ScienceWise - Winter 2011

Does the Earth’s crust yo-yo?

New research suggests that subduction may be much more complex than once thought

Subduction, the process in which one of the Earth’s tectonic plates slides below another, is one of the most important geological processes shaping both the surface of the Earth and the minerals found on it.  When soft sediments or volcanic rocks formed at the surface are driven deep into the Earth by subduction, the temperatures and pressures they are subjected to transform them into rocks with very different physical and chemical properties. Geologists call rocks transformed – or rather metamorphosed – in this way, eclogites.

In most cases, the subduction process simply carries rocks deep into the mantle never to be seen again because with the increasing pressure they become denser and denser and thus sink down into the Earth’s mantle. Fortunately for us what sometimes happens is that these dense eclogites are associated with less dense rocks and together they are able to rise through the upper mantle and reappear close to the Earth surface. This is similar to a stone in water. Normally it would drop to the bottom of the water, however if it is bound up with some floating material such as cork, it might pop back to the surface. Such exhumed eclogites provide many useful minerals and from a scientists’ perspective, invaluable information on processes acting at 50-100 km depth in subduction zones.

It’s hard to imagine something that feels as solid as rock is moving, flowing and floating buoyantly through surrounding rocks. However, the geological forces involved are colossal and so are the time scales. Tectonic plates subduct at a vertical rate of a few cm per year, or 10’s of km per million years. So to reach 70km below the surface and undergo metamorphic transformation can easily take a few million years.

In the simplest case, a piece of the Earth’s crust is subducted and exhumed only once. However recent research at the Australian National University suggests that the subduction process may sometimes be far more complicated than previously thought.

Drs Daniela Rubatto and Joerg Hermann, together with their Honours student Katherine Boston, have just published a paper in the prestigious journal Nature Geoscience, in which they describe a far more intricate process involving multiple cycles of subduction and exhumation. They describe this as “yo-yo subduction” which although suggested theoretically, has never actually been observed in nature until now.

But how exactly do you go about ascertaining what the history of a piece of rock has been?
“The types and compositions of mineral formed in a rock sample can give us clues about the conditions it has been exposed to,” Dr Rubatto explains, “If we see a particular mineral in the rock we can surmise that it must have experienced a given temperature and pressure.”

Working out the timing of these processes is a little more complicated. Some minerals naturally include uranium as they form, incorporating it into their crystal structure. Over millions of years this uranium radioactively decays to lead so by measuring the ratio of uranium to lead you can in principle, determine how old a particular grain of mineral is. Generally this is done on multiple crystals at one time, yielding an average age for the specimen.

What made the current study possible was access to the ANU SHRIMP – A diagnostic machine that bombards a sample with energetic ions and records the material sputtered from the surface. The special thing about SHRIMP is that it can do this at an incredibly sharp focused spot of only 20µm across. This enabled the scientists to work out an age for each region of the tiny zircon crystals that had formed during the rock’s history.

The scientists stumbled upon their discovery whilst looking at other geological processes in rocks from the Italian Alps. ‘I was handed a sample to analyse as part of my honours project,” Katherine says, “and the results we got back were all over the place. I was asking myself, what’s going on?”

What she’d discovered was that the zircon crystals had zones in them, corresponding to multiple stages of being heated and pressurized followed by cooling. The simplest explanation for which is that the rocks had experienced repeated subductions and exhumations over a period of 20 million years.

20 million years sounds like a long time but in geological terms it’s a heartbeat. The fact that the rocks were subjected to repeated cycles over such a short time scale suggests that the movement at the plate boundary was very complex, perhaps even turbulent. Combining this data with other measurements in the region should help scientists get a better understanding of the subduction process in general.

This in turn, will lead to a better understanding of processes like earthquakes, volcanism and mountain building, all of which occur principally at plate boundaries. It may also be of assistance to the mining industry because minerals such as copper ore often concentrate along the seams and folds created by plate tectonics.

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