Alpine Fault Xenoliths

I’m excited about some xenoliths I’ve been looking at from beneath the Alpine Fault… come to my AGU talk to hear more: A few spinel peridotite xenoliths found near the Alpine Fault, New Zealand, exhibit a mylonitic texture and, locally, an extremely fine ~30 micron grain size. The harzburgite xenoliths were emplaced in a ~200 km-long elongate dike zone interpreted as a gigantic tension fracture or Reidel shear associated with Alpine Fault initiation ~25 Ma. The presence of thin (~1 mm) ultramylonite zones with px-ol phase mixing and fine grain sizes, minimal crustal-scale strain associated with the dike swarm, and the absence of mylonites at four of the five xenolith localities associated with the dike swarm indicate that upper mantle deformation was highly localized. Strings of small, recrystallized grains (planes in 3D) are found in the interiors of olivine porphyroclasts. In some cases, bands 1-2 grains thick are traced from the edges of olivine grains and terminate in their interiors. Thicker zones of recrystallized grains are also observed crossing olivine porphyroclasts without apparent offset of the unrecrystallized remnants of the porphyroclasts. We suggest a brittle-plastic origin for these features since the traditional recrystallization mechanisms associated with dislocation creep require much more strain than occurred within these porphyroclasts. Analogous microstructures in quartz and feldspar in mid-crust deformation zones are attributed to brittle-plastic processes. We hypothesize that such fine-grained zones were the precursors of the observed, higher-strain ultramylonite zones. Given the size of the new grains preserved in the porphyroclasts (~100 micron) and a moho temperature >~650°C, grain growth calculations indicate that the observed brittle-plastic deformation occurred <10,000 yrs. prior to eruption. It...

AGU 2016

Excited about a session I proposed at AGU this fall, New insights on igneous and metamorphic processes from non-traditional thermobarometers and geospeedometers. Co-conveners are Matt Kohn, Jay Thomas, And William Nachlas. Mark Ghiorso is an invited speaker. Consider submitting an abstract! “Within the last decade, a variety of new techniques have emerged to better constrain the formation history of minerals and rocks from the shallow crust to the mantle. These approaches utilize experimentally- and empirically-determined thermodynamic, kinetic, and physical properties to retrieve information on the conditions and rates of igneous and metamorphic processes. The introduction of new techniques has necessitated a re-evaluation of results from conventional approaches and also opens the possibility to add new insights to previously-studied problems. We welcome presentations on the development and application of emerging techniques for quantifying the pressure, temperature, and/or temporal history of geologic events. Presentations on the development of such non-traditional thermobarometers and geospeedometers, and comparisons of results with more established techniques in both nature and experiment are especially...

New Website for EAS Master’s program

Happy to have chaired the committee for a new website for the EAS department graduate program! The website has department news, a directory, descriptions of research and classes in the department, FAQ, and opportunities for graduate students. AND it looks great! Assembling all this was pretty fun and a nice reminder of the fantastic opportunities students have at City College.  Find the...

Structural Geology (EAS 227) Field Trip

Some photos from the EAS 227 field trip last week to Catskill, New York Devonian Becraft Limestone comprising mainly grainstone with some greenish shale beds The distinctive brachiopod Gypidula Coeymenensis (pencil for scale in bottom left)  Class...

Mississippi River Commission Maps

I rescued some lovely maps of the Mississippi river from the late 1800’s from the recycling bin a few years ago. I guess the library ran out of room. The maps chart the depth of the river at 100 foot increments. Every 1000 feet or so the surveyors made a transect from one side of the river to the other, from Minneapolis to Louisiana. What I like best about them though is the level of detail of the countryside, including building shapes and names, names of land owners, and even names of the types of trees. I’ve only scanned a few maps from the Minneapolis-St. Paul area, but I’ve got a couple hundred more in storage (not a complete set though). What to do with them? A museum somewhere would be a good home, or you’ve got an idea let me know, I’d like to find a better home for them. This map shows the campus of “Minnesota University” including Pillsbury Hall which is currently the geology building. Here’s scans of the three maps from the Twin Cities (see index below for the locations of each): 187b&w 188b&w 189bw     2015 UPDATE: See http://www.umesc.usgs.gov/data_library/maps_quads_figs/1890s_mrc.html for a full georeferenced set of these maps.  ...

Grain Size Analysis

Interesting post by Stuart Wright on estimating grains sizes using a weighted area average approach:...
Open-access journals

Open-access journals

Wanted to put in a plug for EGU’s open-access journal Solid Earth. I published a paper there last year on Ti-in-Quartz thermobarometry. It was a good experience, and I’d encourage everyone (particularly senior faculty) to give it a try. Solid Earth had an impact factor in 2013 of 2.15. This is the second impact factor that the journal has received in its young life; up from 1.37 the year before. The review process is transparent and published online, and what’s more publishing in Solid Earth is free of charges until the end of the year. Submit your manuscripts to Solid Earth here:...
False Pleochroism

False Pleochroism

Got a microscope a few weeks back on ebay, a used Olympus BX51. Generally I’m very happy with it with one exception… under plane polarized light, grains are appear lighter or darker based on orientation and often look a bit iridescent. The image above of an experimentally deformed quartzite was taken in “plane” polarized light. I put “plane” in quotes because there’s clearly something else going on. The large grains (all quartz) should all be equally illuminated. I’ve seen this effect in a lot of scopes over the years, so I know it is pretty common. I just couldn’t stomach having this problem with my new microscope though so I started disassembling the thing trying to isolate the problem (be careful if you try this at home lest the iris diaphragm on your condensing lens fall out… it took me all afternoon to get that thing back together). It turns out that the problem is with the lower polarizer. If you take it out and put the upper polarizer in its place in the light column this doesn’t happen. I posted this to the microstructure forum on facebook and was told this is known as “false pleochroism.” Apparently it’s a problem unique to Olympus and some other scopes. I’m still waiting for the Olympus contact in my area to respond regarding the availability of a replacement polarizer. This issue can actually be fairly distracting in some thin sections, e.g. Olive turns pleochroic blue. I’m equally interested though in what that polarizer (or it’s coating) could possibly be doing to the light passing through it. This does not make any...