Speed of metamorphism: heating up

Published in the 1830s, Charles Lyell’s ‘Principles of Geology‘ is one of the founding texts of the subject. Part of a generation of Geologists who broke free of Biblical interpretations of the natural world, Lyell was working in an intellectual context seeking to move beyond explanations based on Noah’s flood. The true causes of the earth’s rocks were sought not from sudden catastrophes but instead from slow, gradual processes such as those seen acting upon the modern earth. Lyell expressed this as Uniformitarianism, popularising a concept first expressed by James Hutton.

Such ideas were tremendously powerful and became baked into the way geologists thought. But one of the themes of the last 30 years has been the recognition that sudden events can be important in earth history and that gradual change doesn’t explain everything. The most dramatic example involves vaporised dinosaurs, but the trend is seen even in the less glamorous world of metamorphism.

Mix thoroughly then bake for 50 million years

Metamorphic rocks are often found in discrete areas called belts which are typically highly deformed. The conditions of metamorphism vary across the belts, different zones of rock contain different minerals. A common form of metamorphism is called Barrovian and a neat explanation of it came from a 1984 paper that has since been cited 1489 times. In it, England and Thompson performed some simple modelling of crust affected a collision between two continental plates. The models recreated the Himalayas by first using thrust faults to stack the crust up thick and then waiting for temperatures to reach equilibrium – a process called thermal relaxation, involving slow heating of the buried rocks. The conditions experienced by rocks in the model (expressed in terms of their P-T-t paths) closely matched the conditions estimated from actual metamorphic rocks. Thermal relaxation for thickened crust takes around 50 million years, making it a gradual, Uniformitarian process. Form mountains and you form metamorphic rocks. Slowly.

Back in the 1980s our view of the original Barrovian rocks in Scotland was that they were deformed in the Caledonian Orogeny, that might well have lasted 50 million years. However, as a wrote in a previous post some now distinguish three different episodes of metamorphism within a 50 million year period. Something other than thermal relaxation must be going on.

Very speedy metamorphism

The evidence for their being three distinct metamorphism episodes comes from improved radiometric dating of a variety of rocks (geochronology). This can tell us when a particular mineral grain crystallised from magma, or grew in a metamorphic rock, or cooled below a particular temperature. Combining various data points allows us to measure how long a phase of metamorphism lasted. Another slightly different approach is to use diffusion geospeedometry. This technique relies on our understanding of how quickly different elements move through mineral lattices and how this varies with temperature. Given the right samples, it allows us to estimate for how a long a rock was heated above a particular temperature. There’s more about the details of the technique in this other post on some earlier work. Often estimates from this technique show spikes of temperature that were extremely short-lived, sometimes less than a million years.

A 2016 paper called “On the significance of short-duration regional metamorphism” by Daniel Veite and Gordon Lister is a fascinating overview of this topic1. He defines short-duration metamorphism as being any event that completed in 10 million years for orogenic events (e.g. Barrovian) and 5 million years for subduction metamorphism. These timescales are chosen so that by definition any short duration event must represent a thermal anomaly smaller in size than the scale of the crust or lithosphere. It’s over before there is time to heat the entire crust meaning that the thermal relaxation concepts of England and Thompson cannot explain it.

The paper summarises instances of short-duration metamorphism as discovered either via high-precision radiometric dates or through diffusion geospeedometry.

This figure shows a few different things. Firstly, there is a lot of data showing many examples of short-duration metamorphism. These data are taken from rocks around the world and of various different ages (but nothing PreCambrian2). Also note the scale difference. Geospeedometry consistently gives shorted timescales than estimates from high-precision geochronology. Whether this is because they are measuring slightly different things, or because one is more accurate than the other, is not clear at present.

But these are quibbles – two different forms of evidence show that short-duration metamorphism exists. So what causes it and what does it mean?

Flash steaming, not baking?

England and Thompson’s model shows only the effects of conduction and how it slowly transfers heat around the crust. Advective heat transfer instead is caused by the movement of hot fluids (water or magma, most likely) and it can happen extremely quickly.

The concept of metastability is key to metamorphism. The simple act of viewing high-grade minerals indicates that those minerals remained stable under different metamorphic conditions (as it cooled). Perhaps geospeedometry is measuring only a short instance of mineral growth in rocks that were slowly heated but where metamorphic reactions where suddenly triggered by a brief pulse of advective heating.

The same effect may apply if a gradual long-duration heating event is associated with spikes of heating. As show in the diagram above, we may be measuring the duration of a little red spike using geospeedometry and must be careful not to overinterpret it.

If metamorphic minerals often grow due to transient small scale events, maybe our understanding of metamorphism is faulty. Perhaps metamorphic zones don’t tell us what the steady state condition of the crust is and are less useful than we thought.

One final thought, perhaps temperature isn’t the driver? Recent ideas suggesting that tectonic overpressure is significant raise the possibility that sudden changes of overpressure may be driving rapid metamorphism.

What next?

Viete and Lister’s paper is a review of an exciting area of metamorphic petrology. They summarise convincing evidence that many metamorphic events are of short-duration. The growth of many metamorphic minerals can be seen as more a brief catastrophe than a slow gradual event. It ends with a clear call to action:

Techniques in very high-precision petrochronology that are capable of resolving short- and even extremely short-duration metamorphic activity exist, but are yet to be applied in combination to a single set of young rocks. Such work represents a crucial next step for metamorphic geology

An example of what they have in mind can be found in Veiete et. al (2015). This highlights a technique that allows accurate measurements of the age of small areas of zircons. It’s long been known that thin rims of younger age exist on zircons, but previously they’ve simply been removed so the older bigger core can be dated. The ability to date thin layers of zircon directly adds to the information we can gather. For young rocks (Cenozoic) this technique allows events can be measured to within a million years. Bring all this together with the right rocks (young and showing short-duration metamorphism) and perhaps we can start getting answers to the questions above.

It’s likely this will lead to some very exciting new work in the next few years. Some parts of the world of metamorphic petrology may be about to undergo a sudden transformation into something new and startling.

References

Viete, Daniel Ricardo, and Gordon Stuart Lister. “On the significance of short-duration regional metamorphism.” Journal of the Geological Society (2016): jgs2016-060.

Viete, Daniel R., Andrew RC Kylander-Clark, and Bradley R. Hacker. “Single-shot laser ablation split stream (SS-LASS) petrochronology deciphers multiple, short-duration metamorphic events.” Chemical Geology 415 (2015): 70-86.