Friday, June 15, 2012

At the faucet of June...

Summer is here again in Houston.  It only seemed like yesterday when I was watching migrant warblers on a cool, green, just beginning to bloom Rice campus.  Now the daily temperature is in the mid to upper 90s and the summer heat blanket has descended on Houston.  Time for lap swimming in the pool, watermelon juice, lemonade, and cranking the A/C 24/7.

Let's see.  I've made some progress in the past several weeks.  Lab work, hand-picking minerals and dissolving whole rocks in acid to measure on the ICPMS, has been going on.  Picking minerals has taught me something - how rarely Nature makes a flawless, inclusion-free, surface decomposition-free crystal.  Then when you DO come across one, like a huge, round, perfect red garnet grain, it's a special moment.  I feel more comfortable doing stuff in the lab that involves HF and other nasties.  One thing I have learned from working with tiny samples (xenoliths) and strong acids is that lab work is much more "doable" (and probably safer) using small steps and small containers.  When I hand-pick minerals, I crush a small amount and pick that amount, then put it away.  It would be devastating if I picked a big amount and then accidentally dropped the dish!

I've also come up with a more refined working hypothesis for my current project on continental arc refertilization.  This mostly sprang from a few plots I made when I plotted my Sierran xenolith data on the same graph as peridotites from other places around the world (cratons, SCLM's, massifs, forearcs, etc.).  Doing this also made me realize how special and rare my Sierran xenoliths are - fragments of the mantle wedge beneath a once active continental arc, now cooled and stabilized into mantle lithosphere.  There are hardly any other samples I have found in the literature that represent this tectonic setting.  Xenoliths from the Mexican arc and the Canadian Cordillera may be the closest analogues, but there are no garnet peridotites in either of those places (why not??).  Anyway, when I plotted things like whole-rock CaO vs. MgO or Al2O3 vs. MgO, continental arc peridotites (Mexico, Canadian Cord., Sierra Nevada) plot at distinctly higher CaO and Al2O3 compared to island arcs (Izu-Bonin, Simcoe).  Why are forearc peridotites so depleted?  Why are continental arc peridotites more fertile?  Perhaps that's just the characteristic feature of these two tectonic settings.  But in the Sierran case, the peridotites experienced fairly high degrees of melt depletion, yet appear modally metasomatized - containing excess clinopyroxene and garnet for their degree of melting (why they plot at high CaO and Al2O3).  Is this also the case in other continental arcs?  I looked at the data from Mexico and Canadian Cordillera, and at first glance, the spinels (we use Cr# of spinels as an indicator of melting degree) in those samples don't appear to record as high of a melt depletion as the Sierran spinels, although there is quite a range of Cr#'s.  So, maybe the mantle beneath Mexico and Canadian Cordillera is just more fertile and actually not re-fertilized, like the Sierran mantle?  I wonder if the fact that only spinel peridotites are sampled in Mexico and Canada has anything to do with it.  Also, there are some key differences tectonically between the Sierras, Mexico, and Canada.  Mexico and the Sierras are similar in that they both experienced subduction of the Farallon plate which induced arc magmatism and formed the mountains there.  But, in Mexico it doesn't appear that the slab shallowed (no Laramide orogeny in Mexico?) after the main arc phase; whereas in the Sierras, you had arc magmatism for most of the Mesozoic, then the slab hung around but shallowed and refrigerated the entire lithosphere beneath the western US.  Some people think the slab under Mexico experienced tearing and initiated an influx of mafic magmatism and spreading in the back-arc.  That might explain why there are no garnet peridotites that have made it up, and maybe also why the spinel peridotites don't preserve high Cr# spinel (were they overprinted by ascending melts during backarc spreading?). 

So, one idea is that arcs where the lithosphere is thickened or compressed (e.g., the Sierras) may be more favorable for preserving refertilized peridotites.  This is because it might be easier to trap melts within a thick lithosphere undergoing compression compared to a thin lithosphere (e.g. island arc) undergoing extension.  Also, the persistence of a slab beneath the arc long after cessation of arc magmatism may also promote preserving these refertilized peridotites - they can be "held" in place.