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.

Perhaps not “foot-ball sized”, but still big enough to inspire envy when compared to my whiteschist garnets

Yesterday Chris over at Highly Allochthonous commented that he treats claims of large garnets with skepticism, having been disappointed on more than one occasion when the outcrop failed to show them as large as rumor said they’d be. Therefore I did the logical thing and e-mailed the author of yesterday’s paper to ask for photos.



He graciously agreed to share, and says that the photo on the left "is of a pyrope garnet from the classical locality Case Ramello in the Dora-Maira massif. This is one of the bigger ones (see watch for scale), but there are even bigger garnets there. The slight change in color comes from a decrease in Fe from core to rim. The second garnet is smaller, ca 4 cm in diameter coexisting with large phengite and sugary quartz.”

For contrast, the below photo is of a thin section of the Tasmanian whiteschist; the largest garnet in this photo (upper left corner) is 4 mm wide.

Garnet Envy

While reading a paper* today which combines experimental petrology results with information obtained from natural rocks, I noticed a description of the whiteschists located in Dora-Maira massif of the Western Alps, which says that they “They display spectacular up to football-size, pinkish garnets embedded in a matrix of kyanite, quartz and phengite”. For those of you who remember my post showing the outcrop of Tasmanian whiteschist and how very small the outcrop is, you will see why I now have garnet envy. No way could my tiny little outcrop create foot-ball sized garnets! The largest one I found in my samples was only a couple of millimeters wide, though, like theirs, they are "pinkish". I wish that the article had included photos, I’d like to see those garnets…

*Hermann, J., 2003. Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif. Lithos, 70(3-4), 163-182.

Always double check!

I collected most of my samples for my PhD project in March of 2006, consequently their field numbers start 06___. Those samples all came from the northern fault block of the Collingwood River Metamorphic Complex. In November of 2007 my advisor and I did one additional trip, to the southern fault block of the Collingwood River, in search of the long-lost Tasmanian whiteschist. We found it, and collected a number of other samples from the area as well. Whilst we were out, we also stopped by the northern block and picked up another couple of oriented samples, just for good measure.

In October of 2008 we decided that there was just enough funding left to do some detrital zircon dating for two samples from the Collingwood River. I was told to select "sandy" samples as being most likely to contain zircons. So I picked out half a dozen from both the northern and southern blocks of the Collingwood River which were sandier than the others, though not necessarily sandy enough, and brought them along to my lesson on rock crushing. My teacher du jour narrowed it down to three likely candidates, two with 06___ numbers, and one with a 07___ number. So we, thinking that there might be value in having one from each fault block, chose one of each and we commenced crushing.

Since then I've been thinking in terms of having crushed one from each fault block, so was not too surprised when their detrital zircon patterns were somewhat different from one another. Today, at long last, I decided to make the figure showing the location these two samples were collected from. Yup, you guessed it. They are both from the north block! They were collected from locations not more than 250 meters apart from one another along the highway.

However, since the other 07___ samples were all rejected as not being sandy enough to bother, I can't really complain that I don't have one from each block. There is just that lingering embarrassment that one gets when one realizes that the assumption under which one has been operating doesn’t actually apply. The best cure for that? Admit the mistake publicly—then it won’t ever be repeated!

And always consult the map...

Tasmania's whiteschist

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.