I take great pride in telling people that I have four bombs in East Kilbride that I’ve been using quite regularly. They were quite easy to get hold of, if a little expensive, and are working their magic on my samples. I’m remaining a bit secretive at the moment about the details of the preparation procedure, but we’re having great success at SUERC in East Kilbride with apparent complete dissolution of all samples. We’re using quite a potent mix of acids in the bombs at high temperatures and pressures. The procedure is on a 2-day cycle for 4 samples so it’s going to take a while to get the 40 or so samples from Norway processed in good time. Teaching is coming up soon and so it’s going to be tough juggling everything!
We’re all used to the concept of pressure, but in many different ways. Some think of peer pressure, the pressure of working life, or maybe of your ears popping on an aeroplane. For geologists however, geological pressure along with it’s siblings geological time and temperature describe conditions in the Earth which are unimaginable to most people.
Pressure is a measure of force per unit area, and it’s quite easy to calculate given just a few variables (i.e. area, force, maybe acceleration too). Beyond the base of the Earth’s continental crust, these geological pressures reach 2,500,000,000 pascals or 2.5 GPa, and continue to rise the deeper you go. From this point on, a rock experiences Ultra High Pressure (UHP) and may be metamorphosed (rather than melted). It is, however, difficult to preserve rocks which have been down so incredibly deep as there are very few processes which take rocks down below the base of the crust and bring them back up to the surface quickly enough so that they preserve information on their burial history. Therefore although UHP conditions prevail throughout much of the Earth, not many rocks on the surface ‘remember’ if they were ever there. More than likely they weren’t.
But just how much pressure is felt by rocks when they reach UHP conditions? Well yes we know it’s 2.5 GPa or above, but how can we relate to that? Well, to generate a UHP of 2.5 GPa concentrated on the golden face of the queen on a standard first class stamp, you would need to load that stamp with over 76 million kilograms. That’s equivalent to balancing 10 full-sized Eiffel towers on Lizzie’s Chops! Even that kind of pressure is hard to imagine, but suffice it to say you’d not want to trap your finger under that!
In my eclogites the mineral garnet commonly contains inclusions of quartz. When the garnet bearing rock experiences UHP conditions for long enough, the quartz changes its structure to a more dense form we call coesite. For the UHP rocks in my field area, these rocks inevitably start making their way to the surface. As the pressure starts to drop, the coesite inside the garnet starts to expand. Like the casing of an imminently exploding grenade, however, garnet resists the expansion. Eventually, however, the coesite reverts back to the quartz structure which takes up a larger volume. This shatters the garnet in the surrounding space just like the explosives in a grenade must shatter the casing to continue expanding.
So when I look at my rocks in thin section, I really like to think that I’m looking at tiny quartz grenades exploding on the face of the queen under the weight of 10 Eiffel towers!
There remains much controversy amongst geologists studying the origins of eclogites in Norway. Once the focus of much study in the 60s right through to the mid 90s, much is understood about their metamorphic history in terms of pressure and temperature. However, there remains no clear consensus on the nature of their protoliths and the implications this may have for their entrainment in the country rocks. Recent explorations of their REE geochemistry by myself at the NERC ICP-MS facility have revealed that most have volcanic arc affinities, with a significant minority having N-MORB affinities. For now, it seems like the basalts were created in an oceanic arc with a back-arc spreading ridge. Investigations to prove this are currently underway, but so far it seems likely.