Welcome to GeoUnion, the graduate student body of the Department of Earth, Environmental and Planetary Sciences. GeoUnion strives to supplement the overall graduate student experience at Rice and DEEPS. GeoUnion represents DEEPS in the overall Rice grad student community, acts as a liaison between students and faculty and organizes a number of intra- and inter-departmental events throughout the academic year.
How do you get water back into the mantle? Serpentinites, of course! The main source of water in magmatic arcs like the Cascades comes from altered oceanic lithosphere. Interaction of the seafloor results in extensive hydration of the crust and lithospheric mantle, resulting in the conversion of olivine-rich rocks (peridotite) to a green phyllosilicate-bearing rock known as serpentinite. Studying serpentinites is the key to understanding the global water cycle, at least on million year timescales. What were subduction zones like billions of years ago? There are many examples of Phanerozoic serpentinites, but really old serpentinites are rare. It turns out that we have some ancient serpentinites right here in Texas, in the one billion year Grenvillian orogenic belt that cuts across our state.
Green, green olivine
Peridot is when you’re on show.
Drops of water,
Sooner or later,
You become serpentine.
So the Rice Undergraduate Geology Society (RUGS) organized a one day trip out to central Texas today to look at these 1 billion year old serpentinites. We started out at 6 am in the morning and hightailed out to the Coal Creek serpentinite. We met one of our professors Cin-Ty Lee out there – he had arrived several hours earlier to search for a rare Mexican bird called a Striped Sparrow. A couple of faculty from UT Austin also joined us. The day turned out to crystal clear, with mild temperatures. We could not have asked for a better day weather-wise.
We arrived at the serpentinite outcrop. We immediately noticed that the quarried sections were light green, but the weathered surfaces were red, a common feature of ultramafic terranes. There was almost no vegetation growing on the serpentinite surfaces, in stark contrast to the surrounding amphibolite rocks, which had small live oaks growing on them. We spent a couple hours scrambling over the quarry, trying not to roll our ankles, all the while pondering what type of peridotite protolith this was. Was it oceanic lithosphere, fore-arc lithosphere, sub-arc lithosphere, or back-arc lithosphere? How would we tell? Does the serpentinite represent a suture between the collision of two continents? What we all agreed upon was that this serpentinite looked old! No hints of fresh olivine anywhere, but everywhere, there were hints of serpentinizing fluids in the form of syndeformation veins. We also found a couple of local samples that appeared as if they were originally troctolites based on relic cumulate texture.
After having lunch on these green rocks, we decided to head to Enchanted Rock to look at granites up close and personal. Unfortunately, when we arrived, the park had reached its capacity for the day and they were not letting in anymore visitors until 3 PM. So we went back towards where we had just come from and stopped to look at an unconformity where Proterozoic granites and migmatites were overlain by Paleozoic limestones. A hundred million years missing beneath our feet! We spent a half hour crawling over some beautiful exposed migmatites and discussed what conditions are needed to melt the lower crust.
Daylight was running out, so we thought we would take one more shot at getting into Enchanted Rock State Park. We arrived there with only about 2 hours left of sunlight and a long line of cars inching their way through the entrance. We eventually got in with enough time to climb to the top of one of the granite domes. It was a nice way to end the day! Participants (see our group shot above) included from left to right: Elli Ronay, Xun Yu, Detao He (behind Xun), Larisa LaMere, Sriparna Saha, Michael Farner, Lexi Malouta, Rachel Marzen, Jackie Rios, Adeen Denton, Tierra Moore, Emily Paine, and Cin-Ty Lee (taking the picture!).
A new micro-XRF element mapper (Horiba XGT 7000) has been installed on the third floor. This instrument excites atoms using a Rhodium x-ray source and measures the energy of the re-emitted x-rays using energy dispersive spectrometry. This instrument generates qualitative elemental maps at 50 or 100 micron spatial resolution and can handle sample sizes up to 10 x 10 cm. Minimal sample preparation is needed. All that is needed is a relatively flat surface. It can tolerate a few mm of relief so you do not need a polished surface. Come on by if you’re interested in using it. It is easy to learn and use. Undergraduates Emily Paine, Elli Ronay, Meron Fessahaie, Maya Stokes, and Graham Eldridge have already been working on it!
The above image shows a false-color map of a reaction zone in a granite (actually an orbicular granite), measured by Emily Paine. The long dimension is about 4 cm in length. The red corresponds to sulfur and represents sulfides. The green corresponds to Ti and corresponds to Ti-bearing magnetite. You can use this to investigate sediments, metamorphic rocks, igneous rocks, and many other things.
Department of Earth, Environmental and Planetary Sciences
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