By Larry Larason
Until the 1950s most of the world’s diamonds came from southern Africa. The De Beers cartel had a virtual monopoly on the diamond trade. But diamonds have been discovered all around the world in the last few decades. Russia began mining diamonds in 1957. Australia started up in the 1970s. Canadian diamonds have been produced since the late 1990s. Is there a chance that we might find them in the Four Corners?
When I lived in Louisiana there was an ad for a jewelry store that played on the radio almost every morning as I drove to work. It went something like this: “Millions of years ago a lump of coal got buried. Over time, under great pressure, it was transformed into a sparkling diamond.”
Here’s another story of transformation: The princess bent down and kissed the frog on his wide green forehead. She wiped her lips on the back of her hand and stepped back as the frog transformed into a tall handsome prince.
We have a lot of coal in the Four Corners. Should that produce a lot of diamonds? The frog prince is a fairy tale. (Kissing a frog gets you a diamond only if you’re a princess willing to marry a guy who used to catch flies on his tongue.) The story of the coal becoming a diamond is a fairy tale, as well. Yes, diamond is carbon, and coal is mostly carbon, but you could not bury a lump of coal deeply enough for it to become diamond.
So where do diamonds come from? The answer is deep in the crust of the earth. They formed at a depth of about 90 to 150 miles below the surface. Notice that I used the past tense in that sentence, because diamonds are old. For a couple of decades geologists have been analyzing radioactive isotopes trapped in the crystal lattices of diamonds, and the results show that nearly all of them are more than 2 billion years old, and some are even older than 3 billion years. It’s not at all certain that they are still forming deep in the Earth.
For carbon to crystallize as diamond it needs to be hot and under extreme pressure. In the “diamond zone” the temperature is around 1000o C; that’s nearly 2000o F. The pressure is 30 kilobars or about 440,000 pounds per square inch. Change the temperature or pressure and the diamonds could revert from the hardest known mineral to the softest, graphite. So how did diamonds get to the surface without degrading?
They were brought up by a type of volcanic eruptions known as diatremes. The Four Corners is often touted as having the greatest concentration of diatremes in the world. In the Hopi Buttes Volcanic Field there are about 300. The Navajo Volcanic Field, including Ship Rock, has about 80 volcanic features, almost all of which are diatremes. I’ve written about diatremes before: “Ship Rock” [GJ May, 2006]; and “Diatreme Drive” [GJ September, 2008]. What I haven’t written much about is that there are two kinds of diatremes: shallow, or steam powered, and “deep throat.”
In the steam-powered model, magma rising slowly from the mantle encounters ground water, heating it to steam, which explodes to the surface. Most diatremes are steam powered. All of the ones in the Hopi Buttes and most of those in the Navajo Volcanic Field are of this type. The magma does not make it to the surface or it would cover the evidence of the steam blow out, and we would not see the diatreme.
Ordinary basaltic magma rises from the top of the mantle, sometimes bringing up mantle minerals such as olivine with it. But for a deep throat diatreme the eruption must originate from lower down. Under tremendous pressure the volatile gas rises at supersonic speed, drilling a pipe and carrying a load of stone from the mantle. It shatters all of the crust that it passes through and rips fragments out of the walls of the pipe, including rock containing diamonds. By the time the rocks have tumbled up to the surface they are broken into small pieces or they have been pulverized. Deep throat eruptions commonly carry two types of magma. The best known is kimberlite, a usually bluish mix of minerals. The diamonds of southern Africa are found in kimberlite pipes. The second rock is lamproite. It is less common, but it is the host to Australia’s gem stones. These are probably the deepest rocks that have ever come to the surface.
It is essential that diamonds move quickly up the pipe if they are to survive removal from the temperature and pressure where they formed. The gases propelling stone in the pipe travels at two to four times the speed of sound. This allows the diamonds to cool quickly so they don’t degrade to graphite.
We have a few deep throat diatremes in the Four Corners, although none of them brought up kimberlite or diamonds. [Some sites on the WWW say there is kimberlite in the Four Corners, but it is not true.] You can see one of them, called Green Knobs, just north of Navajo, Arizona. Its color sets it apart from the red Chinle shale and Entrada sandstone that encloses the pipe. The makeup of the green rock is mostly crustal stone that has been reduced to sand by the rough journey up the diatreme pipe. Only about 1% of the material came from the mantle, but you can find tiny pieces of olivine.
So, the Four Corners has no diamonds. Why? 1. The gems seem to be unevenly distributed far below. Even if we had kimberlite pipes, only 14% of them worldwide have diamonds. 2. The Earth is not as active as it used to be. Not only are diamonds old, but so are the deep throat diatremes. The most recent diamond bearing pipe seems to have formed 20 million years ago in Australia during the Miocene Period. Such features are very rare in the last billion years. 3. There is some disagreement about whether diamonds form in the crust or the mantle. If, indeed, they form in the crust, then ours, at about 30 miles thick, is too thin to reach into the diamond zone at 90+ miles below the surface. This is probably the most important reason.
The nearest place diamonds are found is along the Colorado-Wyoming border in a field of about thirty-five diatremes, where they have been commercially mined. Oddly, the site of the great diamond hoax of 1872 was also near the state line, but west of the diatreme field where diamonds really do exist. That hoax involved “salting” a site with gems and selling it to investors. The two swindlers made more than $600,000 on the deal. Geologist Clarence King, who had just completed the survey of the 40th parallel, was sent to investigate. He and his men found gems, but at least one of them had already been cut – a dead giveaway to the fraud. One of the con men was traced and returned $150,000, but the second man was never seen again.
The other source of diamonds in the United States is the Crater of Diamonds State Park in Arkansas. When it was discovered in 1906 it was the only place outside of South Africa where diamonds could be found at their source. Mining at this site was never commercially viable, but the public is allowed to dig in the 95 million year old lamproite pipe and keep whatever they find. I’ve read that it takes about fifteen man-days to find a diamond, but I hope to go there one day, maybe this spring, and try my luck.
A Short Rant
The recent death of Leslie Neilsen reminded me of a pet peeve. Actors become associated with certain lines from their movies. One example, in Neilson’s case: “…and stop calling me Shirley.” You probably recall lines like, “Make my day”; “I’ll be back”; or “You can’t handle the truth.” But we tend to forget that these lines are delivered by actors. Someone gave them that script: the writers.
Movies can be powerful statements and resonate in our minds for decades. That’s why once a movie comes out, we tend to forget the book it was based on. I’m not denigrating actors. It’s their delivery that makes the lines stick in our minds. But don’t forget the writers who gave them those words.