Wednesday, March 30, 2011

A Splash of Sediments: Hot n' Dry Garnet Quartzites and MUDCRACKS

Been a while since I "blogged" in here. What am I doing these days? Mostly working on these enigmatic Sierran garnet-bearing meta-quartzite xenoliths. Yup, garnet-bearing quartzites - sounds weird, and it is! Why am I even looking at such oddities? Well, we think these rocks are evidence of crustal thickening that occurred while the Sierra Nevada batholith was being built by magmatism. Textural and mineralogical evidence (>60% modal quartz, abundant well-rounded detrital zircons) points to a supracrustal origin, but hi TitaniQ temperatures (~750-800 C) and metamorphic garnet (~50Py50Alm) suggest increasing P,T may have occurred - emplacing these sed rocks deep into the mid to lower crust! The story deepens further... We blasted several of the detrital zircons to obtain U-Pb and Hf isotopes simultaneously on the same spot. The set-up was pretty cool, involving 2 mass spectrometers (an ELEMENT XR and a Neptune), and one laser beam split to enter each instrument. U-Pb results show that the detrital zircons are rather unruly, all plotting on discordia. Upper intercept ages are all ancient, indicating provenance from old North American basement terrains. However, all the zircons have similar lower intercept ages, clustering around ~100 Ma - which, as you all know, is the time of peak magmatism in the Sierras. But wait! The trail of clues continues... modeling with THERIAK DOMINO for a given Sierran metaquartzite composition shows that garnet and feldspar are stable between approx 6 kbar to 15 kbar. That is considerably deeper than the maximum thickness estimated for the western Cordilleran miogeocline, which is 3.3 kbar (10 km). (The likeliest protoliths for the metaquartzites are miogeoclinal sediments. Many of these are exposed throughout the batholith - in the form of roof pendants.) Thus, if these xenoliths are indeed originally strata of the miogeocline, what the heck are they doing sitting at >6 kbar? Well... one hypothesis involves magmatic inflation, which can thicken the crust as successive sills are emplaced. Rocks below the level of neutral buoyancy where the plutons rise and stall within the crust will experience an increase in pressure. Perhaps these xenoliths represent such strata. Another idea is that a mega continental underthrust rapidly emplaced a large amount of crust from the east. Yet another idea involves a cryptic mega-strike slip fault that displaced large blocks of the miogeocline northwestward in the early Cretaceous... I find this last hypothesis difficult to test, but the other two and especially the first are interesting.

This project has definitely taken me into a different direction than my main one, which deals primarily with the mantle petrology of continental arcs. I've been learning much more about metamorphic petrology, which I had almost none of during my undergraduate courses (we took petrology in an abbreviated, intense "lagniappe" summer semester after Hurricane Katrina, where we met for 4 days 2 hours a day for 6 weeks... and metamorphic petrology just got squeezed in at the very end). Metapelitic systems are definitely complicated, with many minerals and reactions going on. I am trying my best to "see the forest for the trees" and not get mired down in tiny details.

On another note... here's a picture of some extreme biologically-mediated mudcracking behavior I observed last August, right here in my home parking lot:

I observed this little patch of dirt suffering so and peeling under the heat of the Houston sun in August. It hadn't rained for several days... I wonder how far these things could've peeled up if it hadn't rained.

Wednesday, March 16, 2011

Spring, however fleeting, is here in Houston

March this year felt more like the beginning of spring than other years. And in particular, these past few days, things have been cool, damp, and blooming. There is that pale and vivid green, like "petals on a wet black bough" (T.S. Eliot), which meekly shows itself before the heat weighs down on the trees and gives them their heavy color. On a recent trip to New Orleans 2 weeks ago, I noticed the first signs of spring on our drive back home across the lake, where things are much more wild, and there are swamps and overgrown places, even encroaching on the highway. This is the time when the wild places awake from the hazy winter, but the brief time before the burden of summer heat descends.

This time will not last long - soon it will get hot again. The saucer magnolia has bloomed and gone in a matter of days, soon the pink evening primroses will peek out from among the disturbed places. I'm enjoying the cool nights with the windows wide open while I can.

Saturday, March 12, 2011


I once had a hedgehog, but it died of old age. It was a cool pet.

Last year during the Rodeo (I didn't go, but my roommate did and brought back this contraption), we transformed the hedgehog into a hedgepig:

Now I have a goldfish in a bowl in my office - which is doing surprisingly well (he even outlasted my two basil plants, which were killed in the recent freeze). I think his whole little ecosystem has finally reached steady state, after a bit of a rough start (massive algal bloom, aggressive guppies, not using filtered water). Apparently, according to my office mate, fish have a 3 second memory. That would mean that every time our fish sees us, we're like a new person to him. I guess that's kind of cool in a sense.

Been feeling not as productive lately, and I think it's because I get so easily interrupted during the day with various random things. I'm actually really looking forward to the weekend to being alone, and getting a chance to do some work uninterrupted... once I finish grading 16 more papers. Favorite answer so far: "The satellite (i.e. planetary satellite) is like made of rock and metals." Seriously!