Oct 02, 2017 | No Comments
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...
Jul 06, 2016 | No Comments
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...
May 02, 2015 | 2 Comments
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...
Apr 03, 2015 | No Comments
...
Feb 15, 2015 | No Comments
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. ...
Aug 12, 2014 | No Comments
Interesting post by Stuart Wright on estimating grains sizes using a weighted area average approach:...
