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