Metamorphism: evolving ideas

‘Ontogeny recapitulates phylogeny’ is such a lovely phrase. It was coined as a biological concept, and is now somewhat discredited. The idea was that as an organism develops as an embryo it passes through stages of growth corresponding to stages of its evolution as an species. My excuse for typing ‘ontogeny recapitulates phylogeny’ (twice!) is that is also a useful concept for talking about ideas.

Often the best way of introducing a body of knowledge is to work through the history of the study of that subject. Scientific ideas build on what came before – build and modify.
Take the atom. In my mind it is like a solar system. A tiny nucleus, made up of coloured lumps fused together. A long way off even tinier dots of electrons orbiting around. This model is useful, it worked for Rutherford firing alpha particles through gold in Manchester (it’s his model). Add in the concept of multiple orbits and you get valence and can explain patterns in the periodic table.
But it’s 100 years old, too simple and out of date. Quantum mechanics replaces the image of little electrons whizzing around with probability clouds, the nucleus is a liquid drop and there are cats in boxes somewhere…  Umm. Ultimately I’ll never really understand what’s really going on without grappling with the maths. But thank goodness for the Rutherford model, it marks the limits of my physics education. If I’d been taught nuclear physics by starting with the most modern models, chances are I’d have ending up knowing nothing.

What does this have to do with metamorphic rocks? Well, my previous posts about metamorphism, have taken us up to the mid-1990s. I’ve painted a picture of metamorphism where rocks are closed systems, that achieve chemical equilibrium and go on nice smooth paths through pressure temperature space. This model is simple, useful and demonstrably wrong.

So, building on your understanding of this simple old model, let’s see how the science has evolved since, to understand how reality is more complex.

Staying off-kilter

Thermobarometry relates metamorphic conditions to the minerals in a rock using our knowledge of the chemical equilibria affecting silicate minerals. The assumption is that every mineral, every atom in the rock is in chemical equilibria with every other one. Is this realistic? What things can prevent a rock achieving chemical equilibrium, that can stop atoms finding the place they’d most ‘like’ to be?

It turns out there a few, that can be referred to generally as “reaction kinetics” the processes by which chemical reactions occur.

Our first complication is diffusion. Can the atoms physically get to places they would rather be in time? Along grain boundaries, in the presence of a fluid, this is relatively easy, but in dry rocks, atoms have to wriggle their way through crystal lattices, slowly. In my simple (simplistic) head, diffusion is caused by atoms jiggling about randomly, swapping places and moving around. Heat is the amount of jiggling and temperature the measure of this jiggling. We can estimate rates of diffusion in minerals and it is very temperature dependent. It also depends which minerals and atoms you are talking about. Small atoms that take part in solid-solutions (e.g. Fe-Mg) can move around more easily than big atoms that form part of the mineral lattice (e.g. Al).

So, given enough time, hot rocks are likely to reach chemical equilibrium. Colder rocks, especially in the absence of water may not, which helps to explain why peak assemblages are preserved and not replaced as the rock returns to the surface. For more on this check out my post on the Vredefort impact structure. There I drew on a recent PhD thesis all about studying equilibria in metamorphic rocks.

Getting started is hard

Another obstacle to achieving to chemical equilibrium is nucleation. Minerals consist of atoms arranged into regular structures – lattices. Growing a mineral means adding atoms to its edge and is a relatively easy process. Building the initial chunk of  that lattice (the nucleus) is relatively hard, it can take a long time. You are probably familiar with nucleation in igneous rocks. Obsidian is a rock formed by lava that cooled before substantial nucleation and mineral growth could occur.

Does something similar happen in metamorphic rocks? In a situation where e.g. staurolite is stable, reactions that would create it can’t be active if nucleation hasn’t happened.  Evidence for this can be found in a study of rocks from the Bushveld aureole in South Africa (Waters, D.J. and Lovegrove, D.P. (2002). Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld aureole. Journal of Metamorphic Geology, 20, 135-149.). The Bushveld is a massive mafic igneous complex, the size of Ireland and up to 9 km thick. As you would imagine, it has a massive metamorphic aureole. There isn’t much deformation associated with the intrusion, so wonderful metamorphic textures are preserved. Also we can accurately model the heating associated with it, making it a wonderful natural laboratory. So Dan (“the man”) Lovegrove, assisted by Dave Waters, his doctoral supervisor were able to do things backwards. Starting with a set of known P-T-t paths (different paths at different distances from the intrusion) they then did a thorough thermobarometric study of the metamorphic rock and compared the two sets of results.

They showed a clear difference. The observed and predicted sequences of metamorphic reactions are different, and minerals first occur at higher temperatures than estimated based on chemical equilibria. Their conclusion is that ‘overstepping’ is an important factor. The difficulties of nucleating new phases means that they first occur at higher temperatures than expected, up to 40°C is estimated. The scale and rates of heating around the Bushveld aureole are typical of many orogenic belts (roughly 0.1 to 1°C per thousand years), so these results can be applied generally.

Dan also studied the affect of heating rates on grain size (slower heating = bigger porphyroblasts). More details can be found on Dan’s own website.

So our lovely theoretical model doesn’t apply perfectly to real rocks. No surprise there. Does this mean that this model is wrong? No. On the right rocks, thermobarometry works just fine. Also metamorphic petrologists are starting to get a handle on reaction kinetics as well, meaning that we can extract even more meaningful information from the rocks. Here’s a flavour of that from a real metamorphic petrologist, Dave Waters.