting them with each other, and with the other samples in the area. Interprets the results to indicate that the quartzite may have taken up much of the low-T deformation in this area
Paragraph 62 gives the EBSD results for samples from the RLFZ because they feel that understanding deformation in the fault zone will help interpretation of the role of the fault in burial and exhumation. They point out the similarities between these samples and the above.
Here ends the microstructural analysis section. I found it slightly hard to follow in terms of understanding which samples/areas were being discussed when, but that was because I hadn’t made notes about the sample names/locations in my pre-reading familiarization session. Nonetheless, I think that sub headings might have helped.
The next section will cover the 40Ar/39Ar results.

Thursday, April 1, 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.