The 184 diamonds in the Seahawks Super Bowl Championship rings can tell us a thing or three about Earth's inner self. We're still interrogating those valuable, shiny rocks (which aren't actually forever). Here's the story so far:
You need just a few things for diamonds to form. For one, you need carbon. That's a diamond, yeah? Chemical formula C. Simple, right? We learnt the last time how that carbon came to be. But it turns out the simple story has a few plot twists.
Current research shows there's probably more than one kind of carbon that goes in to making diamonds. Some of it's the original star stuff (possibly including carbon from carbon stars, which are just neat). Carbon atoms, graphite grains, and nanodiamonds formed in the blastwaves of supernovae condensed with all the other stuff into Earth. Some of it ended up in the mantle, and hasn't seen the surface. It's called juvenile carbon, and it exists in the form of hydrocarbons like methane. But we suspect there's another kind that's much better traveled. It may have been up on the surface, and some of it ended up as biology (life on Earth is carbon-based, after all!). Some of that surface carbon, locked up in various deposits, rocks, and stones created from critters (like limestone), hitches a ride on a subducting slab, and ends up recycled into the mantle. And that carbon could, potentially, become diamond.
Only thing is, you and the Seahawks probably aren't wearing diamond rings containing carbon atoms that had a previous life as clams in the Ordovician - we'll see why in a moment.
So, you've got carbon hanging about in the mantle, but it's not diamond yet. That takes enough carbon in one spot to form a solution just supersaturated enough to start precipitating out. Have you ever made rock candy from sugar water? Same principle, really, only instead of a string, diamonds just need a nice little mineral seed to get started on, and there are plenty down there in the deep mantle. How deep? We're talking at least 140-190 kilometers (87-118 miles) down. At those depths, the pressure is about 45-60 kilobars. Just to give you an idea of how much pressure this is, imagine standing on Earth at sea level, with all the atmosphere above you. Not so bad, right? Now imagine adding between 44,999-59,999 more Earth's atmospheres. Dat's heavy.
Diamonds not only need all that pressure, they need the right temperature - around 900-1300°C (1650-2370°F). There's only two places on Earth that we know of where those conditions occur. One's in the upper mantle below cratons, the continental bits that have spent the last billion or more years being steady and dependable. No one puts it better than Metageologist:
[C]ratons are like great-grandmothers at family gatherings, while younger crust moves excitedly around them, they sit quietly, occasionally remarking on how different things were when they were young.
This may have led you to believe that the other place where the conditions for diamond formation are met is great-grandfatherly, but not so much. More like a long-lost great-nephew who crashes the family reunion with a bang.
However, the kinds of diamonds suitable for Super Bowl rings are not likely to have been created in a flash. They need around one billion to over three billion years to grow from bit to bling. And we're not actually positive those sorts of diamonds are busily forming beneath Great-Grandma Craton right at the moment - the best we can say is that they definitely were in the Archean and Proterozoic. And the little buggers probably aren't usefully uniformly distributed. You know how geology is: rather contingent.
Right, so we've got these nice crystals of diamond, all growed up. But they can't take a sentimental journey up through the crust, stopping to see the sights along the way. If the magma transporting them has got oxygen in it, at those temperatures and pressures, the lovely hard glittery clear-carbon rocks will have turned to carbon dioxide long before they ever make it to a mine. How weird is that, realizing that one of the hardest substances on Earth, something we think of as pretty much indestructible, is so fragile that it shouldn't ever have made it up here without becoming a greenhouse gas in the process?
Diamonds need rapid transit through those nearly two hundred kilometers from mantle to surface in order to survive. And just like there's only two kinds of places on Earth where diamonds can form, there's only two kinds of process that can transport them in super-express fashion. Find a panic handle to grab, my darlings, because it's about to get pretty explosive around here.
Davies et al (2004): "Inclusions in diamonds from the K14 and K10 kimberlites, Buffalo Hills, Alberta, Canada: diamond growth in a plume?" Lithos, Vol. 77, Issue 1-4, pp. 99-111.
Kelley, S.P. and Wartho, J-A. (2000): "Rapid Kimberlite Ascent and the Significance of Ar-Ar Ages in Xenolith Phlogopites." Science, Volume 289, Number 5479, pp. 609-611.
Mitchell, R.H. (1991): "Kimberlites and Lamproites: Primary Source of Diamond." Geoscience Canada, Volume 18, Number 1, pp. 1-6.
Sparks et al (2006): "Dynamical constraints on kimberlite volcanism." Journal of Volcanology and Geothermal Research, Volume 155, Issues 1–2, pp. 18–48.