Wednesday, January 11, 2012

End of 2011, Rio Grande Birding Redux, Pyroxenites, Cooling of deep roots

2011 has been gone for almost 2 weeks now, and it does feel like a new year in ways, especially that I have some exciting new research goals for the semester. Despite my general feeling that new, refreshing winds are starting to blow through the start of 2012, the beginning of the spring semester at Rice began in a rather inauspicious manner. A giant rainstorm pelted campus on the very first day of school, causing flooding, rafting of large islands of drought-compacted mud and leaf debris onto sidewalks, vortices in parking lots, and 5-foot high wakes behind cars splashing through the flooded streets. But the next day dawned bright and clear and cold, a nice herald to a new school year.

2011 seemed to have gone by so fast. I can't believe I'm a 4th year PhD student! It is both sort of scary and exciting. I don't feel ready at all to graduate yet; I think I have still have a way to go in terms of becoming more independent (scientifically speaking), and I still have much more to pound in my head when it comes to everything it seems... Sometimes I feel so inadequate when I look at everyone who has trodden the path before me. Will I ever make it to where they stand one day? I try not to think/worry about that too much, because I can only see it going down into depressing avenues. One can only do their best... mistakes are made, we learn from them, and hopefully it makes us a better scientist AND person in the end. "Adversity is the path to truth" - a fortune out of a fortune cookie that is now pinned up on my bulletin board.

So, what did I do in 2011... I submitted my first paper to Journal of Petrology on New Years Day. After nearly 8 months, I finally got reviews back, sent it back, and it is now finally in press, over a year later. I wrote a second paper on metasedimentary xenoliths and their zircons. Hopefully will be submitted soon. This paper provided me with my first real foray into isotope geochemistry, which is a beast all its own. I TA'ed The Planets for Adrian Lenardic in the spring. I attended CIDER for the 2nd time, this time held in Berkeley and not Santa Barbara. I went on a short recon trip with Cin-Ty and other students down to the Peninsular Ranges Batholith, where we looked at mafic enclave-host magma relationships, metasedimentary wallrock-pluton contacts, and lots of other cool stuff in preparation for a field mapping class. We also visited the Yerington Copper mine in Nevada, which was crazy. Not just crazy mineralogically (huge neon-yellow skarn garnets!), but also crazy because at one point we found ourselves next to a bullet-ridden wall when some dude with guns pulled up in a pickup truck with his wife, who may or may not also have had guns (maybe he likes to shoot sericitized monzonites from a distance). Next, in August, I went to Prague for my first ever Goldschmidt conference, and my first ever time in Europe! Then I came back to Houston and Rice, finished up the metasediment paper, started some other projects, and went from 0 - 60 mph in 10 seconds (figuratively speaking) in terms of becoming a real birder, thanks to Cin-Ty's exciting daily observations of what birds were migrating through the intramural fields near the geology building every day (and they were all really cool birds!!!). So I started birding on a more regular basis (instead of the occasional trips to High Island during migration and field trip birding). It's a great stress-reliever sometimes, just walking outside early in the morning with no one around, gives you a calmness to start the day. And then to catch a glimpse of that elusive sparrow, it's worth every second you spent stalking it. Then to end 2011 with a bang, we went down to the Rio Grande twice in December (see previous post for the first time). Second time was with Blake Dyer, former Rice undergrad/hardcore birder. Combined over the two trips, I got 29 lifebirds! My favorite bird down there was the black-vented oriole, shown below (photo of black-vented oriole taken by Blake Dyer):

And every time I saw an Altamira oriole, I wanted a glass of tangerine juice...

So, that's the big stuff I did in 2011. What do I hope to do in 2012? One is to get into the next phase of the Evolution and Development of Deep Sierran Arc Lithosphere: by what, how, and when was the deep lithosphere refertilized? For this, I'm moving into the realm of pyroxenites. Lu-Hf dating of the deepest pyroxenites, garnet websterites interleaved with residual peridotites, could help better constrain the time of sub-batholithic root formation. If the websterites yield Lu-Hf ages coeval with batholith formation, would that indicate that a root >3 GPa thick was already present during the peak of arc magmatism? If the websterites yield an age younger than batholith formation, could this age correspond to a refertilization/metasomatic overprint, possibly associated with the Laramide Orogeny? Either way, both outcomes would be interesting.

The websterites themselves are interesting from a petrologic standpoint. Why are they interleaved with peridotites? (They are thought to be interleaved with peridotites based on similar final equilibration P, T conditions). Do the websterites represent actual early and deep igneous cumulates (a deeper analogue to the high-MgO pyroxenites), or do the websterites instead represent products of melt-rock reaction within a residual peridotite matrix? The story is further complicated because of the possibility that much of the garnet we observe in the websterites now is the result of subsolidus cooling and exsolution, probably related to root thickening. So, garnet may not even have been on the liquidus at the time the cumulates crystallized, instead pyroxene would have been the primary cumulate phase. Anyway, aside from that, I am not sure how well one could distinguish a deep cumulate origin from melt-rock reaction origin. If the garnet mineral chemistries are different between high MgO, low MgO, and websterite, that could tell you something about the melt that fractionated the garnet (but just from a very quick look at some old Sierran garnet data, all the garnets seem to be pretty well equilibrated at high T conditions, obliterating any vestiges of a metasomatic origin...). Regardless of what the true origin of the Sierran websterites are, they still represent the deepest igneous products of arc magmatism, which in itself is interesting.

I'm tired, and rambling.

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