Posts Tagged ‘photomicrographs’

The deadline for Accretionary Wedge #43 – “my favorite geological illustration” was extended, which finally kicked me into deciding what to post.   There are a number of diagrams I love that people have already used (Erik’s bubble figure, the Wooster fun with chemographic diagrams, MK’s subduction zone — which I’ve drawn on tables at Italian restaurants & on onsies at baby showers).   And then there were the ones that were finalists (Tharp ocean map is awesome, the USGS Volcanic Hazards poster is basically reproduced by students during intro classes, the different types of silicates), but in the end, I had to go for what I truly know — metamorphic thin sections.   Though photomicrographs are gorgeous, to truly “see” what’s going on with textures, you need to draw them by hand.   The old-fashioned pen & ink drawings draw your eyes to the key features — ah, for the days when every department had an in-department scientific illustrator.

The following illustrations are of the progressive syntectonic metamorphism of a volcanic graywacke from New Zealand. The original illustrations are from Best (1982): Igneous and Metamorphic Petrology (W. H. Freeman, San Francisco).

original volcanic rock

burial of our volcanic rocks, which turns up the heat & pressure a bit:

breakdown of some of the hydrous material, recrystallization of material to start to form definite foliation

rock continues to be buried, which increases the amount of metamorphism:

continued breakdown of hydrous phases; beginning of segregation into "felsic" vs. "mafic" lithologies

as metamorphism continues, we finally get to the “good stuff” i.e. garnet 🙂

down to just muscovite & biotite as hydrous phases, with a higher mode of anhydrous garnet + garnet + oligoclase dominating; segregation is more pronounced

My students don’t really appreciate my insistence that they have to draw fields of view during mineralogy & petrology, but the process really helps them “see” what’s going on so much better.   And though most of them are not in the running to become scientific illustrators in the long run, I do really enjoy grading those labs.

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(post two of three.   The first gives background on metamorphic reactions.)

Starting point: at some period of time in the distant past, a collection of clays, small quartz grains (less than sand sized), maybe some carbonates and oxides collected in a relatively quiet sedimentary basin.   If we were to sample the sediment, it would feel like mud in our hands (though it might be slightly gritty if we tasted the seds).

Core samples from the continental shelf off of Monterey Canyon. From http://www.mbari.org/news/homepage/2005/sand-channelmud2-215.jpg

Let’s take a moment and looks at the composition of our components:

  • quartz (maybe also some chert) – SiO2
  • clays: usually kaolinite – Al2Si2O5(OH)4, montmorillonite – (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O, and/or illite – (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]
  • carbonates: normally calcite / aragonite – CaCO3, though you may have some ankerite – FeCO3 or dolomite – CaMg(CO3)2
  • oxides: hematite – Fe2O3, limonite – FeO(OH)·nH2O and/or goethite – FeO(OH)
  • other random things that may or may not be present: albite – NaAlSi3O8, organic material – C + H2O

What this boils down to is that our quiet depositional environment seds are Si + Al + water-rich with some K, Ca, Na, Fe, Mg, CO2 mixed in.   Whatever minerals are going to be stable in the future of this rock, they’ll probably be Si + Al-rich.

The first event to occur is more sediment being deposited on top of “our” seds.   This new weight forces compacts the seds, driving off some water +/- carbon dioxide, reducing the porosity of the rock, and increasing the density.  If you look at the image above, the clays on the left are “looser” and on the right “denser” due to their placements (towards top & towards bottom) within the core.

(Here’s a link to a great animation of a filling basin over time to give you an idea of what occurs over time within a depositional environment.)

As more & more sediment is deposited in the basin, our seds are compacted more & the temperature starts to gradually go up simply due to burial.   More than driving off water, though, some of our clays (and some of the other random minerals such as goethite, aragonite, etc.) become unstable.   At this point, usually the clays are replaced by other clay minerals that are slightly more dense and contain a bit less water.   If aragonite was present, calcite may form.   Goethite would be replaced by an anhydrous oxide such as hematite.

The further compaction, driving off of water / carbon dioxide, and recrystallization of some of the phases takes our loose seds and turns them into a sedimentary rock–in this case, a shale.   Shales are still very, very fine grained so that individual minerals are not visible to the naked eye.   They are also “fissible” or form in thin layers along which they are easy to break in relatively smooth planes.

Photomicrograph from a thin section of the Proterozoic Rampur Shale (Proterozoic of India) from Schieber et al. (2010)

If our basin simply continues to fill, our shale may lose more water / carbon dioxide, but it won’t become “interesting” (at least to a metamorphic petrologist).   In order to take that next step, we need to either heat the rock by intruding a pluton (igneous body) next to the shale or involve the shale within an orogenic event that will increase both the P & T.

Let’s first deal with contact metamorphism:

contact aureole around an igneous pluton from Winter (2010)

The rocks within the contact aureole will be heated up with higher temperatures near the igneous body & lower temperatures further away.   Frequently, there is a minimal P change associated with the intrusion of an igneous pluton, but it may cause a differential pressure field.

Normally the changes from unmetamorphosed to low-T contact metamorphism to moderate-T to high-T are gradual and may be difficult to pin down exactly to as easily identifiable line in the field.   Detailed work with samples made into thin sections is usually need to pin down exactly where a new mineral becomes stable (in-isograd).   The rock may either be unfoliated (no alignment of the grains due to a differential stress) or foliated.

If unfoliated, we’ll see a sequence of “hornfelses” in which our fine grained shale will become more & more coarse grained.   The other main characteristic as the temperature increases is that the rock will lose more & more water / carbon dioxide.   The clays / carbonates / oxides may no longer be stable and instead plagioclase, chlorite, muscovite &/or biotite will form.   Quartz is still stable.   (Just as a quick check – micas + quartz + plag are Si + Al rich with some Fe + Mg + Ca + Na, so compositionally we’re on track.)

low temperature hornfels that contains biotite, muscovite, quartz, and plagioclase (http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/hornfels.UX.html)

As the temperature continues to go up, the rock become more & more anhydrous and instead of micas (chl, bt, mu) either anhydrous minerals (e.g. plagioclase, andalusite, garnet, sillimanite, K-feldspar) or minerals with only very low amounts of water (e.g. cordierite, staurolite) become stable.   Note the increase in grain size below.

Photomicrograph of cordierite hornfels rich in orthoclase, from lower part of Silver Hill formation, near contact with Cable batholith; shows large poikilitic crystal of cordierite and small crystals of andalusite, sillimanite, tourmaline magnetite zircon and biotite (dark, partly transparent), in a matrix composed essentially of polyhedral grains of orthoclase. (http://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20Calkins,%20F.C.%20146)

How hot the contact aureole will get (and therefore how high grade of metamorphism will be present) depends on a few things:

  • what is the temperature of the igneous body itself?   granites will be colder than diorites or gabbros
  • how large is the igneous body?  small bodies will lose their heat quickly & therefore won’t be able to heat as large a region or to as high a temperature
  • is there free fluid in the system?  fluid-flow convection around a pluton transfers heat much more efficiently than simple conduction–wider contact aureole that may reach higher temperatures simply because the heat reaches the country rocks before the pluton can cool off much
  • how deep within the Earth is the system?  shallow intrusions cool off much quicker because the surrounding country rocks are colder and absorb the heat almost instantly (in geological timescales), deeper intrusions have a less drastic temperature differential between the pluton & the country rocks and allow for a slower cooling of the igneous material and a protracted timing of metamorphism — also if the rocks are warmer to start with, it doesn’t take as much heat to bump them up to a higher grade of metamorphism

If the conditions are right, the rocks directly in contact with the pluton may start to melt and form migmatites.   I’m going to talk about migmatites at the end of the regional metamorphism post, so you’ll have to wait a sec…

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The name of this blog is “Life in Plane Light” and to live up to the name, we need more thin section pictures 🙂

Last week’s mineral of the week was quartz.   The optical properties for quartz are fairly simple and, unfortunately, overlap with a number of other minerals.

In plane light, quartz is clear and has low relief.

In crossed polars, quartz has 1st order whites to greys.

Ok, so what could we mistake it for and how do we avoid that?

  • plagioclase: in igneous rocks, plagioclase will tend to have lamellar twins in crossed polars; in sedimentary & metamorphic rocks, things become more complicated since twinning is rare; metamorphic plagioclase may have deformation twins or core-rim zoning (I’m looking for a photomicrograph), but will not have the undulatory extinction or subgrains that can occur in quartz; when thin sections are slightly too thick, quartz will be 1st order yellow in XPL, but plagioclase will remain whites to greys; plagioclase in a water-rich environment may result in sericite (a fine-grained white mica), which doesn’t happen in quartz

deformation twinning in plagioclase; Fig. 3.19 in Winter (2010)

undulose extinction in quartz; Fig. 3.22 from Passchier & Trouw, 2005

1st order yellows due to thick section, http://www.nslc.ucla.edu/pet/thins/jpgs/1.099.jpg

  • K-feldspars: in igneous rocks, sanidine / orthoclase usually have simple twins and microcline has tartan twins; in metamorphic rocks, we start having similar issues as with plagioclase; K-feldspar can also alter to become sericite like plagioclase and deformation twins may also form in K-feldspar, but this is also a problematic mineral for metamorphic petrologist
  • cordierite: found in relatively few rocks (low pressure, moderate to high temperatures, Al-rich metamorphics) it may also have deformation twins, but the great salvation is that cordierite will form pleochroic haloes around U/Th-rich minerals (e.g. monazite, zircon), which won’t happen in either of the feldspars or quartz

cordierite with yellow pleochroic halo around monazite; NEK-97-13 from my field area in the eastern portion of the Victory Pluton's contact aureole in Northeastern Vermont; field of view is 1.2 mm

What are you’re options if you happen to be a metamorphic petrologist?

  • Find an SEM
  • Technically, we could use the fact that quartz is uniaxial and the others are biaxial to distinguish between them using an optic axis figure, but finding grains with vertical optic axes tends to be difficult in metamorphic schists

The next mineral of the week is hematite / magnetite, so the next photomicrograph post is going to be “interesting.”

Oh, and if anyone has a good link to a core-rim metamorphic plagioclase zoning picture, please leave me a comment!

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