12 November 2015

## Dialing in 5 GPa on a diamond anvil cell

Posted by Callan Bentley

Peek into the middle of this contraption, and tell me if you see anything interesting:

Did you notice the gem-quality diamond in there?

This is not the latest in fashionable jewelry, but a scientific tool called a diamond anvil cell. (Well, half of one, anyhow.)

The exceptional hardness, strength, and clarity of the diamond allows pressure to be cranked up on a sample squeezed between two diamonds, and laser light to heat the sample up, approximating conditions in Earth’s interior. Extraordinary temperatures and pressures can be achieved in such an apparatus. And if you’re an experimental petrologist, you can watch what’s happening through the window of the diamond itself. The two images above show open ones: half of the apparatrus. I got to examine them courtesy of Steve Gramsch of the Carnegie Institution’s Geophysical Laboratory during my visit to the Carnegie a few weeks ago. He then handed a closed diamond anvil cell to me. I hefted it and asked how much pressure it was under. Steve casually replied “60 gigapascals or so.”

That shocked me – not that it could get up that high, but that it was that high, just sitting there at that extraordinary pressure. Suddenly, I felt as if I had a grenade in my hands.

Consider the following:

• A newton is the force needed to accelerate a mass of 1 kilogram by 1 meter per second per second.
• A newton of force spread over a square meter of area is one pascal.
• One billion pascals is one gigapascal (GPa).
• Setting aside tectonic stresses and only considering overlying rock mass and the acceleration due to the force of gravity, the pressure in Earth’s interior goes up by about 30 megapascals (MPa) per kilometer of additional depth: that’s 0.03 GPa/km.
• 60 gigapascals is therefore a pressure equivalent to about 2100 kilometers of depth in the planet – most of the way through the mantle, though not quite to the outer core (which is at ~2900 km depth).
• A pressure cooker cooks at 0.0001 GPa.
• Your car’s tires are inflated to a pressure of 0.0002 GPa (2 bars, or ~30 psi).
• 60 GPa is a lot more than 0.0002 Gpa.

Steve brought out two Allen wrenches. I put one in the red Allen bolts (on the left – a dated Cold War reference there) and one in the blue bolts (on the right), and gave each a gentle turn through about 5 degrees of arc. It was easy. I was done in about 8 seconds.

By turning that Allen wrench, I dialed up the pressure on that diamond anvil cell by 5 gigapascals (GPa); that’s 25,000 times the pressure that your tires are under, added to the diamonds’ pre-existing pressure before I walked into the room. The diamonds were now squeezing each other at 65 Gpa. If you accept a pressure gradient of 30 Mpa per kilometer, that means my turn of these four bolts was equivalent to plunging the experimental rock sample another 167 km deeper into the lower mantle – an hour and a half of driving time, if you could drive at highway speed straight down through bridgmanite, (which of course you cannot, Aaron Eckhart’s and Hilary Swank’s best efforts aside).

That blew my mind. It was such a small motion on my part, imparting so little force to the Allen bolts, but that translated into a mighty change on the tip of that diamond.

Our campus’s geology club is having Dr. Gramsch out to speak about mineral physics tomorrow – and I think these diamond anvil cells will be props in his lecture. I hope dearly for my students to be similarly impressed at this tiny little microcosm of our planet’s deep interior.