Wednesday, February 1, 2012

New Beautiful Rocks, Winter

I haven't been as productive this week as I wished. The GSA proposal that I submitted today took a while to write mostly because it is a new project. The shatterbox has been down still (the technician is still waiting on the part), so I can't grind any more samples, and our fume hood needs to be cleaned so I haven't started on wet chemistry yet. However I did send off the East African Rift samples and 2 of Monica's blueschists to get XRF data, so hopefully in a month there will be some good stuff coming back. I've been working on a presentation for the Chile field trip class on tectono-magmatic evolution of Chile. Mostly I've been reading some papers about the Andes, and trying to sort out the complex evolution of the orogenic belt. I'm actually really excited to work on these samples from the East African Rift. Of course I'm not putting the Sierras behind, but it feels a bit refreshing to get out into a new place (and by that I mean, looking at new rocks). So I was looking under the microscope at some of the thin sections we got back recently. Cin-Ty originally only had 5 or so xenoliths out of the 20 made into thin sections, so I sent the rest of the samples out to get thin sections made. While many of the samples are pyroxenites, I realized (and this was later confirmed by Peter Luffi after I asked him about the texture I will discuss below) that equally as many are peridotites. Check out this sample below (field of view ~5 mm):

So this is a very nice example of a poikilitic texture, where large olivine crystals are poikilitically enclosing orthopyroxene crystals (the small grey round blebs). What was interesting to me about this texture, though, was the extreme roundness of some of the opx. I've seen cumulates where the early formed crystals still retain more or less euhedral shapes, but in this case none of the opx are euhedral. So an alternative hypothesis for this texture could be that it formed by melt infiltration. This rock, which has high modal olivine, could be on its way to becoming a reactive dunite, and the opx blebs might be the last vestiges of original opx. Imagine melt infiltrating along grain boundaries of opx in a harzburgite, the melt will begin reacting with the opx and dissolve it to form olivine. The big olivines poikilitically enclosing the opx may have originated in this way, and left behind small opx pockets. It's a beautiful texture, isn't it?

On another note, I got a new camera! Well, not exactly new. I bought Blake Dyer's old Nikon D90 (body) (and he upgraded to the D7000). I was only able to afford one lens, so I settled for a 70-300 mm since I already have a high-end point and shoot for wide angle stuff. This past Sunday Ben and I went birding, and I got a chance to try out the new camera. Here are some photos of first the wintery landscape at Highland Reservoir where we unsuccessfully chased Smith's Longspurs (but got a Sprague's Pipit and Le Conte's sparrow as consolation prizes!), followed by a palm warbler (a lifebird for me) at San Jacinto Monument, a female cardinal at the Houston Arboretum, a tufted titmouse (Arboretum), a chipping sparrow (also Arboretum), a Forster's tern at Lynchburg ferry, and a great egret in wintery landscape at Sheldon Lake State Park. One has to get pretty close to the bird with a 70-300 mm, but the fun is in the stalking and seeing how close the bird will let you get!







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