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Posts Tagged ‘metamorphism’

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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(I’m on spring break working writing like crazy on two papers, but I decided to take a bit of time and catch up on some blog posts I promised.)

Over a week ago while I was at NE-NC GSA in Pittsburgh, several tweets resulted in me promising blog posts.   Today’s sequence of posts is a result of Dana Hunter’s request to understand better how garnet schists form.   I was originally going to do this in one post, but its too ridiculously long.   Instead we’ll have this post on the theory behind metamorphic reactions.  A second on contact metamorphism of a shale.   And our final one on the formation of a garnet schist from a shale.

The base concept for metamorphic petrology is that thermodynamics tell us what should be present at any given pressure (P), temperature (T), water conditions, etc. (the item with the lowest energy = most stable), but kinematics gives us an idea how quickly a reaction will take place & whether or not the predicted phases will be present.   However, one of the most important thing to understand about metamorphism is that you have deal with the hand your dealt–the chemical components (Si, Fe, H2O, etc.) that are in protolith (original, unmetamorphosed rock) will either have to be in your subsequent metamorphic rock or leave the system in a believable fashion.   This is one of the reasons why memorizing the chemical formula of the common rock-forming minerals is actually useful–it gives you an idea about what could have been & what may have occurred to produce the rock now in your hand.

Before I move on to how the reactions work, let’s take a second and talk about “leave the system in a believable fashion.”   Some elements and compounds on our planet are very mobile and can move in & out of a system (a user-specificied volume that’s being studied).   When we metamorphose a rock, its easy to believe that these mobile components can drift in or out depending on the P & T conditions.   Other elements are relatively immobile and rarely travel any great distance.   In metamorphic systems, these are the elements we simply have to incorporate both in the original minerals & any subsequent minerals that form.

Frequently mobile components:

  • water (H2O)
  • carbon dioxide (CO2)
  • any other gases (Ar, methane, etc.)
  • Na+ or K+

Usually immobile components in metamorphic rocks:

  • Al3+, Cr3+, Ti4+, Si4+
  • Fe2+ or 3+, Mg2+, Mn2+, Ca2+

P&T will control how mobile an element is (hotter = more movement possible), but also the presence of fluid can make elements more mobile.   For instance, uranium is fairly mobile in the presence of a fluid phase — but there’s usually not much uranium in a rock to start with (usually in the parts per million (ppm) or parts per billion (ppb) range), so its not a huge concern when we try to balance a system.

Some general rules hold true:

  • colder rocks will generally have more fluid in them than warmer rocks
  • fluid is usually driven off during prograde (increasing T and/or P) metamorphism
  • when the fluid leaves or enters the system, it may have K+ or Na+ with it
  • during retrograde (decreasing T and/or P) metamorphism, the introduction of a fluid into a system may drive reactions that involve the formation of lower T&P minerals (retrograde minerals) at the expense of erasing the higher P&T mineral assemblages

As P&T changes along a prograde path, our rock looses water and/or carbon dioxide and some of the components begin to be less stable than a new group of minerals.   Though occasionally one mineral will simply switch to being a new mineral that is more stable, usually that instant switch is restricted to polymorphs (two different minerals with the same composition) that only need to change minor things in the structure (e.g. alpha-quartz to beta-quartz).   Most of the time, one (or more) mineral(s) will slowly lose an ion here and an ion there and a then new mineral(s) will use the released ions to grow.   These ions can either move around the outside of grains either with or without a fluid phase present or through a mineral–the latter is much slower, even though it might be a more direct path.

In the sequence of pictures above, thermodynamics dictated that the red mineral was no longer stable and that the purple mineral became stable at P2 & T2.   However, its kinetics that dictate how long it takes the red mineral to break down into its individual components (Ca2+, Si4+ and Al3+ in this case) and how quickly the purple mineral will grow.   Since the components have to diffuse (migrate from one location to another), there’s going to be some lag between when the red mineral breaks down and before the purple mineral starts to grow (picture #2).   The speed of the diffusion depends on:

  • temperature -> higher T’s = faster
  • presence or absence of a fluid phase -> fluid-present = faster
  • what the pressure is & whether or not its lithospheric (uniform in all directions) or differential (varies depending on direction) -> more complicated influence on diffusion, but minor compared to T & fluid-presence, so let’s skip on by for the moment
  • chemical gradients within the system; if one area is Ca2+ poor and another Ca2+ rich, then the Ca2+ will shift to try to get the Ca2+ evenly distributed throughout the system -> more of a difference between poor & rich regions = faster

If we had suddenly taken the rock after step 2 or 3 back up to the surface of the Earth, instead of preserving an equilibrium assemblage where all of the minerals are stable, we would have captured a moment of disequilbrium and a reaction frozen in its progress.   For most metamorphic analyses, equilibrium is what we are trying to find because we can calculate via thermodynamics what it should be.   However, in my mind, its the disequilibrium of frozen reactions that are more fascinating.

cordierite-staurolite-sillimanite-garnet schist from Vermont in PPL; several reactions were concurrently frozen creating the mix of non-ideal of grain shapes

I think that’s enough theory for now.   Let’s start dealing with a real rock.

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