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His particular specialty involves seeking out the rare individual diamonds that contain tiny and specific sulfide inclusions, painstakingly removing the inclusions and analyzing their contents—in this case, trace amounts of the radioactive isotopes of the rare metals osmium (Os) and rhenium (Re)—to determine ages on the order of billions of years.
An isotope of a chemical element is simply an atom with the same atomic number as that element, but with a slightly different mass, resulting from a different number of neutrons in its nucleus.
“We have to go to a mine or some place that’s being very aggressively prospected so that they’re processing large amounts of kimberlite for diamond grade.” Diamonds are trace minerals in the rock—kimberlite, or more rarely a lamproite as in Australia’s Argyle mine—that carried them up from the mantle.
“A diamond in a kimberlite occurs at the part-per-billion level,” says Shirey, “so the average person walking around on a kimberlite is not going to find a diamond sitting there—that’s an extremely rare occurrence.” Once researchers have traveled to suitable mining or exploration operations, where large amounts of diamond-bearing ore are produced, they have to pick through the production.
The process begins with suitable inclusion-bearing diamond crystals.
As Shirey tells us, obtaining such diamonds for study is quite a challenge.
“We have to get the inclusion out without breaking it,” says Shirey.
“We need to recover the whole inclusion, and we also need to characterize the diamond as fully as possible.
If scientists know the rate of radioactive decay, and have instrumentation capable of measuring the different isotopes present, they can calculate the age of an item very accurately.These “superdeep” diamonds provide Shirey and other scientists a tantalizing window into the workings of the deeper mantle.Peter Johnston © GIA paper “Recent Advances in Understanding the Geology of Diamond,” coauthored by Shirey and GIA’s Dr. Shirey is a one of a collaborative group of geoscientists from institutions all over the world using diamonds as a means to sample the deep earth.The isotopes Shirey seeks provide a much longer reach back into the earth’s history.These radioactive isotopes are like tiny, slow-ticking clocks captured in the fabric of a diamond crystal.
“So in one mineral species,” he says, “you have the deepest, the oldest, and the most resistant to secondary effects mineral that you can get, so it makes them very, very unique specimens.” Shirey tells us that the osmium-rhenium radioactive decay system allows researchers to date individual inclusions.