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Research using the rhenium-osmium decay system proves that some diamonds are of remarkable antiquity, says Shirey.
Recent discoveries show that diamonds might form beneath subduction zones at great depths, and that very rarely plumes in the earth’s mantle move them up to where kimberlites can bring them to the earth’s surface.
“Our job is not just to measure the isotope composition of the inclusion, but also to get the whole picture of how the diamond formed, before we end up destroying the diamond to get the inclusion out.” The first step is to slice the diamond very precisely across its growth zones using a diamond-cutting laser.
“It would probably drive everybody in the gem business crazy to understand that we take a beautiful rough diamond and slice the center out of it,” says Shirey with a wry smile.
“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.